Warmer temperatures are expected to
raise sea level by expanding ocean water, melting mountain glaciers, and
melting parts of the Greenland Ice Sheet. Warmer temperatures also increase
precipitation, as described below. Snowfall over Greenland and Antarctica is
expected to increase by about 5 percent for every 1°F warming in
temperatures. Increased snowfall tends to cause sea level to drop if the snow
does not melt during the following summer, because the only other place for
the water to be is the ocean. (The amount of water in the atmosphere is less
than the water it takes to raise the oceans one millimeter). Considering all
of these factors, the IPCC estimates that sea level will rise 20 to 86 cm by
the year 2100. A recent EPA
study estimated that global sea level has a 50 percent chance of rising 45
cm (1-1/2 ft) by the year 2100, but a 1-in-100 chance of a rise of about 110
cm (over 3-1/2 ft).
Over the longer run, more substantial
changes in sea level are possible. Some scientists believe that the West
Antarctic Ice Sheet could slide into the oceans after a sustained warming, or
if other factors raised sea level. The vulnerability of this ice sheet is
poorly understood. It contains enough ice to raise sea level 6 meters (20
feet), and coastal scientists generally agree that sea level was 20 feet
higher than today during the last interglacial period, which was only slightly
warmer than today. While some scientists have suggested that there is fossil
evidence on the polar ocean floor that this ice sheet collapsed during the
last interglacial period, there is no scientific consensus on this question.
An EPA
study solicited the opinions of 8 US glaciologists on the vulnerability of
this ice sheet. All but one concluded that Antarctica is most likely to have a
negligible contribution to sea level over the next century. Nevertheless, they
all agreed that there is some risk that a catastrophic collapse of the ice
sheet could occur over a couple of centuries if polar water temperatures warm
by a few degrees. Most of the scientists estimated that such a risk had a
probability of between 1 and 5 percent. Because of this risk, as well as the
possibility of a larger than expected melting of the Greenland Ice Sheet, the EPA
study estimated that there is a 1 percent chance that global sea level
could rise by more than 4 meters (almost 14 feet) in the next two centuries.
Sea level along the US coast is likely
to be somewhat greater than the global average. The EPA
study includes a set of projections that coastal residents can use to
calculate how much sea level will rise in specific communities. Along the
coast of New York, which typifies the US Coast, sea level is likely to rise 26
cm (10 inches) by 2050 and 55 cm (almost 2 feet) by 2100. There is also a 1
percent chance of a 55 cm rise by 2050, a 120 cm rise (4 ft) by 2100, and a
450 cm rise (15 feet) by the year 2200.
Climate Models
Scientists generally agree on the likely rise in the average global
temperatures over the next century . Unfortunately, projecting the change in
particular regions is more difficult. Nevertheless, there is a general
consensus that temperatures will warm throughout the United States. However,
scientists are unable to say whether particular regions will receive more or
less rainfall; and for many regions they are unable to even state whether a
wetter or a drier climate is more likely.
Virtually all published estimates of how
the climate could change in the United States are the results of computer
models of the atmosphere known as "general circulation models."
These complicated models are able to simulate many features of the climate,
but they are still not accurate enough to provide reliable forecasts of how
the climate may change; and the several models often yield contradictory
results. For the time being, however, these models are about all we have to
say how the climate may change in particular areas.
Given the unreliability of these models,
researchers trying to understand the future impacts of climate change
generally analyze different scenarios from several different climate models.
The hope is that, by using a wide variety of different climate models, one’s
analysis can include the entire range of scientific uncertainty. For all of
these reasons, EPA reiterates the warning provided by all climate modelers to
people considering the impacts of future climate change: the projections of
climate change in specific areas are not forecasts but are reasonable examples
of how the climate might change
Climate model projections fall broadly
into two categories which are known as "CO2
doubling" and "transient" scenarios. The "CO2
doubling" scenarios represent the climate model’s estimate of how the
climate would change if the level of CO2 in the
atmosphere was doubled and the climate had several decades to reach a new equilibrium.
These scenarios were particularly common with older versions of the climate
models, which generally analyzed how climate might change without attempting
to calculate how the ocean currents might change. They generally do not
consider the cooling effect of sulfates or other aerosols.
More recently, elaborate models of the
ocean currents have been added to the climate models. The transient scenarios
mostly use these more elaborate "coupled ocean-atmosphere" models.
Instead of simply calculating how the climate and oceans would respond to a
doubling of CO2, these models use the historic and
projected changes in concentrations of greenhouse gases and calculate how the
climate might change each year until some date in the remote future. Many of
these model calculations include the cooling effects of sulfate aerosols.
