7.1. What extreme events could global warming explain ?
"As the world warms, some extreme climate events,
like the frequency of heat waves and very heavy precipitation, are
expected to increase, but it remains uncertain whether or not to
expect changes in the frequency of some other extremes. Moreover,
it is important to note that it is not possible to link any particular
weather or climate event definitively to global warming .
The causal linkage, if any, between the frequency
of extreme events and global warming only can be determined through
statistical analyses of long- term data, because the natural climate
system can produce weather and climate events that often appear
to be uncharacteristic of the recent climate."
7.2. Is the occurrence of extreme temperatures increasing ?
"Data on climate extremes in many regions of the world are
inadequate to draw definitive conclusions about possible changes
that may have occurred on a global scale. However, in some regions
where good data are available, there have been some significant
increases and decreases in extreme events over time. For example,
there has been a clear trend to fewer extremely low minimum temperatures
in several widely separated areas in recent decades (e.g., Australia,
the United States, Russia, and China). The impact of such changes
can manifest itself in fewer freezing days and late season frosts,
such as have been documented in Australia and the United States.
Indeed, we expect that the number of days with extremely low temperatures
should continue to decrease as global temperatures rise.
Widespread, extended periods of extremely high temperatures are
also expected to become more frequent with continued global warming
, such as the unprecedented high nighttime temperatures during
the 1995 heat wave in Chicago, Illinois, and the midwestern United
States that caused an estimated 830 deaths. However, the global
frequency of such heat waves has not been analyzed at this time."
"Higher temperatures lead to higher rates of
evaporation and precipitation. As the Earth warms, we expect more
precipitation and it is likely to fall over shorter intervals of
time, thereby increasing the frequency of very heavy and extreme
precipitation events. Analyses of observed changes in precipitation
intensity have been conducted only for a few countries. Perhaps
the best evidence of increases in extreme and very heavy precipitation
events comes from data in North America as depicted for the United
States in Figure
8.1. In Australia, which is historically prone to heavy precipitation,
an increase in rainfall amount from major storms has also been observed.
Analyses for South Africa also show increases in extreme precipitation
rates. In another area, China, where data have been analyzed for
the last several decades, no obvious trends are apparent, but high
concentrations of air pollution (such as sulfate particles that
can cool the climate) may be counteracting such changes in this
region.
There is as yet no evidence for a worldwide rise
in the frequency of droughts. In the future, however, it is expected
that many regions will experience more frequent, prolonged, or more
severe droughts, primarily due to the more rapid evaporation of
moisture from plants, soils, lakes, and reservoirs.
This is expected to occur even as precipitation increases and heavy
precipitation events become more common."
"Blizzards and snow storms
may actually increase in intensity and frequency in some colder
locations as atmospheric moisture increases. In more temperate latitudes,
snowstorms are likely to decrease in frequency, but their intensity
may actually increase, as the world warms. Observations show that
snowfall has increased in the high latitudes of North America, but
snow accumulations have melted faster because of more frequent and
earlier thaws.
There is evidence
of an increase in the frequency of intense extra- tropical storms
in the northern North Atlantic and adjacent areas of Europe, such
as the British Isles, but there has been a decrease in such events
in the southern North Atlantic (south of 30°N) over the past
few decades. It remains uncertain as to whether these changes are
natural fluctuations or relate to global warming , because there
is little consensus about how global warming will affect these non-tropical,
yet powerful storms.
There is little evidence to support any significant
long-term trends in the frequency or intensity of tropical storms,
or of hurricanes in the North Atlantic during the past several decades.
Although the hurricane frequency
was high during 1995 and 1996, an anomalously low number of hurricanes
occurred during the 1960s through the 1980s, including those hitting
the United States during that period (Figure
8.2). Reliable data from the North Atlantic since the 1940s
indicate that the peak strength of the strongest hurricanes has
not changed, and the mean maximum intensity of all hurricanes has
decreased.
There is also some evidence for a decrease in the
frequency of cyclones in the Indian Ocean during the past two decades
relative to earlier records and an increase in the frequency of
typhoons in the western Pacific. Wide variations in the total number
of tropical storms including hurricanes, typhoons, and cyclones
occurring per decade have been observed, with no apparent long-term
trends in most ocean basins. There is little consensus about how
global warming will affect the intensity and frequency of these
storms in the future."
US
Global Change Research Information Office
