Scientists & Engineers for America

Climate Change: Science and Policy Implications

Forecasting future climate conditions is challenging, and some major processes
remain poorly understood. However, methods are improving to characterize the
risks. Scientists have found it is very likely that rising greenhouse gas
concentrations, if they continue unabated, will raise the global average temperature
above natural variability by at least 1.5o Celsius (2.7o Fahrenheit) during the 21st
Century (above 1990 temperatures), with a small likelihood that the temperature rise
may exceed 5oC (9oF). The projections thought most likely by many climate
modelers are for a greenhouse gas-induced temperature rise of approximately 2.5 to
3.5oC (4.5 to 6.3oF) by 2100. However, the magnitude, rapidity, and details of the
change are likely to remain unclear for some time. Future climate change may
advance smoothly or sporadically, with some regions experiencing more fluctuations
in temperature, precipitation, and frequency or intensity of extreme events than
others. Some scientists emphasize potential beneficial effects of climate change, or
count on the ability of humans to adapt their behaviors and technologies to manage
climate change in the future; other scientists argue that the benefits of climate change
may be limited, even accounting for probable adaptation and its costs, and that there
are risks of abrupt, surprising change with accompanying dislocations.

The continuing scientific process has resulted in a better understanding of
climate change and generally confirms the broad conclusions made in previous
decades by the preponderance of scientists: that human activities emit greenhouse
gases that influence the climate, with potentially serious effects. Details have been
revised or refined, but the basic conclusion of the risks persists. The principal
questions remaining for the majority of scientists concern not whether greenhouse
gases will result in climate change, but the magnitude, speed, geographic details, and
likelihood of surprises, and the appropriate timing and options involved in addressing
the human components of climate change.

Introduction

CLIMATIC EFFECTS OF POLLUTION: Carbon dioxide is being added to the
earth’s atmosphere by the burning of coal, oil and natural gas at the rate of 6
billion tons a year. By the year 2000 there will be about 25% more CO2 in our
atmosphere than at present. This will modify the heat balance of the atmosphere
to such an extent that marked changes in climate, not controllable through local
or even national efforts, could occur. Possibilities of bringing about
countervailing changes by deliberately modifying other processes that affect
climate may then be very important.

President’s Science Advisory Panel, 1965

For more than a century, scientists have known that adding carbon dioxide and
certain other gases to the atmosphere could warm the Earth, as expressed in the quote
above. During the past decade, mounting scientific evidence and public debate have
generated interest in the U.S. Congress to understand climate change, and potentially
to address related emissions of carbon dioxide, methane, and additional gases
generated by human activities. These human-related emissions have accumulated in
the atmosphere, raising concentrations dramatically. For example, carbon dioxide
emissions associated mostly with fossil fuel combustion and land clearing have
increased atmospheric concentrations by one-third since the Industrial Revolution,
from about 280 parts per million (ppm) in 1850 to 380 ppm today.
Investment in science and technological research has been the cornerstone of the
federal strategy since the 1960s. Great strides have been made to collect observations
of relevant Earth processes; to develop a variety of models to analyze and forecast
atmospheric, ocean, land, and related economic and energy systems; and to
understand the potential impacts of climate change on humans and ecosystems.

Research has been conducted by universities, governmental agencies, research
institutions, and the private sector. Most U.S. funding has gone to researchers through the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), and the Department of Energy (DOE), as well as smaller amounts through other U.S. agencies and the
governments of other nations. Research collaboration across countries, gaining
efficiencies and insights across boundaries, has proceeded through the International
Biosphere/Geosphere Program, the World Meteorological Organization (WMO), and
others. The products — data, models, and analyses — have been peer-reviewed,
repeated, reproduced or contradicted, debated, and updated. Dozens of assessments
of various aspects of climate change science have been conducted by public multidisciplinary
bodies, including the National Academy of Science (NAS), the U.S.
Congress’s former Office of Technology Assessment, and the Intergovernmental
Panel on Climate Change (IPCC).

The IPCC is the preeminent international body charged with periodically
assessing technical knowledge of climate change. It draws on thousands of scientists
with expertise on all aspects of climate change. The Fourth Assessment Report
(AR4) of the IPCC assessment is being issued as several reports in 2007. A summary
of selected key findings of the science assessment is provided below in
“Intergovernmental Panel on Climate Change — Climate Change 2007: The Physical
Science Basis.” Many governments use the IPCC assessments as one input to their
policy deliberations. The U.S. government has been a primary supporter of the IPCC
assessment, by sponsoring much of the research and monitoring that underpins the
assessment, as well as many experts representing a broad range of views on specific
topics, and by coordinating both external expert and governmental peer reviews of
the reports.

The continuing scientific process has resulted in an understanding of climate
change that is increasingly robust and has withstood a multitude of challenges; in
most respects, the evolving science confirms the broad conclusions made in previous
decades by the preponderance of scientists. Many details and complexities, however,
remain nebulous. Although most scientists are confident in their current
understanding of climate change, they are less certain about the future magnitude,
timing, and geographic details.

Climate Change Science Is a Process

The evolution of scientific understanding of climate change is an example of the
process of science. It is iterative, beginning with statements of hypotheses,
followed by testing and observations, scrutiny by other scientists, reproduction or
repudiation of results, and revisions to the state of knowledge. The practice of
critique and revision is desirable to support continuous improvement of
knowledge; some comments provide insights while others may be refuted.
Research may decrease certainty, or enhance it. The objective, like most science,
is to be able to produce ever more reliable predictions.

Such has been the case following the discovery described by Joseph Fourier in
1827 that Earth’s atmosphere acts like the glass of a greenhouse, letting in solar
energy, then trapping some of its heat, hence influencing the temperature of the
Earth’s surface. Related findings have been tested, critiqued, refuted, revised and
improved over almost two centuries. As a result of the scientific process, most
scientists are confident in their current understanding of climate change and the
role of human activities, including emissions of so-called greenhouse gases.
Ongoing research, debate, and refinement have yielded broadly accepted
agreement on climate change science.

Intergovernmental Panel on Climate Change
Climate Change 2007: The Physical Science Basis

In February 2007, the Intergovernmental Panel on Climate Change (IPCC) released
its fourth assessment of the science of climate change, updated with research
reported over the previous six years. Selected major findings from this report
include the following:

Carbon dioxide emissions associated with land-use change are
estimated to be 1.6 GtC [0.5 to 2.7] per year over the 1990s,
although these estimates have a large uncertainty.”

Climate change has become a highly debated issue in which the U.S. Congress
has maintained an active and continuing interest. Congress has provided tens of
billions of dollars for research and additional programs (almost $5 billion in
FY2006). It established the Global Change Research Program (USGCRP) in the
Global Change Research Act of 1990 (P.L.101-606), aimed at understanding and
responding to global change, and requiring a scientific assessment for the President
and Congress at least every four years, as well as annual reports on activities and
budget. The Senate in 1992 ratified the Framework Convention on Climate Change,
which contains commitments from the United States and other parties to improve the
science and cooperate internationally on it, as well as to communicate about climate
change to the public.

The first and only national assessment was released by the executive branch in
2000. Our Changing Planet, an annual report to Congress, outlines climate-related
research progress and strategies. Various committees have held hearings to oversee
existing programs and to consider further options. There has been general consensus
regarding the benefits of additional scientific and technological research; however,
long-standing debate exists about whether and how to attempt to achieve mitigation
or adaptation through federal legislative action. As additional action is considered,
an understanding of climate change science may help inform consideration of relative
priorities, the scope of action, and timing, among other issues.

This report presents an overview of the science of climate change and its
potential impacts. It provides highlights on the impacts of climate change itself; it
does not address the mitigation of climate change (e.g., by controlling greenhouse gas
emissions) nor the economic issues associated with it. This report is organized into
four topics:

This report represents a snapshot of the science of climate change, which will
continue to evolve. It will be updated periodically to reflect new peer-reviewed
evidence and the developing state of scientific understanding.

