New Federal report
Changes in extreme weather and climate events have significant impacts and are among the most serious challenges to society in coping with a changing climate.
Many extremes and their associated impacts are now changing. For example, in recent decades most of North America has been experiencing more unusually hot days and nights, fewer unusually cold days and nights, and fewer frost days. Heavy downpours have become more frequent and intense. Droughts are becoming more severe in some regions, though there are no clear trends for North America as a whole. The power and frequency of Atlantic hurricanes have increased substantially in recent decades, though North American mainland land-falling hurricanes do not appear to have increased over the past century. Outside the tropics, storm tracks are shifting northward and the strongest storms are becoming even stronger.
It is well established through formal attribution studies that the global warming of the past 50 years is due primarily to
human-induced increases in heat-trapping gases. Such studies have only recently been used to determine the causes of some changes in extremes at the scale of a continent. Certain aspects of observed increases in temperature extremes
have been linked to human influences. The increase in heavy precipitation events is associated with an increase in water
vapor, and the latter has been attributed to human-induced warming. No formal attribution studies for changes in drought severity in North America have been attempted. There is evidence suggesting a human contribution to recent changes in hurricane activity as well as in storms outside the tropics, though a confident assessment will require further study.
In the future, with continued global warming, heat waves and heavy downpours are very likely to further increase in
frequency and intensity. Substantial areas of North America are likely to have more frequent droughts of greater severity. Hurricane wind speeds, rainfall intensity, and storm surge levels are likely to increase. The strongest cold season storms are likely to become more frequent, with stronger winds and more extreme wave heights.
Weather and climate extremes (Figure ES1)
have always posed serious challenges to society.
Changes in extremes are already having
impacts on socioeconomic and natural systems,
and future changes associated with continued
warming will present additional challenges.
Increased frequency of heat waves and drought,
for example, could seriously affect human
health, agricultural production, water availability
and quality, and other environmental condi-
tions (and the services they provide) (Chapter
1, section 1.1).
Human-induced warming is known to affect
climate variables such as temperature and
precipitation. Small changes in the averages
of many variables result in larger changes in
their extremes. Thus, within a changing climate
system, some of what are now considered to
be extreme events will occur more frequently,
while others will occur less frequently (e.g.,
more heat waves and fewer cold snaps [FiguresES.1, ES.3, ES.4]). Rates of change matter since
these can affect, and in some cases overwhelm,
existing societal and environmental capacity.
More frequent extreme events occurring over
a shorter period reduce the time available for
recovery and adaptation. In addition, extreme
events often occur in clusters. The cumulative
effect of compound or back-to-back extremes
can have far larger impacts than the same events
spread out over a longer period of time. For
example, heat waves, droughts, air stagnation,
and resulting wildfires often occur concurrently
and have more severe impacts than any of these
alone (Chapter 1, section 1.2).
Climate models indicate that currently rare extreme
events will become more commonplace.
For example, for a mid-range scenario of future
greenhouse gas emissions, a day so hot that it is
currently experienced only once every 20 years
would occur every three years by the middle of
the century over much of the continental U.S.
and every five years over most of Canada. By
the end of the century, it would occur every
other year or more (Chapter 3, section 3.3.1).
Extreme precipitation episodes (heavy downpours)
have become more frequent and more
intense in recent decades over most of North
America and now account for a larger percentage
of total precipitation. For example,
intense precipitation (the heaviest 1% of daily
precipitation totals) in the continental U.S.
increased by 20% over the past century while
total precipitation increased by 7% (Chapter 2,
section 2.2.2.2).
Heavy precipitation events averaged over North
America have increased over the past 50 years,
consistent with the observed increases in atmospheric
water vapor, which have been associated with human-induced increases in greenhouse
gases (Chapter 3, section 3.2.3).
Projected Changes
On average, precipitation is likely to be less frequent
but more intense (Figure ES.4), and precipitation
extremes are very likely to increase
(see Table ES.1; Figure ES.5).
http://downloads.climatescience.gov/sap/sap3-3/sap3-3-final-ExecutiveSummary.pdf
Weather and Climate Extremes in a Changing Climate
http://www.noaanews.noaa.gov/stories2008/20080619_climatereport.html
http://downloads.climatescience.gov/sap/sap3-3/Brochure-CCSP-3-3.pdf
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