[Note: A later (1997) updated version of this report is available as PDF file and is available in hardcopy from the GCRIO Online Catalog]
the Environmental division, Office
of Science and Technology Policy, Executive Office of the President,
Washington, DC, March 1995.
The Earth's climate is predicted
to change because human activities are altering the chemical composition
of the atmosphere. The buildup of greenhouse gases-primarily carbon dioxide,
methane, nitrous oxide and chlorofluorocarbons-is changing the radiation
balance of the planet. The basic heat-trapping property of these greenhouse
gases is essentially undisputed. However, there is considerable scientific
uncertainty about exactly how and when the Earth's climate will respond
to enhanced greenhouse gases. The direct effects of climate change will
include changes in temperature, precipitation, soil moisture, and sea
level. Such changes could have adverse effects on ecological systems,
human health, and socio-economic sectors.
Human-induced climate change is a complex problem, which can impact the
economy and the quality of life for this and future generations. The lag
time between emission of the gases and their impact is on the order of
decades to centuries; so too is the time needed to reverse any effects.
Thus, policy decisions in the near term will have long-term consequences.
A natural greenhouse effect
keeps the Earth 33 degrees C. warmer than it otherwise would be. Without
this, life as we know it would not be possible. Water vapor, carbon dioxide,
and other trace gases trap heat as it is re-radiated from the Earth back
to space. However, since pre-industrial times, human activities have added
to the natural greenhouse effect by releasing additional greenhouse gases
to the atmosphere. The burning of fossil fuels (coal, oil, and gas) for
energy is the primary source of emissions; changing land-use patterns
through agriculture and deforestation also contribute a significant share.
Current global emissions of carbon dioxide from energy use are approximately
6 gigatons of carbon (GtC) per year.
Future greenhouse gas emissions are sensitive to changes in demographic,
economic, technological, policy, and institutional developments. By the
year 2025, world emissions could range from 8 to 15 GtC per year. In the
year 2100, world emissions are projected to range from 5 to 36 GtC per
year, depending on energy use. The U.S. and the rest of the OECD countries
currently contribute about 40 percent of global carbon dioxide emissions.
Future growth in emissions from OECD countries is predicted to be significantly
smaller than growth in developing countries and countries with economies
Since the pre-industrial era, atmospheric concentrations of carbon dioxide
have increased by nearly 30 percent, methane concentrations have doubled,
and nitrous oxide concentrations have risen by 15 percent.
These increases result in
a radiative forcing or heat-trapping of energy equivalent to about 2.8
watts per square meter (Wm-2). A significant fraction of warming may have
been masked by increased levels of traditional air pollutants-sulfates
and carbonaceous aerosols, particularly in the Northern Hemisphere - which
reflect incoming solar radiation and alter the reflective properties of
clouds. Aerosols are short-lived and vary regionally, hence they should
not be regarded as a simple offset to greenhouse gas forcing. Calculations
suggest that the projected increases in atmospheric concentrations of
greenhouse gases alone will result in an additional radiative forcing
of about 3 ­p; 8 Wm-2 by 2100. For a given concentration of greenhouse
gases, the resulting increase in radiation can be predicted with precision;
but the resulting impact on climate is more uncertain.
Model calculations, based on plausible ranges of future emissions and
climate sensitivities, suggest that the global surface temperature could
increase an average of 0.9 ­p; 5.0 degrees C. by 2100, with significant
variation by region. This estimate does not account for offsets from aerosols
which would somewhat lower these values. Global-average temperature changes
of this magnitude would be greater than recent natural fluctuations and
would occur at a rate significantly faster than any observed changes in
the last 10,000 years. The U.S. and high latitudes are projected to warm
more than the global average.
Model calculations also suggest that the rate of evaporation will increase
as the climate warms, leading to an increase in average global precipitation.
While precipitation at high latitudes is expected to increase, much of
the precipitation increase may fall over the oceans. The models also suggest
that the frequency of intense rainfall will increase and there will be
a marked decrease in soil moisture over some mid-latitude continental
regions during the summer.
Sea level is projected to increase by several tens of centimeters by the
end of the next century, due primarily to the thermal expansion of the
oceans and the melting of glaciers and ice sheets.
Calculations of climate change at the regional scale are significantly
less reliable than average global values, and it is unclear whether climate
will become more variable. The frequency and intensity of extreme weather
events of critical importance to ecological systems (droughts, floods,
frosts, cloudiness, the frequency of hot or cold spells, and the intensity
of associated fire and pest outbreaks) could increase.
Global mean surface temperatures have increased between 0.3 and 0.6 degrees
C. over the past century, despite marked regional, seasonal and diurnal
There is consistency between the observed global average temperature trend
over this period and model simulations of the warming due to greenhouse
gases if allowance is made for the increasing evidence of a cooling effect
due to anthropogenic aerosols and stratospheric ozone depletion. However,
the warming is also at the upper end of the range of temperature fluctuations
observed in the pre-industrial record. Thus, the observed temperature
increase could be largely due to natural variability; alternatively, variability
and other human factors could have offset a still larger human-induced
warming. The unequivocal detection of the enhanced greenhouse effect from
observations is likely to require another decade or more of data.