Regional Temperatures
The historical temperature record shows that a rise in the global average
temperature does not automatically imply that every part of the world warms.
The cooling from sulfates may offset the warming in some areas. Moreover,
natural fluctuations in the jet stream and other factors often can cause the
Eastern United States to be unusually cool when the West is unusually warm (as
well as the reverse). During the summer of 1988, when the East suffered
severely hot and dry weather, cold relatively deep ocean water began to flow
to the sea surface off the mid-Atlantic Coast, keeping the coastal zone
unusually cool. Scientists have not ruled out the possibility that global
warming could induce such shifts, which could lead to little or no warming in
some areas while other areas warm by much more than the 1.0-3.5°C (3-8°F)
expected for the world as a whole.
The region of the United States that has
been the most thoroughly examined is the area from 35-50°N and 85-105°W. The
transient climate model results suggest that if global temperatures warm
2.6°C, the combined impact of aerosols and greenhouse gases is likely to warm
this region approximately 1.5-3.5°C (3-6.5°F) during winter. The same models
suggest, however, that summer temperatures will warm between 0 and 0.5°C
(less than 1°F). Our actual uncertainty for future temperature change is
probably at least twice as great as these ranges suggest, because global
warming is also uncertain.
Moreover, the cooling effect of aerosols
may prove to be less than assumed by the climate models. When the effect of
aerosols is eliminated, however, the same models estimate that summers could
warm by 3.5-5°C (6-9°F), and winters by 4-5°C (7.5-9°F). Other climate
models, which have estimated the impact of greenhouse gases but not aerosols,
suggest that summers in that region could warm 1.2-4.4°C and winters by
1.2-5.8°C.
Temperature projections vary for other
regions as well. For example, the Max Plank Institute’s model suggests that
California will warm approximately 1°C (2°F) in summer and 3°C (5°F) in
winter, while the United Kingdom’s Hadley Centre estimates that both winter
and summer could warm by about 3°C (5°F).
Regional Precipitation
The nation's water resources are sensitive both to rising temperatures and
changes in precipitation. Although scientists expect global temperatures to
rise approximately 0.5 to 1.5°C (1-3°F) by the year 2050, most climate
models suggest that warming over land--including the continental United
States--will be greater than the warming over the sea. Because higher
temperatures increase evaporation and plant transpiration, rainfall would
generally have to increase just to maintain current levels of water
availability. Holding other factors constant, the potential for evaporation
and transpiration increases about 5-10% per degree (C) throughout most of the
United States (Waggoner and Revelle 1990).
There is a general consensus that annual
worldwide precipitation and evaporation will increase a few percent for every
degree of warming. But there is considerably less certainty about rainfall in
particular locations, and whether the rainfall will increase enough to offset
the increased evaporation. Many scientists, however, believe that middle
latitudes such as that of the United States will see drier summers: Assuming
that the land warms more than the sea, evaporation over the land will increase
by more than the evaporation over the sea that produces rainfall. Thus, summer
rainfall may not increase by as much as evaporation.
For specific locations, however, it is
currently impossible to confidently project even the direction, let along the
magnitude or timing, of the seasonal or even annual changes in precipitation.
In the Central North American region, the two models that include the effect
of sulfates estimate that rainfall may increase slightly more than
evaporation, leading to modest increases in soil moisture during both winter
and summer.
A more pessimistic picture emerges,
however, from the nine climate models that have considered the implications of
greenhouse gases without the cooling effect of aerosols. During winter,
precipitation changes range from a decline of 15 percent to an increase of 18
percent; during summer, the changes range from a decline of 8 percent to an
increase of 6 percent. The scenarios that show an increase in precipitation
also project warming of 4-5°C (7-9°F), which would generally cause
evaporation to increase by 20 to 50 percent. Thus, all of the scenarios
suggest that summers will be drier if the cooling effect of sulfates does not
occur. Some of these models also suggest that winters could be drier, while
others project wetter winters.
Whether or not annual or seasonal
rainfall increases, many climate models project that rainfall will occur in a
smaller number of heavier storms, and that the number of dry days is likely to
increase. An Australian climate model, for example, projects that total
rainfall in the Midwest will decline by about 5 percent, but that heavy
rainstorms would occur 2 to 5 times as often. The National Center for
Atmospheric Research also expects fewer but heavier rainstorms. The Max Plank
Institute estimates that in central North America, 3-month-long dry spells
could become about 50 percent more frequent with a 2°C warming.