Changes Observed in the Earth’s Climate

Scientists agree that the Earth’s climate is changing. The average global surface
temperature has increased since the Industrial Revolution, by about 0.6 to 0.9o
Celsius (1.1 to 1.6o Fahrenheit) from 1880 to today. The U.S. temperature has risen
at roughly twice the global average rate since the 1970s. The global climate of the
past few decades is likely approaching, or has already passed, the warmest since the
rise of human civilizations around 12,000 years ago. Precipitation in the 20th
Century has increased by about 2%, more in the high northern latitudes, while drying
has occurred in parts of the tropics, especially in the Sahel and southern Africa.
Extreme precipitation events have increased in most of the few locations where data
are adequate for analysis.

The following sections cover observed climate change. The initial section
describes changes in the climate measured around the globe, first in temperature, then
precipitation, then extreme events. The subsequent section describes climate changes
specific to the United States. The last section on observed climate change identifies
findings from the study of climate centuries to millenia ago that have relevance to
consideration of recent and potential future climate change.

Global Climate Changes

Global Temperature. The average temperature of the Earth’s surface, the
global mean temperature (GMT), has increased about 0.6 to 0.9oC (about 1.1 to
1.6oF) from 1880 to 2004. Warming occurs over land and sea
surfaces. The warming during the 20th century, however, was not smooth. Global
warming occurred from around 1910 to 1945, followed by a period of slightly
declining or stable temperatures into the 1970s. Since 1979, warming has returned
to about twice the rate of the 20th Century average, at about 0.18oC per decade (0.32oF/decade). A panel of the National Academy of Science has reconciled earlier
disagreement in confirming that warming has occurred also in the mid-troposphere,
though less than at the surface.

Globally, 2005 was the warmest in nearly 130 years of direct measurements;
2006 was the sixth warmest year on record. The 10 warmest years on record have
occurred since 1994.

Surface temperatures have increased nearly everywhere,
except for cooling in parts of the North Atlantic. Warming has been greatest in the
northern high latitudes, such as Alaska, Canada, Northern Europe, and Russia.
Although warming over most land areas has occurred year-round, the increases have
generally been greater in the Northern Hemisphere during winter and spring.

Strong evidence of global warming since 1955 comes from measurements of
heat content of the world’s upper oceans, overall by 0.04oC since 1955. Because
oceans store about 84% of the heat on Earth, this small warming is considered a
strong signal of long-term change. Additional evidence of global warming comes
from the detection of increasing continental temperatures (measured by boreholes
into rock below the surface), by about 0.02oC, during the past five decades. Both
ocean and continental warming corroborate the elevated surface air temperatures.

Global Precipitation. Humans and ecosystems are affected by many aspects
of climate, including precipitation, which has increased over the past century. This
observed increase is consistent with scientific understanding that as warming
temperatures increase evaporation, precipitation will increase to maintain balance in
the water cycle. Over land, precipitation has increased by about 2% since 1900, but
the patterns are highly variable across time and in different locations. Only a few
regions show significant changes. Most of the United States and other
high latitudes, except eastern Russia, have seen greater wetness, whereas
precipitation has decreased in the sub-tropics, such as the Sahel in Africa.

Climate Extremes. Few patterns of change have emerged globally in the
frequency or intensity of most types of extreme events, according to NOAA’s
National Climatic Data Center (NCDC). No trend in global thunderstorm
frequencies has been identified. Though there has been a clear trend toward less
frequent extremely cold winter temperatures in some locations, there is no trend in
the frequency of extremely high temperatures. Researchers at NOAA’s National
Center for Atmospheric Research (NCAR) have found that “[w]idespread drying
occurred over much of Europe and Asia, Canada, western and southern Africa, and
eastern Australia. Rising global temperatures appear to be a major factor.” Drought
area increased more than 50%, mostly due to conditions in the Sahel and southern
Africa over the past few decades. Researchers have found that great floods
worldwide increased significantly during the 20th Century, especially in the latter half
of the period. The frequency of floods exceeding the 200-year flood levels also
increased significantly, while the frequency of floods having return periods shorter
than 100 years did not increase significantly. In most regions, insufficient data
remain a challenge for assessing trends in climate variability, because of the
infrequency of events (by definition) and their spatial variability. In regions that have robust precipitation data, extreme precipitation has increased; in a few regions, such as the Sahel and East Africa, extreme precipitation has decreased.

Contentious debate continues regarding trends in hurricane or cyclone frequency
and intensity. For about 85% of the world’s oceans, data are inadequate to detect
long-term changes. Only in the extra-tropical Atlantic basin has research
established a positive relationship between sea surface temperatures and increased
number and severity of hurricanes or cyclones. Since the mid-1980s, satellite data
reveal a distinct increase in tropical cyclone activity associated with higher eastern
Atlantic sea surface temperatures, as well as other factors. Much longer series of
high-quality observations and improved understanding of tropical cyclones are
needed to provide definitive attribution of changes in hurricane activity to natural
variability, greenhouse gas (GHG) forcing, or other processes. A November 2006
meeting of international experts on tropical cyclones, convened by the World
Meteorological Organization, concluded that “[d]espite the diversity of research
opinions on this issue, it is agreed that if there has been a recent increase in tropical
cyclone activity that is largely anthropogenic in origin, then humanity is faced with
a substantial and unanticipated threat.”

Climate Changes Observed in the United States

At the global scale, average annual temperature and precipitation have increased
over the past century, especially since the 1970s. At a regional scale, such as for the
United States, significant climate changes have also been measured.
In the United States, both temperatures and precipitation have increased during
the 20th Century, but with important regional variations. The mean
temperature for the contiguous United States has increased by 0.6oC (1.1oF) since
1901 (0.06oC or 0.1oF per decade), generally following the global oscillations. From 1979 to 2003, the rate of warming in the United States, at 0.33oC (0.6oF) per decade,
has been about twice the rate of the global average. Two regional exceptions to this
overall warming were (1) cooler winters and springs in the Southeast from 1901 to
1978, which reversed to warming after 1979 (except in Florida), and (2) generally
stable or cooler summer and autumn months in the South and central United States.

The warmest year for the United States in the 20th Century was 1998 (with
temperatures boosted by El Nino), followed by 1934; 2006 was the third-warmest
year in the U.S. record, about 1.1oC (2oF) above the 20th Century average.
U.S. average precipitation has increased 6.1% since 1895, but with more
variability and a less distinct pattern than temperature. The increase in
precipitation was particularly pronounced in the central, south, and east north-central
climatic regions. The Northwest has had large and increasing inter-annual cycles of
precipitation since the 1970s, associated with ENSO (El Nino-Southern Oscillation)
events. In contrast, the Southwest and Hawaii have had decreases in precipitation,
although the trends are not statistically significant because of high inter-annual
variability. The 20th Century was also marked by strong and extensive droughts in
1931 to 1938 and 1951 to 1956 in the United States.

The frequency of extreme precipitation events in the United States has
increased since the 1920s and 1930s, although frequencies in the late 1800s and early
1900s were about as high as in the 1980s and 1990s. The number of events “much
above normal” has increased by 20% since 1910. Extreme precipitation events
lasting from one to seven days increased at a rate of about 3% per decade from 1931
to 1996. In the Southeast, very heavy precipitation not associated with hurricanes
has increased by about 2.6% per decade on average in the 20th Century. Although
such increases in precipitation intensity and duration tend to increase the risk of
flooding, some land uses and investments in flood management actively work to
reduce such risks. Tornado frequency since 1955 has not changed much, although
the record is complicated by reporting uncertainties.

Climate Lessons from the Distant Past

Climate change since the Industrial Revolution has been measured both
globally and across the United States, as increases in temperature and changes in
precipitation. Whether this constitutes natural variability, and how to interpret the
observed change, can be informed by putting it in the context of climate changes that
have occurred over the past thousand, ten thousand and hundreds of thousands of
years.

