Global Warming Reports
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Executive Summary
Scientists expect that global warming will cause a variety of
changes to precipitation patterns in the United States. Many areas will
receive increased amounts of rain and snow over the course of a year;
some areas will receive less. But scientists expect that, all across
the country, the rainstorms and snowstorms that do occur will be more
intense – increasing the risk of flooding and other impacts. In
this report, we evaluate trends in the frequency of storms with extreme
levels of rainfall or snowfall across the contiguous United States over
the last 60 years. We analyze daily precipitation records spanning from
1948 through 2006 at more than 3,000 weather stations in 48 states. We
then examine patterns in the timing of heavy precipitation relative to
the local climate at each weather station. We find that storms with
extreme amounts of rain or snowfall are happening more often across
most of America, consistent with the predicted impact of global
warming. Scientists expect global warming to increase the
frequency of heavy precipitation. As the earth warms, temperate regions
of North America will face a growing risk of storms with extreme levels
of rain or snowfall. Global warming increases the intensity of
precipitation in two key ways. First, by increasing the temperature of
the land and the oceans, global warming causes water to evaporate
faster. Second, by increasing air temperature, global warming enables
the atmosphere to hold more water vapor. These factors combine to make
clouds richer with moisture, making heavy downpours or snowstorms more
likely. The consequences of increasingly intense rainstorms may
include flooding, crop damage, pollution of waterways with runoff,
erosion, and other environmental and economic damage. During the 20th
century, floods caused more property damage and loss of life than any
other natural disaster in the United States. An increase in the number
of downpours does not necessarily mean more water will be available. Scientists
expect that extreme downpours will punctuate longer periods of relative
dryness, increasing the risk of drought. In the Southwest, for example,
total annual precipitation is projected to decline – amplifying the
impact of periods of little rainfall between heavy storms. Even in the
rest of the country, where total annual precipitation is expected to
increase, more of that precipitation will fall in heavy rainstorms or
snowstorms, paradoxically increasing the potential for drought. As
temperatures rise, precipitation will become increasingly likely to
fall as rain rather than snow, increasing runoff and likely reducing
water supplies in areas dependent on snowpack. Weather records
show that storms with extreme precipitation have become more frequent
over the last 60 years. Consistent with the predicted impacts of global
warming, we found that storms with extreme precipitation have increased
in frequency by 24 percent across the continental United States since
1948. (According to a statistical analysis of the data, with 95 percent
confidence, the increase has been between 22 and 26 percent.) New
England and the Mid-Atlantic experienced the largest increase in
extreme precipitation frequency. New England and the Mid-Atlantic saw
storms with extreme precipitation levels increase in frequency by 61
percent and 42 percent, respectively. At the state level, Rhode Island,
New Hampshire, Massachusetts, Vermont, New York and Louisiana all saw
extreme precipitation events increase in frequency by more than 50
percent. In the contiguous United States, 45 states experienced
increasingly frequent storms with extreme precipitation, and the trend
reached statistical significance (or 95 percent confidence) in 40 of
these states. Only three states (Oregon, Florida and Arkansas) showed a
decline in frequency of storms with extreme precipitation, and in only
one of those states (Oregon) did the data reach statistical
significance. See the report appendices for a full list of results by
region, state and metropolitan area. Climate divisions covering
more than half of the land area of the United States show a
statistically significant trend toward more frequent storms with
extreme precipitation. We also looked at the trend in frequency of
storms with extreme precipitation within climate divisions, which are
boundaries used by climatologists since the 1950s to aggregate weather
observations. Figure ES-2 presents these trends, showing that the
largest increases occurred across New England, New York, much of the
Great Lakes area, the upper Midwest, plus Louisiana, New Mexico,
northern Washington and southern California. Climate regions covering
more than half of the surface area of the contiguous United States show
a statistically significant increase in the frequency of storms with
extreme precipitation levels. In contrast, the data show statistically
significant decreases in extreme precipitation frequency for climate
regions covering only 4 percent of the area of the United States.
(Oregon, the northwestern corner of North Dakota, central Arkansas, the
southern tip of Lake Michigan, and northern Florida.) These
findings are consistent with previous studies of extreme precipitation
patterns, both in the United States and across the globe. For example:
Scientists have observed warmer weather, higher atmospheric moisture
content, increased formation of storm clouds, and an increase in
thunderstorm activity over the contiguous United States in recent
decades. In 1999, researchers at the Illinois State Water Survey and
the National Climatic Data Center (NCDC) found that storms with extreme
precipitation became more frequent by about 3 percent per decade from
1931 to 1996. Our findings are consistent with this result. In 2004,
scientists at NCDC concluded that most of the observed increase in
storms with heavy and very heavy precipitation levels since the early
1900s had occurred in the last three decades. In other words, they
found that the change in extreme precipitation frequency is unusual and
relatively recent. Moreover, NCDC found that extremely heavy storms are
increasing in frequency more rapidly than very heavy storms – which in
turn are increasing in frequency more rapidly than heavy storms. The
severity of the trend toward more intense downpours in the future
depends upon our emissions of the pollution that drives global warming.
Climate models predict that the trend toward increasingly frequent
storms with heavy precipitation will intensify in the future. Some
amount of change is inevitable given the global warming emissions
humans have already created. However, we still have the ability to
prevent the worst-case scenarios. By halting the increase in total U.S.
global warming emissions now and reducing emissions by at least 80
percent by mid-century, we can limit the increase in major storm
frequency – and thus reduce future risks of flooding and other serious
consequences of extreme rainstorms. To address global warming,
America should limit emissions of global warming pollution, while
improving energy efficiency and increasing the use of renewable energy.
To protect future generations, the United States should adopt a
mandatory cap on global warming pollution that reduces total U.S.
emissions by at least 15 to 20 percent by 2020 and by at least 80
percent by 2050. If policymakers choose a cap-and-trade program to
achieve this goal, it should include auctioning 100 percent of emission
allowances, rather than giving allowances away to polluters. By
auctioning allowances, we can reduce the cost of achieving emission
reduction goals, making it more likely that America will succeed. The
United States should also adopt complementary policies to improve
energy efficiency and increase the use of clean, renewable energy.
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