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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization




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Are Recent Extreme Weather Events, Like the Large Number of Atlantic Hurricanes in 1995, Due to Global Warming?

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.

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.

Figure 8.1

The area (expressed in percentage) of the United States, excluding Alaska and Hawaii, with an unusually large amount of total annual precipitation coming from extreme precipitation events (those with more than 5.08 cm {2 inches} of rainfall {or equivalent if precipitation is snowfall} in 24 hours) is displayed. The smooth curve shows the same data, but averaged over periods of about 10 years.

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.

Figure 8.2

Number of all hurricanes and the most severe hurricanes making landfall in the United States, excluding Alaska and Hawaii.

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