A number of lessons can be gleaned from paleoclimatology, the study of past
climates, by piecing together extensive and disparate sets of proxy indicators, such
as ice cores, tree rings, fossil records, and the chemical composition of shells. First,
modern human civilizations have developed and thrived in the stable and relatively
temperate climate of the past 12,000 years, a period called the Holocene period.
Global average temperatures have varied fairly narrowly during the past 10,000 years,
within about 2oC, with the maxima around the current global mean temperature and
the minimum temperatures occurring in the 16th and 17th centuries, during the Little
Ice Age, about 1oC cooler than current temperatures. According to NOAA, no
evidence demonstrates that global annual temperatures at any time during the
Holocene were warmer than today.

Second, some climate shifts can be rapid, occurring on time scales of only
years to decades. Paleoclimatological records show abrupt shifts that have included
changes in hurricane frequency, flooding, drying of lakes, and several mega-droughts
lasting decades to centuries. Recently, evidence shows that the main warming events
ending the last ice age (about 15,000 years ago) took place in less than a decade.
Regionally, the shift was rapid and extreme, with Greenland’s temperature rising in
one step of around 8oC in a decade or less.

This revelation that the Earth’s climate can change abruptly, with triggers,
amplifiers, and bounds that are not well understood, has precipitated grave concern
among many scientists. A number of studies have found that major ecological
restructuring has accompanied major climatic shifts in the past. Although human
societies have proven adaptable to moderate inter-annual variability and smooth
change, research in several regions indicates that significant structural, and
sometimes catastrophic, reorganizations of regional civilizations (e.g., the Mayas in
the 9th Century, African civilizations) have been triggered by past significant
climate changes.

Observed Impacts

Scientific research has revealed a number of changes in the Earth’s climate
system in the past century. This section addresses changes observed in human and
ecological systems that may be associated with, and perhaps caused by, the observed
climate changes.

Impacts of climate change over the past few decades are visible on human
activities and ecosystems. There is a likely bias that favors the reporting of detected
changes, rather than the reporting of no changes. Nonetheless, observations confirm
that most biological and physical systems studied are responding to warming and
other climate changes over the 20th Century in ways scientists would expect, but there
have also been some surprises.

The northern boundary of successful corn production in the United States has
migrated north by 100 miles over the past three decades, according to an official of
the DuPont Corporation. One study explained roughly 25% of corn and 32% of
soybean yield trends in certain counties in the Midwest and Northern Great Plains by
observed temperature trends from 1982 to 1998. Some salmon fisheries in parts of
Alaska have seen record catches, benefitting from warmer temperatures, while in
western Alaska, the Pacific Northwest and Canada, salmon stocks have decreased,
having passed the upper limits of their temperature tolerance, according to the Alaska
Regional Assessment Group for the U.S. Global Change Research Program.

In the U.S. West, decreasing trends in mountain snowpack and earlier
snowmelt have altered the timing of stream flows. This has significant implications
for flood control, irrigation and summer drying of vegetation.
Studies continue to conclude that higher temperatures increase the risks of
heat-related illnesses and deaths, that these vary by location, and that the risks are
elevated in some regions, older and younger age categories, and impoverished
populations. In Europe during the summer of 2003, as many as 52,000 people died
prematurely in the most severe heat wave in at least 500 years. One study concluded
it was very likely that human-driven climate change had more than doubled the risk
of occurrence of such an extreme heat event. Significant preparedness and
emergency response systems in a number of cities have substantially lowered the
local risks of mortality during heat waves over the past two decades.

Significant impacts of the warming climate are reported in ecological systems
on every continent. Of more than 1,600 species analyzed by two researchers, more
than half show changes in their phenologies, the timing of their life events (such as
egg-laying or blossoming dates), or their distributions (where they are found),
systematically and dominantly in the direction expected from regional climate
changes. Through the 1990s, oceanic plankton productivity has varied with sea
surface temperatures, with warming significantly lowering productivity. This raises
concerns among scientists because plankton are a major food source for many marine
species.

Coral bleaching, triggered by some heat episodes, has become increasingly
widespread in many reef regions, including Hawaii, the Caribbean, and Australia’s
Great Barrier Reef. In addition, elevated concentrations of carbon dioxide in the
atmosphere are absorbed by, and are acidifying, the world’s oceans, posing risks to
shell-forming organisms and marine food chains.

Warming and drying in southeast Alaska and the western United States from
the late 1980s to the present have resulted in pest outbreaks and fires, destroying
property, increasing fire management costs and loss of life, reducing economic forest production, and emitting severe air pollution with consequent health impacts.48
Record high temperatures and droughts have also triggered extensive fires in
Argentina, Greece, South Africa, and other locations. Some species, populations,
and individuals have shown benefits from warming conditions, whereas others that
are more reliant on cool habitats have been adversely affected or lost
competitiveness.

Mountain glaciers have contracted worldwide over at least the past 200 years,
with evidence that the rate of melting or flow has accelerated in recent decades,
including in Argentina, Bhutan, Canada, India, Kyrgystan, Nepal, Switzerland,
Tanzania, Uganda, Venezuela, and the United States. The glacial ice sheets of
Greenland are melting overall, with an apparent acceleration in the period 2003 to
2005. In western Antarctica, accelerated ice melting and loss contrasts with
accumulating ice in East Antarctica because of increased precipitation. With loss
of buttressing sea ice, glacial flows have sped up in the past decade in parts of
western Antarctica. Record low Arctic sea ice extent in 2005, at 5.6 million square
kilometers, was 20% less than the 1970 to 2000 median. Polar bears, which rely
on sea ice to access food, have experienced a significant decrease in cub survival
rates from 2001 to 2006 in the Beaufort Sea area. Rising absolute sea levels (unaffected by vertical land movement) is attributable to expansion of the oceans’
waters as they warm, and inflow of water from melting glaciers.

Likely Causes of Global Climate Change

The evidence is strong that the Earth’s climate is changing; the forces thought
to be driving observed climate changes are discussed below, including the evidence
that human activities, particularly greenhouse gas emissions, have contributed a large
influence on top of ongoing natural variability.

The Earth’s climate is driven by the energy balance of the Sun’s radiation
coming into and leaving the Earth’s atmosphere. The more active the Sun, the closer
the Earth to the Sun, or the greater the ability for the Sun’s energy to penetrate the
atmosphere and be absorbed by the Earth, the greater will be the warming tendency
on Earth. On the other hand, the less active the Sun, the farther the Earth is from the
Sun, the more the Earth’s atmosphere or surface reflect the radiation back out to
space, the greater the cooling tendency. The tilt of the Earth’s axis in its orbit around
the Sun, making one or the other hemisphere closer to the Sun most of the year,
drives the heating and cooling of the seasons outside of the tropics. Scientists
understand well the fundamental drivers of the Earth’s climate through geologic time;
for example, how the pattern of an irregular orbit around the Sun has led to regular
climate swings in and out of ice ages, or how massive volcanic eruptions can spew
particles into the atmosphere that block incoming radiation and cause temporary
cooling.

Until human populations grew to large numbers — from a population of
about 5 million around 10,000 years ago to over 6 billion today — the global climate
was almost certainly not influenced by human activities. However, human clearing
of land, use of fossil fuels for energy, and other activities have greatly changed the
surface of the Earth and the composition of the atmosphere, leading almost certainly
to changes the Earth’s climate through the past 150 years. During this period, the
population grew from about 1.3 billion in 1850 to about 6.5 billion today,
associated with land clearing, increasing affluence, a switch from wood to fossil
fuels, and industrialization — all increasing greenhouse gas emissions and
atmospheric concentrations. The IPCC science assessment concluded in 2007 that
“most of the observed increase in globally averaged temperatures since the mid-20th
century is very likely due to the observed increase in anthropogenic greenhouse gas
concentrations.”

The Concept of Radiative and Other Forcing of the Earth’s Climate

To compare the contributions of different agents to the balance of incoming and
outgoing energy, scientists use the concept of radiative forcing, which quantifies
the direct or indirect effect an agent has on global mean temperature. This concept
has proved successful in helping to predict global temperatures. A shortcut for
radiative forcing that is easier to compute and considered broadly reliable and,
hence, is often used to compare greenhouse gases is Global Warming Potential
(GWP). GWP is an index of how much a greenhouse gas may, by its potency and
quantity, contribute to global warming over a period of time, typically 25, 75, or
100 years. Non-radiative forcing is an as-yet-unquantified concept of the effect
on the Earth’s energy balance that does not directly and immediately involve
radiation, such as the effects of an increase in evaporation resulting from
agricultural irrigation.

