Past and Future US
Observations from 1200
weather stations across the US show that temperatures have increased over
the past century, on average by almost 1Â°F (0.6Â°C). The coastal
Northeast, the upper Midwest, the Southwest, and parts of Alaska have
experienced increases in the annual average temperature approaching 4Â°F
(2Â°C) over the past 100 years. The rest of the nation has experienced
less warming. The Southeast and southern Great Plains have actually
experienced a slight cooling over the 20th century, but since the 1970s
have had increasing temperatures as well. The largest observed warming
across the nation has occurred in winter.
Average warming in the US is
projected to be somewhat greater than for the world as a whole over the
21st century. In the Canadian model
scenario, increases in annual average temperature of 10Â°F (5.5Â°C) by
the year 2100 occur across the central US with changes about half this
large along the east and west coasts. Seasonal patterns indicate that
projected changes will be particularly large in winter, especially at
night. Large increases in temperature are projected over much of the South
in summer, dramatically raising the heat index (a measure of discomfort
based on temperature and humidity).
In the Hadley
model scenario, the eastern US has temperature increases of 3-5Â°F
(2-3Â°C) by 2100 while the rest of the nation warms more, up to 7Â°F (4Â°C),
depending on the region.
In both models, Alaska is
projected to experience more intense warming than the lower 48, and in
fact, this warming is already well underway. In contrast, Hawaii and the
Caribbean islands are likely to experience less warming than the
continental US, because they are at lower latitudes and are surrounded by
ocean, which warms more slowly than land.
The annual average
of minimum and maximum temperatures are compiled from the daily
lows and highs. These graphs show the lows and highs, averaged
over the year and over the lower 48 states. The green line shows
observed temperatures while the red and blue lines are model
projections for the future.
The minimum and
maximum temperatures are important because, far more than the
average, they influence such things as human comfort, heat and
cold stress in plants and animals, maintenance of snowpack, and
pest populations (many pests are killed by low temperatures; a
rise in the minimum often allows more pests to survive).
Both the Canadian and
Hadley model scenarios project substantial warming during the 21st
century. The warming is considerably greater in the Canadian model, with
most of the continental US experiencing increases from 5 to 15Â°F. In
this model, the least warming occurs in the West and along the Atlantic
and Gulf Coasts. In the Hadley model, annual temperatures are projected
to increase from 3 to 7Â°F, with the largest warming occurring in the
western half of the country.
Changes in Precipitation
Average US precipitation has
increased by 5-10% over the last century with much of that due to an
increase in the frequency and intensity of heavy rainfall. Precipitation
increases have been especially noteworthy in the Midwest, southern Great
Plains, and parts of the West and Pacific Northwest. Decreases have been
observed in the northern Great Plains.
For the 21st century, the
Canadian model projects that percentage increases in precipitation will be
largest in the Southwest and California, while east of the Rocky
Mountains, the southern half of the nation is projected to experience a
decrease in precipitation. The percentage decreases are projected to be
particularly large in eastern Colorado and western Kansas, and across an
arc running from Louisiana to Virginia. Projected decreases in
precipitation are most evident in the Great Plains during summer and in
the East during both winter and summer. The increases in precipitation
projected to occur in the West, and the smaller increases in the
Northwest, are projected to occur mainly in winter.
In the Hadley model, the
largest percentage increases in precipitation are projected to be in the
Southwest and Southern California, but the increases are smaller than
those projected by the Canadian model. In the Hadley model, the entire US
is projected to have increases in precipitation, with the exception of
small areas along the Gulf Coast and in the Pacific Northwest.
Precipitation is projected to increase in the eastern half of the nation
and in southern California and parts of Nevada and Arizona in summer, and
in every region during the winter, except the Gulf States and northern
Washington and Idaho.
In both the Hadley and
Canadian models, most regions are projected to experience an increase in
the frequency of heavy precipitation events. This is especially notable in
the Hadley model, but the Canadian model shows the same characteristic.
While the actual amounts are
modest, the large percentage increases in rainfall projected for the
Southwest are related to increases in atmospheric moisture and storm
paths. A warmer Pacific would pump moisture into the region and there
would also be a southward shift in Pacific Coast storm activity. In the
Sierra Nevada and Rocky Mountains, much of the increased precipitation is
likely to fall as rain rather than snow, causing a reduction in mountain
snow packs. This would tend to increase wintertime river flows and
decrease summertime flows in the West. Across the Northwest, and the
central and eastern US, the two model projections of precipitation change
are in less agreement. These differences will be resolved only by
improvements in climate modeling.
Changes in Soil Moisture
Soil moisture is critical
for both agriculture and natural ecosystems. Soil moisture levels are
determined by an intricate interplay among precipitation, evaporation,
run-off, and soil drainage. By itself, an increase in precipitation would
increase soil moisture. However, higher air temperatures will increase the
rate of evaporation and, in some areas, remove moisture from the soil
faster than it can be added by precipitation. Under these conditions, some
regions are likely to become drier even though their rainfall increases.
In fact, soil moisture has already decreased in portions of the Great
Plains and Eastern Seaboard, where precipitation has increased but air
temperature has risen.
Since soil moisture
projections reflect both changes in precipitation and in evaporation
associated with warming, the differences between the two models are
accentuated in the soil moisture projections. For example, in the Canadian
model, soil moisture decreases of more than 50% are common in the Central
Plains due to the combination of precipitation reductions exceeding 20%
and temperature increases exceeding 10Â°F. In the Hadley model, this same
region experiences more modest warming of about 5Â°F and precipitation
increases of around 20%, generally resulting in soil moisture increases.
Increased drought becomes a
national problem in the Canadian model. Intense drought tendencies occur
in the region east of the Rocky Mountains and throughout the
Mid-Atlantic-Southeastern states corridor. Increased tendencies toward
drought are also projected in the Hadley model for regions immediately
east of the Rockies. California and Arizona, plus a region from eastern
Nebraska to Virginia's coastal plain, experience decreases in drought
tendency. The differences in soil moisture and drought tendencies will be
significant for water supply, agriculture, forests, and lake levels.