SUBTLE INFLUENCES Changes in the position of the jet stream can be induced by the “El Niño/La Niña-Southern Oscillation”, a five- to seven-year climate pattern located over the Pacific Ocean. Where the jet stream flows can have a significant impact on thunderstorm development, both directly, by providing the venting mechanism described above, and by initiating or otherwise influencing the movement of extra-tropical cyclones and frontal systems, both of which are potential lifting mechanisms.
Of course, severe thunderstorms are also subject to significant year-to- year and even day-to-day natural variability.
THE EFFECTS OF CLIMATE CHANGE A primary indicator of climate change is global average surface
temperatures. The observational record of the past century shows these temperatures to be increasing. A major question—and concern—is how will this warming climate affect the conditions and mechanisms that lead to the development of severe thunderstorms?
WEAKER WIND SHEAR = FEWER
SEVERE THUNDERSTORMS As the earth’s average global surface temperature rises, the polar regions
are expected to become warmer. As a result, the difference between the temperature of the air moving away from the poles and the temperature of the air moving away from the equator is expected to decrease. Since this difference fuels thunderstorm activity, a smaller temperature difference suggests that fewer thunderstorms will develop.
Similarly, vertical wind shear also should weaken, since shear too is a
product of temperature differences—thus also reducing the probability that severe thunderstorms will develop and intensify.
INCREASED MOISTURE AND VERTICAL INSTABILITY =
MORE SEVERE THUNDERSTORMS At the same time, increased warming is also expected to produce at least
two other major effects: increased moisture in the air and increased vertical instability. Both of these conditions, however—abundant moisture and vertical instability—promote the generation of severe thunderstorms. Thus, climate change characterised by rising temperatures, holding all else constant, could increase the probability that severe thunderstorms will develop.
CONSENSUS OF CURRENT RESEARCH These conclusions appear to be contradictory, the one pointing to elevated risk and the other to reduced risk. However, both are based on anticipated
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conditions whose eventual characteristics are not independent of the totality of meteorological changes that may develop in the end. At present, there is no consensus as to which effects will “win” over the others, or further how other unknown or less well understood factors will contribute.
The United Nations’ “Intergovernmental Panel on Climate Change”
(IPCC) 2007 Report states: “There is insufficient evidence to determine whether trends exist in small-scale phenomena such as tornadoes, hail, lightning, and dust storms.” This simply reflects the fact that the current state of science cannot attribute changes in small-scale weather phenomena such as tornadoes directly to changes in larger-scale climate phenomena such as jet stream patterns and the ENSO, especially since data from before the 1990s is less certain.
WHAT OF THE OBSERVED
INCREASE IN LOSSES? All the same, the historical record does demonstrate an increase in
losses caused by these storms in recent years. A number of explanations, unrelated to climate change, are very credible. Especially persuasive is how patterns of living and behaviour have changed over the past several decades. The Great Plains, which as late as the 1950s were predominantly rural and had few large cities, in recent years have become more densely populated, more urban, and the farms and ranches located there more consolidated and corporatised. In brief, the total insured value at risk in areas where severe thunderstorms occur has been increasing.
In this understanding, at least three factors have contributed to increasing
losses from severe thunderstorms: increased population in formerly rural areas, increasing exposure values in today’s urban and suburban areas, and severe storm activity that has directly impacted urban areas, many of which extend much further away from city centres than in decades past.
CONCLUSION The latest research on the influence of climate change points in two conflicting directions and further study is needed for scientists to be able to reach a consensus. In the near term—which may extend for decades or longer—which of the competing factors “wins out” may change from one year to the next, making it nearly impossible to attribute specific changes in the hazard to changes in climate until a very long and dependable historical record becomes available.
What is clear is that people are moving to and building homes in areas
of higher risk. In comparison to climate impacts, these trends are more easily quantified and can more readily explain at least part of the upward trend in losses.
Peter Dailey PhD is director, atmospheric science at AIR Worldwide. Ioana Dima PhD is a senior research scientist at AIR Worldwide. For more details, please visit
www.air-worldwide.com
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