A 2005 panel of the National Academy of Science (NAS) concluded that the
concept of radiative forcing is too limited to express contributions of different
agents to regional climates, variability, or aspects of the climate system other than
mean global temperature. For example, there is no measure of the influence of
agents on precipitation, winds, or other important aspects of climate that may
changedifferently than temperature. The NAS panel concluded that broader
concepts are needed to more fully describe the influences of different agents on
multiple aspects of climate. (National Research Council, Radiative Forcing of
Climate Change: Expanding the Concept and Addressing Uncertainties,
Washington: National Academies Press, 2005.)

The remainder of this section summarizes scientific knowledge of how human
activities influence climate change; the human components are described because
these are most readily addressed by public policies. In addition, greenhouse gases are
particularly implicated in recent climate change and are the target of numerous
programs and proposals intended to stabilize climate change. Natural forcings, over
which humans generally have limited control, are discussed in Appendix A.
Methods to compare the relative roles of human and natural forcings and conclusions
that attribute a large part of observed warming to human activities are discussed at
the end of this section.

The terms radiative forcing, non-radiative forcing, and GWP (see “The
Concept of Radiative and Other Forcing of the Earth’s Climate” above), will be used
in the following sections to explain and compare the contributions of different agents
to observed and future climate change. A forcing may lead to a change in climate.
In response to the climate change, other components of the Earth system may also
adjust, resulting in feedbacks to the climate that can either amplify (positive
feedbacks) or dampen (negative feedbacks) the initial change in climate. A number
of researchers have concluded from observing natural forcings and variability that
large climate changes can be triggered by very small changes in forcings because of
feedbacks that amplify the initial change.

Human Activities that Influence Climate Change

Virtually all climate scientists agree that human activities have changed the
Earth’s climate, particularly since the Industrial Revolution. Consumption of fossil
fuels and clearing of land, as well as industrial and agricultural production release socalled greenhouse gases (GHG). Other human-related influences on climate include
air pollution, such as tropospheric ozone and aerosols (tiny particles), land use
change, paving and urban development, and airplane emissions. The different ways
in which humans are affecting climate change are discussed in the following sections.

Greenhouse Gases. Greenhouse gas concentrations in the Earth’s
atmosphere have increased dramatically since the Industrial Revolution, with carbon
dioxide growing from about 280 ppm in 1850 to about 380 ppm today. The
presence of greenhouse gases is critical to trapping the Sun’s energy and warming the
planet to habitable temperatures. Human activities, such as use of fossil fuels,
production of crops and livestock, and manufacture of various products, now emit
certain gases in sufficient quantities to have raised concentrations higher than they
have been for hundreds of thousands of years; the elevated concentrations are
changing the balance of solar radiation in and out of the Earth’s atmosphere and,
consequently, altering the Earth’s climate.

Greenhouse gases (GHG) in the atmosphere allow the Sun’s short wavelength
radiation to pass through to the Earth’s surface, but once the radiation is
absorbed by the Earth and re-emitted as longer wave-length radiation, GHG trap the
heat in the atmosphere. The best-understood greenhouse gases include carbon
dioxide (CO2), methane (CH4), nitrous oxide (N2O), and certain fluorinated
compounds, including chlorofluorocarbons (CFC), hydrochlorofluorocarbons
(HCFC), hydrofluorocarbons (HFC), perchlorofluorocarbons (PFC), and sulfur hexaflouride (SF6). These greenhouse gases remain in the atmosphere for decades
to thousands of years and are generally well-mixed around the globe; hence, their
warming effects are largely global. (See “The Concept of Radiative and Other
Forcing of the Earth’s Climate” above). Moreover, the long atmospheric residence
time and the cumulative effects of gases have important implications for possible
policy responses. (See “Time Lags in the Climate System” below). Because these
GHG affect radiative balance of the Earth in similar ways, they can be compared
using measures of radiative forcing or Global Warming Potentials (GWP), the
latter being an easier but imperfect approximation.

The following human-related sources of the principal greenhouse gas
emissions have been identified:

The share of emissions coming from each sector varies greatly by gas. Agricultural production contributes very little to carbon dioxide emissions (aside from land clearing), but is about 62% of nitrous oxide emissions globally, mainly from fertilizer use.

For the year 2000, CO2 constitutes approximately 72% of the human
contribution to GHG emissions; CH4 is about 18% and N2O is about 9%. There is
considerable uncertainty regarding some of the historical estimates, especially prior
to the 1950s.

Although most of the GHG occur naturally to some degree, the human-driven
emissions of GHG are increasing above the rate of their natural removals from the
atmosphere. Scientists are certain that GHG emissions from human activities have
increased GHG concentrations in the atmosphere to levels unprecedented for
hundreds of thousands, possibly even millions, of years. Over the past 150 years,
CO2 concentrations have increased globally by more than one-third, from about 280
ppm to current levels of about 380 ppm. Methane has increased by
about 150%, although the rate of increase has declined over the past decades, down
to essentially no growth (varying slightly) in recent years. N2O concentrations have
increased by 16% since the Industrial Revolution.

Tropospheric Ozone. Ozone is another greenhouse gas, but it is not
emitted directly by humans. Although it occurs naturally, tropospheric ozone is
elevated by polluting emissions, such as nitrogen oxides from fuel combustion or
volatile organic compound (VOC) emissions from fuel leakage, solvent evaporation,
etc. Tropospheric ozone concentrations, both background levels and episodes of high
concentrations, have been increased, perhaps 50%, by polluting emissions since the
Industrial Revolution. Ozone forms and dissipates quickly, so its concentrations
are unevenly distributed in time and space; hence it is difficult to compare the forcing
of troposphere ozone with other GHG through Global Warming Potentials.
Tropospheric ozone pollution drifting into the Arctic region may be responsible for
one-third to one-half of the warming observed in its springs and summers. In many
countries, ozone concentrations are controlled by regulations that limit air pollutant
emissions, such as the Clean Air Act in the United States.

Sulfur and Carbon Aerosols. Aerosols are tiny particles suspended in
the air; some are there from natural sources, such as volcanoes and forest fires,
whereas others result from human pollution, such as emissions from powerplants or
vehicles. The principal aerosols of concern to climate change are sulfates, black
carbon, and organic carbon. Aerosols can scatter or absorb light, with cooling or
warming effects, respectively, depending on the size, color, composition, and other
characteristics of aerosols. Black carbon aerosols are thought primarily to warm the
atmosphere; organic carbon aerosols (emitted largely by forest fires) are thought to
have mostly a cooling effect.

Sulfur aerosols (sulfates) scatter incoming solar radiation and have
consequent cooling influence on climate. This has been well known for decades but
only included in climate modeling since the early 1990s. Sulfate aerosols are a byproduct
of sulfur emissions, largely from the burning of coal and oil, as well as some
industrial processes. Sulfur emissions and their aerosols have increased dramatically
over the past century.

Aerosol effects on temperature are both regional and short-lived (as particles
typically remain suspended in the atmosphere for days to weeks). Aerosol
concentrations in the atmosphere fluctuate greatly, are difficult to measure, and
consequently are uncertain by a factor of two or more. Aerosols are also understood
to affect precipitation patterns downwind of their emissions, although research is just
beginning to reveal the processes involved; they may influence monsoon water
cycles as well. Aerosols may having amplifying or dampening effects when
interacting with such factors as sea surface temperatures and snow cover. The role
of aerosols in driving various aspects of climate is one of the major uncertainties
being tackled by monitoring and research.

Emissions from Aviation. Emissions from fuel consumption by aircraft
and water vapor emissions in their exhaust both contribute to climate change in
special ways. First, these GHG are emitted at high altitudes, where few other GHG
are present, and therefore do not overlap other gases’ absorbing spectra, increasing
their relatively small contribution to global surface temperatures. They also affect
the vertical distribution of temperatures in the atmosphere. More complex, the
emissions of small particles and water vapor form ice crystals in aviation contrails
that can produce more clouds in the upper troposphere. These clouds can have a
cooling or warming effect depending on the characteristics of the ice crystals; most
scientists believe that the overall climate effect of contrails is a net warming. These
are not globally distributed and therefore have stronger regional than global effects.