The nine warmest years this century have all occurred since 1980. 1994
was the third or fourth warmest year on record, suggesting the atmosphere
has rebounded from the transient cooling of 0.5 degrees C. caused by Mt.
Pinatubo and simulated by the models.
Several ancillary pieces of
evidence consistent with warming, such as a decrease in Northern Hemisphere
snow cover, a simultaneous decrease in Arctic sea ice, continued melting
of alpine glaciers, and a rise of sea level, have also been corroborated.
The frequency of extreme rainfall events has increased throughout much
of the country, suggestive of an intensification of the hydrologic cycle.
Carbon cycle models imply that limiting atmospheric concentrations of
CO2 to any level below 750 parts per million by volume (ppmv), about three
times the pre-industrial level, would require emissions worldwide to eventually
drop below 1990 levels. The longer emissions continue to increase, the
greater reductions would eventually have to be to stabilize concentrations
at a given level.
While much is already known about the greenhouse effect, substantial reduction
of key uncertainties (detailed quantification of the timing, magnitude
and regional patterns of climate change) may require a decade or more.
Accurate predictions are limited by our knowledge of the future emissions
and atmospheric concentrations of carbon dioxide, aerosols and other greenhouse
gases, the role of clouds and water vapor, and the role of the oceans.
Climate change poses threats
to resources both domestically and internationally. These threats could
have significant, but uncertain, socioeconomic consequences. Changes in
temperature, precipitation and sea level driven by climate change can
add to existing stresses on resources caused by other influences such
as population growth, land-use changes, and pollution.
Overall, various strategies for coping with climate change can be identified
for "intensively managed" systems (such as agriculture, water
resources, and developed coastlines). For these systems, technological
and management options exist to some extent today, although they may be
costly to implement. By comparison, fewer options can be identified for
natural systems (such as wetlands and wilderness areas).
Temperature changes of the magnitude expected from the enhanced greenhouse
effect have occurred in the past, but the previous changes took place
over centuries or millennia instead of decades. Rates of natural migration
and adaptation of species and communities appear to be much slower than
the predicted rate of climate change. As a result, populations of many
species and inhabited ranges could change as the climate to which they
are adapted effectively shifts northward or to higher elevations.
One of the consequences of global climate change may be increases in the
frequency of extreme weather events, particularly droughts and floods.
These events, when they do occur, can be costly. For example:
> Damages from the Mississippi River flooding in 1993 are estimated
to range from $10 billion to $20 billion. Almost $4 billion in federal
payments went to farmers suffering crop losses during the 1988 drought.
Examples of the kinds of effects that may result from climate change include:
> Changes in precipitation
and increased evaporation from higher temperatures can affect water supplies
and water quality, posing threats to hydropower, irrigation, fisheries,
and drinking water.
>Floods are more likely
because the frequency of intense rainfall is predicted to increase.
> Droughts are likely to be more severe because warmer temperatures
increase evaporation rates and thereby lead to drier soils during periods
of little or no rain.
> Climate change would be likely to add to the stress in several U.S.
river basins such as the Great Basin, California, Missouri, Arkansas,
Texas Gulf, Rio Grande, and Lower Colorado.
> Water scarcity in Middle Eastern and African countries is also likely
to be exacerbated by climate change.
> A 50 cm rise in sea level
by the year 2100, which is within the range projected by the IPCC, could
inundate more than 5,000 mi2 of dryland and an additional 4000 mi2 of
wetlands in the U.S. if no protective actions are taken.
> Areas at highest risk from sea level rise are areas currently experiencing
high erosion rates and those with very low elevations, such as parts of
the U.S. Atlantic and Gulf coasts.
> Internationally, many low- lying areas such as parts of the Maldives,
Egypt, and Bangladesh would be completely inundated and uninhabitable
by a similar sea level rise.
> Climate change may shift
the range of infectious diseases, with likely increased risks of malaria
and dengue in the United States. Changing temperatures and precipitation
patterns may produce new breeding sites for pests and pathogens.
> Climate change may increase heat-stress mortality, particularly in
the very young and very old.
> Large areas of the eastern
and central United States face moderate to severe drying. Drought could
become more frequent, particularly in the Great Plains.
> Changes in management practices and technological advances might
reduce or eliminate many of the potentially negative impacts of climate
change in the agricultural sector.
> Agriculture production in developing countries is likely to be more
vulnerable to climate change given that they have relatively fewer economic
> Climate change over the
next several decades might shift the ideal range for some North American
forest species by as much as 300 miles to the north, exceeding the ability
of forests and other ecological communities to migrate.
> Forest damage from fire
and diebacks driven by drought,insects and disease could increase.
> The most vulnerable forest resources are those in regions subject
to increased moisture stress, as in the dry continental interiors.
> Warmer temperatures will
increase cooling demand but decrease heating requirements.
> Changes in water availability may affect reliability of hydropower
> Warming should lead to fewer disruptions of winter transportation,
but increased droughts and floods may interfere with water transport.