Land Surface Changes. Although the Earth’s land surface changes
naturally, as part of ecosystem processes, humans have had a major impact on land
cover and land uses that, in turn, affect the climate system. At least one scientist has
provided evidence that human forest clearing and rice production, beginning roughly
8,000 and 5,000 years ago, respectively, may have significantly affected carbon
dioxide and methane concentrations, as could the carbon sequestration from forest
regrowth following abandonment of farms in Medieval times after the bubonic
plague.

Land Clearing. When humans clear land, as happened in the United States
from its early years and well into the 20th Century, CO2 is emitted mostly through
burning or decomposition, increasing CO2 concentrations in the atmosphere. As
vegetation regrows, it absorbs CO2 from the atmosphere, albeit more slowly than
emission occurred. Net deforestation is occurring globally, mainly in developing
countries. The United States, like many other countries, cleared land decades ago but
abandoned some of it with industrialization and migration to more fertile lands;
regrowth of forests on abandoned lands is, in net, estimated to be removing CO2 from
the atmosphere. In other words, U.S. forests overall are a net sink for carbon, not a
source at this time, on the order of 780 million metric tonnes of CO2 per year.

Agriculture currently covers about one-third of the Earth’s land surface. Agriculture
can also alter the evaporation and transpiration of plants on land, and can alter local
to regional climates, and, via atmospheric circulation, possibly modify global climate
as well.

Land Cover Feedbacks. Land cover change also results from climate
change, and therefore can be a feedback within the climate system. On the one hand,
CO2 in the atmosphere is effectively a nutrient to plants, and this higher carbon
fertilization will tend to increase vegetation growth and remove more CO2 from the
atmosphere. Where precipitation increases, and, to a lesser degree, where currently
cool locations warm, vegetation is expected to increase, creating a negative feedback
to climate warming. To the degree that vines and other weedy plants thrive better in
higher CO2 and warmer temperatures than woody trees, the enhanced carbon uptake
may be short-lived. Moreover, warmer temperatures and greater moisture will tend
to speed up decomposition, and even potentially cause die-back at high levels,
generating a positive feedback to climate warming. In addition, trees and other
vegetation transpire water vapor (another GHG) into the atmosphere. Also, land
cover can alter the amount of dust raised by wind into the atmosphere.

Albedo. The reflectivity of the Earth’s surface is called albedo. Where the
Earth’s surface has low albedo (i.e., is not very reflective), the Sun’s radiation is
absorbed and warms the surface. Particles deposited on the snow/ice surface (e.g.,
from pollution) can darken the surface and increase melting. In places covered with
snow or ice, the surface has very high albedo; as the extent of snow and ice
diminishes with climate warming, the reflectivity decreases and creates a positive
feedback to climate, leading to more warming. Land cleared of its vegetation may
reflect light more than the dark leaves that previously shaded it, increasing reflection
of solar energy, and having a cooling impact. When snow is on the ground, the
removal of trees can have a particularly strong albedo cooling effect, but mostly in
winter or locations with permanent snow or ice. Land clearing tends to warm
temperatures near the equator and cool them at high latitudes.

Methods To Compare Human and Natural Causes

Multiple factors simultaneously influence the Earth’s climate, and scientists
have developed a variety of methods that help determine which forcings are
contributing, and are likely most important, at any period. Several different lines of
evidence discovered in the past decade have led a large majority of scientists to
conclude that human-related greenhouse gas emissions have contributed substantially
to the increase in global mean temperature and other climate changes observed since
the 1970s, and probably over the past century. Additional factors also contribute,
including solar variability, volcanoes, and natural variability.

The simplest method of analyzing the role of greenhouse gases in climate
change is to compare CO2 concentrations in the atmosphere with surface
temperatures. Through the past million years or more, CO2 and CH4 concentrations
have been tightly correlated with global temperatures in the paleologic records.

This correlation may be insufficient to discern the triggering cause of the changes,
but most scientists are confident that once warming has been initiated, there are
strong positive feedbacks to CO2 levels and again to climate warming, leading to a
strongly amplifying effect of the initial cause. This would explain instances of
timing mismatches, where rising CO2 concentrations lag behind rising temperatures,
evidenced in the paleoclimatological record. However, some scientists also raise
problems in the proxy records, pointing to time lags among the complex interactions
within the climate system, and to additional drivers of change that, at times, may
exceed the forcing of CO2 in the atmosphere.

Another method scientists use to attribute climate change to various sources,
or to project future changes, relates to the concept of causative radiative forcing.
The apparent strength of greenhouse gas
forcing has steadily grown and seems to dominate other known forcings in its effects
on warming the Earth’s climate. Volcanic emissions of stratospheric aerosols are
also major but episodic and short-lived drivers. Tropospheric aerosols, including
sulfates, black carbon, and organic carbon have had a much smaller but growing
cooling effect. From these data, the effect of solar variability on recent temperature
change is apparent, but small. Some scientists, however, contend that solar
variability has a larger role.

A third method scientists have developed, especially since 2000, is the
concept of fingerprinting the patterns of radiative forcing and observed change, and
comparing them. Different forcing agents produce different patterns of climate
change over time and space, and even vertically in the atmosphere. For example,
volcanoes spew aerosols that persist only a few years in the atmosphere, creating a
temporary cooling effect over both land and oceans. Reductions in solar irradiance
can last decades and affect land temperatures more than oceans. These patterns are
very different from, for example, the influence of long-lived greenhouse gases that
are expected to exert long-lived, global influence.

The observed warming of the climate, described above in “Changes Observed
in the Earth’s Climate,” corresponds to expected greenhouse gas-induced patterns of
warming greater in winter than summer, more at night than daytime, and of generally
increasing precipitation, and more warming at high latitudes than low latitudes.
Another piece of evidence is the observed cooling, as expected, at 50 km and higher
in the atmosphere, which cannot be explained by seasonal or solar cycles. Also, the
increasing heat content of the oceans cannot be explained by urban heat islands or
other placement-related problems with measurement stations. Fingerprint methods
have proven critical in attributing the climate change over the past decades to
greenhouse gas emissions versus natural forcings.

Several studies have tried to explain historical climate change by running
computer models — mathematical simplifications of how scientists understand
climate processes to work — with different combinations of forcing agents. (See
“Use of Models for Climate Change Analysis” below.) Scientists have repeatedly found that they cannot reproduce
the warming of the last half century with natural forcings alone, but can generate
warming patterns similar to observed climate changes when including greenhouse
gases and aerosols. These studies constitute one of the important lines of evidence
that lead scientists to conclude that recent climate change has been caused in large
part by greenhouse gases.

Attribution of Climate Change in the 20th Century

In conclusion, a variety of natural and human-related forces have influenced
observed climate change throughout the 20th Century, including greenhouse gases
accumulating in the atmosphere, other air pollutants, land use change, solar
variability, and volcanoes. The relative importance of each of these factors is not
well quantified. Nonetheless, relying on a variety of tests, a preponderance of
scientists have concluded that the observed climate change over the 20th Century
cannot be explained without including the effects of rising concentrations of
greenhouse gases. A panel of the National Academy of Science, at the request of
President George W. Bush, reviewed the established research in 2001 and concluded
That the changes observed over the last several decades are likely mostly due to
human activities, but we cannot rule out that some significant part of these
changes is also a reflection of natural variability.

The attribution to greenhouse gas forcing of significant climate change since
the 1970s has been strengthened since the NRC 2001 findings by a number of
additional studies, including matching of the spatial and temporal patterns of
greenhouse gas forcing with observed ocean heat distribution. The IPCC science
assessment concluded in 2007 that “most of the observed increase in globally
averaged temperatures since the mid-20th century is very likely due to the observed
increase in anthropogenic greenhouse gas concentrations.”

Projections of Future Human-Driven
Climate Change

Much of the discussion of future climate change is based on projections
produced by computer models that represent as completely as possible the relevant
factors that are today understood to influence the climate (including the effects of
past climates). These models are incomplete, as scientific understanding of the
relevant factors and processes is continuously developing. However, climate models
have improved substantially over the past decade, and experts believe that many now
do a better job of representing the current and historical climates. It is disagreement
about the ability of these models to predict future climate change that drives much
of the current climate change debate. This section explains how projections of
climate are produced and provides the range of forecasts provided by the many
climate analysis institutions around the world.

Most studies indicate, and experts generally agree, that growth of greenhouse
gas forcing, if it continues unabated, will raise global average temperatures well
above natural variability. The 2007 IPCC scientific assessment concluded, “[F]or the
next two decades a warming of about 0.2oC [0.36oF] per decade is projected for a
range of [SRES] emission scenarios. Even if the concentrations of all greenhouse
gases and aerosols had been kept constant at year 2000 levels, a further warming of
about 0.1oC [0.2oF] would be expected.” It further found, “[T]he best estimate for
the low [SRES] greenhouse gas emission] scenario (B1) is 1.8oC (likely range is
1.1oC to 2.9oC) [3.2oF (likely range is 2.0oF to 5.2oF)], and the best estimate for the
high scenario (A1F1) is 4.0oC (likely range is 2.4oC to 6.4oC) [5.2oF (likely range is
4.3oF to 9.5oF)].” It will be many years to decades before the wide range of
uncertainty in global average temperature increases can be narrowed with confidence.
(See “Use of Models for Climate Analysis” below.)

Climate models generally predict more heat waves, droughts, and floods;
extreme cold episodes are predicted to decrease; the centers of continents are likely
to experience summer warming and dryness. Scientists expect precipitation will be
more intense when it occurs (therefore also increasing runoff and the risk of
flooding). But it will be substantially harder to establish the range of possible
changes in the hydrologic cycle — or even direction of change for some regions —
both because there are fewer historical observations on which to build scientific
understanding, and because the physical constraints are weaker. Scientists expect
atmospheric and ocean circulation are likely to change as well.
Studies have found that future climate change will not be evenly distributed
geographically or temporally: even if the global mean temperature were to change
very little, regional climate changes could be dramatic because of the uneven
distribution of forcings by different agents and the connectedness of regions within
the climate system. Although almost all regions are expected to experience warming,
some regions are projected to become wetter while others become drier.

Future climate change is not likely to proceed smoothly, as often depicted by
averaged model results, but to swing up and down around a rising average, as has
occurred in the past. This variability around a rising average may complicate the
detection and prediction of change. Though the ability of scientists to understand and
model changes is advancing, there will remain major uncertainties in the forecasting
of local and seasonal climate changes that accompany global warming. Climate
models are not yet adept at capturing extreme events or abrupt changes, and there is
significant potential of important climate “surprises” that models may not predict.
It is unclear how serious future changes may be, given the climate variability to
which humans and ecosystems are already adapted.

Use of Models for Climate Change Analysis

In deciding whether to take action to address climate change, and what actions may be
effective, decision-makers seek projections of what to expect in the future. Scientists
cannot rely only on analogies to past climates because the Earth’s current biological,
chemical, and geologic systems, and human activities, have no precedent. Scientists use
models, first, to understand the system they are studying, building from theory and
validating with experiments and observations of the past, in order to interpret the causes
of past variability and to use that understanding to forecast the future.

Models are simplified representations of systems. Almost all of the models used for
climate change analysis are mathematical, and they are developed using a wide variety
of disciplines, including physics, atmospheric chemistry, economics, engineering,
ecology, and others. Over time, and especially in the past decade, different disciplines
have joined expertise and tools to provide more integrated — and complete — models
for analyzing climate change. As important, while climate models a few decades ago
were built primarily on theoretical understanding, the recent expansion of monitoring,
computing capacity, and funding has allowed data assimilation, or the use of real-world
observations, to improve the models.

Rigorous comparisons of models help to validate their performance by reproducing
observations of today’s climate, although good performance on this test does not
guarantee reliable future projections of climate changes or their patterns. Models are
also tested in their ability to reproduce paleoclimatic events, and are compared in detail
to understand why models respond differently to forcings.

Since the 1990s, climate models perform significantly better in reproducing current and
historical climates, although models diverge in important ways in the patterns of climate
that they produce. Since 2000, important improvements have been made in modeling
changes in some regions, while large discrepancies exist for a few regions. Climate
models are generally less successful in reproducing observed precipitation than
temperature, perhaps because of its higher natural variability, and extreme events and
local climate predictions require a higher resolution than global climate models currently
offer. Also, different models produce different results. In long-term projections of
climate change, the differences between climate models for a given GHG emission
scenario can be larger than the differences produced by one model running the range of
future GHG emission scenarios. Furthermore, there is some scientific opinion that,
while research is critically important for improving scientific understanding of the
climate system and for possible future changes, research may well increase the range of
uncertainty, as new processes are uncovered or existing structures are tested and revised.

Methods and models are improving in their ability to characterize important
uncertainties and to support risk assessment and risk management decisions in spite of
the unknowns, just as in other sectors such as finance, security and medicine. The
application of risk assessment and management techniques to climate change decisionmaking is nascent, but is providing useful insights for incorporating uncertainties into
decision-making.

Another complication in forecasting climate change is the importance of
feedbacks — both positive and negative. A National Research Council report
concluded that feedbacks in the climate system have, many times in the geologic
record, amplified small initial climate perturbations into major climate cycles with
global mean temperature swings of 5o to 6oC. The close linkage, over hundreds of
thousand of years, between past temperature swings and carbon dioxide and methane
concentrations in the atmosphere strongly suggests that temperature change can
trigger strong positive amplifications through the carbon cycle and water vapor
feedbacks. Current models include such feedbacks, but they are very uncertain. The
most important and uncertain feedbacks affecting future climate projections include
water vapor feedback, cloud feedbacks, vegetation feedbacks, and albedo.

Thus, while most climate scientists conclude with high confidence that future
climate change, forced by greenhouse gases, land use change, and natural factors, is
probable, the magnitude, rapidity, and details of the changes are likely to remain
unclear for many years, or even decades. There is near unanimity among climate
model projections that (1) past human emissions have committed the climate to some
change over the next few decades, and GHG emissions emitted from now on will
begin to dominate global warming by mid-century, and (2) feedbacks to the carbon
cycle tend to be positive, amplifying initial warming by greenhouse gases. The latter
suggests also that climate change may reduce the effectiveness of carbon uptake by
oceans and vegetation, and that more warming would require proportionately greater
GHG emission reductions to stabilize the climate system.

Impacts of Projected Climate Change

Projected impacts of future climate change indicate that there will be winners
and losers among regions, sectors, and income groups. Some groups may benefit
from a certain amount of climate change, whereas others may suffer harm. Regions
that fare relatively well may be negatively affected by changes in other regions
through trade, security, and humanitarian demands and immigration pressures.
Future generations are likely to experience more change, but may also be wealthier
and hence better able to adapt, although not uniformly so. Many species may become
extinct, while others are likely to flourish. The local effects of climate change may
contribute more to decision-making than national or global aggregates.
In April 2007, the IPCC released its fourth assessment of the impacts of
climate change and vulnerability to these impacts. Selected key findings from that
report are provided in the box below.

IPCC Climate Change 2007: Selected Key Findings on Impacts,
Adaptation and Vulnerability

Evidence from all continents and most oceans show that many natural systems
are being affected by regional climate changes, such as —

Impacts will depend on changes in temperature as well as precipitation, sea levels
and ocean circulation, and concentrations of carbon dioxide, as well as other
features of the climate. The ability to adapt to climate change, to reduce
vulnerability, is expected to be more constrained for low-income populations,
especially in developing countries.

By mid-century, average annual river runoff and water availability are projected
to increase by 10-40% at high latitudes and some wet tropical areas, while
decreasing by 10-30% over some dry regions at mid-latitudes and in the dry
tropics, some of which are already water-stressed. Drought extent, heavy
precipitation events, and flood risks are expected to increase.

The resilience of many ecosystems is likely to be exceeded by an unprecedented
combination of climate change, associated disturbances (e.g., flooding, drought,
wildfire, insects, ocean acidification), and other changes (e.g. land use change,
pollution, over-exploitation of resources).

Crop productivity is projected to increase at mid- to high latitudes for local mean
temperatures up to 1-3oC (1.8-5.4oF) and then decline beyond that in some
regions. Especially in seasonally dry and tropical regions, crop productivity is
projected to decrease for even small local temperature increases. Adaptations
allow yields to be maintains for modest warming.

Coasts are projected to be exposed to increasing risks due to sea level rise, coastal
erosion, human-induced pressures, more frequent bleaching of corals, loss of
wetlands, and increased flooding.

Climate change may affect the health status of millions of people through increases
in malnutrition, increased deaths, disease and injury due to heat waves, floods,
storms, fires and droughts; increased air pollution and altered distribution of some
infectious diseases.

For agriculture, most models project overall benefits over the next few decades,
largely due to increased fertilization by CO2 in the atmosphere, although negative
impacts might occur in some regions and for some sub-populations. As climate
change progresses — several models suggest turning points at 2 to 4oC warmer than
1990 — projected impacts on crop agriculture become negative in most regions
except for the high latitudes. Adverse impacts on Africa may be of particular
concern. Research to date on agricultural impacts cannot be considered conclusive.

Few studies of agriculture have incorporated the effects of climate variability, or the
spread of pests, crop diseases, and weedy plants that could be favored by warmer
temperatures and higher CO2 concentrations. Very little research has been applied
to other important food sources, such as fruits and vegetables, livestock production,
fisheries, and crops grown to produce oil, which constitute the fastest growing shares
of agriculture. Biotechnological products and cropping flexibility have only partially
been included in agricultural impact studies. Experts believe impacts will also
depend on migration of agricultural production to regions favored by climate change,
with implications for land values and shifts in labor forces.

Climate change and the fertilization of vegetation by higher levels of CO2 in
the atmosphere are projected to have both positive and negative effects on forests.
However, as species reach their higher temperature tolerances, stress and
susceptibility to disease, pests, and drought are likely, possibly resulting in die-offs
such as those currently being experienced by forests in parts of the western United
States and Canada. If forests and vegetation are able to migrate or expand in conjunction with projected climate change, the composition of land cover would
likely be altered, with significant agricultural, economic, cultural, and ecological
consequences.

One risk appearing in some climate model projections is the possibility of
dieback of the Amazon rainforest, resulting in a self-reinforcing cycle of greater
drying and further dieback. This could result in an amplification of greenhouse-gas
induced climate change, as well as ecological change.

Models show a wide range in the projected decrease of Arctic sea ice extent,
from very little to, as most models show, an ice-free Arctic in summers by the end
of the century or sooner. Arctic sea ice melting is consequential for the global
climate. It would have ecological effects on polar bears, seals, bird populations, and
marine life, as well as on humans, including native cultural and subsistence systems,
and might raise national security and sovereignty issues.

Recent research on the melting of ice sheets and accumulation of snow and
ice at higher elevations of ice sheets has reduced scientists’ confidence in related
projections and implications for sea level rise over several centuries. While most
global models project somewhat lower rises in sea levels with future warming, some
scientists assert that these results contradict recent evidence in some locations of
faster melting than predicted. Understanding of dynamics of ice sheets is weak and
a source of large uncertainty regarding future sea level rise.

Major declines of live coral cover for reef systems around the world are
expected by many scientists, because of combined effects of greater frequency of
high temperatures and to higher ocean acidity from elevated CO2 concentrations.
(See “Carbon Dioxide and Ocean Acidification” below.) To the degree that live
coral reef cover declines, losses up the related food chain could be expected, with
possible economic consequences for fisheries and human food security in parts of the
world.

Models predict that the northern tier of the United States, Canada, and most
of Europe are likely to experience more days with heavy precipitation (above 0.4
inches) by the late 21st Century. Some areas would undoubtedly benefit from
increases in precipitation. More of this is likely to fall as rain rather than snow, and
snow is likely to melt earlier. For some areas and systems, these changes would be
positive. Many scientists conclude that it is likely that there will be some increase
in tropical cyclone intensity if the climate continues to warm.

Projected climate change is expected to have additional major repercussions
for ecological systems. The specific reorganization of ecosystems, and effects on
particular populations, species, landscapes, and ecosystem services to humans are
beyond reliable prediction, given relatively little monitoring and research and the
rudimentary state of models for understanding these processes. The effects are
expected to be highly localized, though some will be widespread and be linked to
changes in other regions through food chains, nutrient flows, atmospheric and ocean
circulations, etc. Some populations and species are likely to flourish in a more
temperate and humid environment. Some will be able to adapt and/or migrate to stay
within a hospitable habitat. Others will be affected by obstacles or patchiness of
suitable pathways, or migration rates slower than the movement of the appropriate
biome, disruptions in food chains or other critical dependencies among species, and
increased competition. Many ecologists expect high rates of extinctions and loss of
biological diversity if climate change projections are accurate.

Human health may benefit or suffer with future climate change. It is
uncertain at what point increased heat stress impacts may outweigh the declining cold
stress benefits; that turning point will be sooner the faster temperatures rise and the
slower effective prevention and response adaptations are established. Other adverse
health impacts that may increase include incidence of food- and vector-borne
diseases. Warmer temperatures may increase air pollution, as well as boost growth
of fungi, mold, and other allergens. In terms of mortality, the potential expansion
of malaria is by far the most critical health impact studied, though its growth may be
mitigated as incomes rise by the improvement of public health systems,
environmental modifications and pesticides, and possible vaccines.

Carbon Dioxide and Ocean Acidification

Climate change research has revealed that elevated concentrations of carbon dioxide
in the atmosphere are changing the chemistry of the oceans to a state not witnessed
for at least 55 million years, prompting concern by many scientists about potentially
large effects on the marine food chain. As the oceans absorb CO2 from the
atmosphere — a normal process in the carbon cycle that helps to lower CO2 in the
atmosphere and slow climate change — the dissolved CO2 makes the normally
alkaline seawater more acidic. One study has found that the oceans have become
about 30% less alkaline since pre-industrial times. This process is popularly
referred to as ocean acidification. The high dissolved CO2 decreases carbonate
ions, which are needed by shell-forming creatures to make calcium carbonate for
their shells.

Research on more than a dozen species of corals and plankton indicates that net
calcification (the process of shell-building) would decrease by -4% to -56% if CO2
atmospheric concentrations were to double from pre-industrial levels (around 550
ppm), forecast by some models to occur as early as mid-century. Above varying
thresholds, the decrease in growth turns to dissolving of their shells. Some species
that show small effects may indicate unusual adaptive responses that may not be
needed at lower CO2 concentrations. In some cases, resource managers may be able
to help protect species in some locations. However, the few forecasts available
suggest potentially large impacts on the ability of coral reefs to grow under future
conditions. Moreover, scientists believe the current acidification is essentially
irreversible on human-time scales, as the natural processes to raise alkalinity operate
far too slowly to reverse the current trend.

The main unknown in how rapidly ocean acidification will occur is the future
trajectory of CO2 concentrations in the atmosphere, which depends largely on future
human-related emissions of CO2, mostly from fossil-fuel burning. Concerns for
impacts are particularly great for areas where the form of carbon needed for shelling
building (aragonite) is already low (i.e., the high latitudes) or where the ecosystems
are dominated by shell-building organisms; in the case of the Southern Ocean
encircling Antarctica, both conditions occur.

Implications of Climate Change for the Federal Government

The prospects of a changing climate have a variety of implications for the
U.S. federal government. First, scientific attention and public questions about
climate change have increased pressure for research to provide answers, and for
elected officials to make decisions about whether and how to address climate change.

The U.S. government invests around $6 billion yearly on climate change research,
voluntary programs, and financial incentives to advance low-GHG-emitting
technologies. Setting clear and realistic objectives for research and programs remain
a near-term challenge for federal programs, but such measures facilitate continuing
oversight of program performance and improvements. Of particular interest may be
questions about the rate at which science (also integrated with economics) can
narrow uncertainties about the magnitude, rate, geographic distribution, and other
characteristics of climate change, and the degree to which changes may be
predictable, hence facilitating effective and timely adaptation. These questions bear
importantly on the trade-off between acting sooner with imperfect information versus
delaying action in expectation of reducing uncertainties.

Second, the federal government manages many assets that are potentially
affected by climate change. For example, climate change could bring benefits or
threats to public lands, particularly to national parks and other physical and biological
assets valued for their high natural and cultural amenities. As climate change alters
grasslands, forests, fisheries, and other resources, their values will change, as may
appropriate management objectives and plans. Similarly, climate change may affect
the demand and supply of energy for the operations of government, with implications
for infrastructure planning, expenses, and supply choices.

The federal government may find it desirable to redefine objectives and
provide for institutional flexibility and adaptive management as climate, and the
resources that depend on specific conditions, change. For example, such ecological
impacts as competition among species may lead to conflicts among endangered
species or other resource management goals. Evaluations of objectives and
practices in light of potential future climate change may enhance future successes of
resource management.

Although some climate changes and their impacts may occur relatively
smoothly, ecosystems frequently exhibit abrupt changes in response to incremental
pressures. In terms of socioeconomic consequences, abrupt changes may constitute
emergencies or even disasters that require federal responses and possibly financial
and other resources. Preparedness for, and managing aftermaths of, hurricanes,
droughts, pest infestations, fires, epidemics, coastal erosion, and deterioration of permafrost are a few examples of the physical pressures climate change may bring.
Such changes can also create social dislocations and strife. The Dust Bowl of the
1930s, as an illustration of a regional environmental disaster, resulted in one of the
largest dislocations in U.S. history.

Beyond immediate responses, the federal government also frequently acts as
the insurer of the last resort; this role could expand if private insurance becomes less
available or more costly, or if people do not procure adequate insurance. The
dramatic increase in economic losses since 1980 resulting from extreme weather
events makes clear that economic exposures to extreme weather events, such as
droughts, floods, hurricanes, tornadoes, and others can run in the billions in the
United States, and much higher globally.

Many experts suggest that all levels of government may find it useful to
consider possible climate change implications when planning their long-lasting
infrastructure or other projects, including energy procurement, water supply and
flood control, investment in buildings and transportation systems, etc. This could
lessen the problem of obsolete, expensive, or stranded assets that could arise with
possible climate and policy changes.

Further, expected climate changes outside the borders of the United States
could have important implications for the U.S. federal government as well. For
example, climate change impacts in Mexico, and many other developing countries,
could be more adverse than in the United States, due partly to more severe projected
climate changes and partly to lower capacity to adapt. The differential between
United States and foreign direct impacts may increase pressure on migration and
terms of trade; it may also increase demands for disaster relief, development
assistance, and other types of interventions.

Implications for Policy

The federal government and other institutions have invested billions of
dollars in researching climate change over more than five decades. Human emissions
of greenhouse gases have increased exponentially since the Industrial Revolution,
leading to higher concentrations of carbon dioxide, methane, and other gases than
have existed for hundreds of thousands (maybe millions) of years. Measurements
demonstrate with increasing confidence that the Earth’s climate has been warming,
especially in the past few decades, and changing in other ways. Science cannot
explain the recent patterns of climate change without including the influence of
elevated greenhouse gas concentrations. Although many uncertainties remain
concerning the future magnitude, rate, and other details of climate change, a
preponderance of scientists conclude that greenhouse gases emitted by fossil fuel use
and other human activities are virtually certain to induce global average warming by
at least 1.8oC (3.2oF) over the 21st Century — or as high as 4oC (5.2oF), and possibly
more than 6oC (more than 9oF). These scientists assert that some warming is virtually
inevitable, because of the accumulation of past emissions in elevated atmospheric
concentrations of GHG. The time lags inherent in the climate system are an important aspect affecting the effectiveness of various policy options. (See “Time Lags in the
Climate System” below).

Time Lags in the Climate System

Timing of changes: There are long time lags between emissions of most
greenhouse gases, their accumulation as rising concentrations in the atmosphere,
and the induced climate change that may last from decades to millenia.
Atmospheric lifetimes different greatly among GHG — from minutes to tens of
thousands of years, with CO2 typically assumed to have half remaining in the
atmosphere after about 100 years. The long-lived gases continue to influence the
climate as long as they remain in the atmosphere. Thus, emissions of, say, CO2
today are expected to continue to affect climate for hundreds of years. Conversely,
slowing the growth of concentrations, or stabilizing them, would require
proportionately large cuts in emissions or enhancements of removals (e.g., through
uptake by trees). Dominant influence over the next 50 years is likely the
accumulation in the atmosphere of past GHG emissions; changes in climate would
continue for many decades to centuries after GHG concentrations cease to grow.

Several points made by climate experts are important:

These conclusions raise a number of questions that may help direct further
research or science to support public policy decisions:

Current and emerging science also provides key information for policymakers
that may help to resolve the details of specific decisions, such as:

There are a number of bills before the 110th Congress proposing a diversity
of approaches to address climate change and greenhouse gas reductions. There also
likely will be more hearings, both legislative and oversight. Thus, assuring that the
best scientific underpinning is available for policy-related deliberations will remain
a priority.

Pages: 1 2

Edit this PageWIKI:

[1] On December 11, 2008, California adopted America's first comprehensive climate change plan. It calls for increased cuts in emission, a regional cap and trade program, and a gradual movement to a system where 100% of greenhouse gas permits will be auctioned off.

[2] Senator Jeff Bingaman has stated that Congress will most likely not act on a global warming bill until 2010 because of the transition to a new Presidential administration and the impending economic crisis. He believes it will take at least a year for any climate change bill to go through.

[3] A National Teach-In on Global Warming has been established for the date of February 5, 2009 to engage a broad population of Americans to educate and expand the climate change movement.

[4] The Pew Center on Global Climate Change posted an article about the climate change issues being discussed in Congress. They list proposals to Congress as well as “bill analysis and a complete overview of the floor debate.”

[5] The National Resource Defense Council (NRDC) has put out an issue summary for the McCain-Lieberman Climate Act as well as all a comparison of climate change legislation introduced in the 110th Congress.

[6] On Apr 24, 2008 Representative Dave Reichert gave a press release to clarify his his global warming legislation.

[7] On Aug 8, 2007 Representative Nick Lampson gave a press release addressing the need for Air and Water Pollution Control Facilities.

[8] On August 6, 2007 Representative Heath Shuler put out a press release regarding energy independence and climate change.

[9] On August 1, 2007 Representative Ciro Rodriguez put out a press release supporting Renewable Electricity Standard.

[10] In March of 2007, Al Gore testified before Congress about global warming and the importance of legislation being passed to help the problem.

[11] On January 30, 2007, Congress held its first global warming hearing, led by Senator Barbara Boxer. [1]

[12] The 2006 documentary An Inconvenient Truth by former Vice President Al Gore addresses global warming and its implications.

[edit] Endnotes

  1. ↑ Traci Watson. "Global warming issue gains 'traction' with new Congress." USA Today. January 30, 2007. http://www.usatoday.com/news/washington/2007-01-29-global-warming_x.htm
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Issue Contents

  1. Introduction
    1. Climate Change Science Is a Process
  2. Changes Observed in the Earth’s Climate
    1. Global Climate Changes
  3. Climate Changes Observed in the United States
  4. Climate Lessons from the Distant Past
  5. Observed Impacts
  6. Likely Causes of Global Climate Change
    1. The Concept of Radiative and Other Forcing of the Earth’s Climate
  7. Human Activities that Influence Climate Change
  8. Methods To Compare Human and Natural Causes
  9. Attribution of Climate Change in the 20th Century
    1. Use of Models for Climate Change Analysis
  10. Impacts of Projected Climate Change
    1. Carbon Dioxide and Ocean Acidification
  11. Implications of Climate Change for the Federal Government
  12. Implications for Policy
    1. Time Lags in the Climate System

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