Lloyd’s of London recently released a report that suggests global warming could either increase or decrease thunderstorm risk in the United States, but a rise in sea levels on the east coast could increase losses from storms similar to Hurricane Sandy.
Catastrophe Modelling & Climate Change, which was produced by the exposure management and reinsurance team at Lloyd’s, was released May 8. In it, the authors review the latest research in climate change and the history of catastrophe modelling. Another section includes case studies provided by catastrophe modelling vendors.
“For many extreme perils the natural variability to date is larger than the underlying climate change tendency,” Lloyd’s notes. “Future projections show that in the coming decades the underlying tendency is expected to emerge more clearly.”
The contributing authors are Ralf Toumi, an atmospheric physics professor at Imperial College London and Lauren Restell of Lloyd’s Exposure Management. They warn that the levels of carbon dioxide, methane and nitrous oxide levels in the atmosphere “are higher than at any time” during the last 800,000 years.
“The Summary for Policymakers of the IPCC’s Fifth Assessment Report (2013) reports an unequivocal warming of the climate system,” Lloyd’s noted. “Changes are observed in atmospheric and oceanic temperatures, the extent of ice and snow coverage, and the concentration of greenhouse gases in the atmosphere.”
But this is not necessarily contributing to an increase in thunderstorms in the United States, according to a case study, provided by Verisk Analystics Inc.’s Boston-based AIR Worldwide unit, in the Lloyd’s report.
The AIR Worldwide authors – senior research scientist Ioana Dima and Shane Latchman, AIR’s manager of research and consulting and client services – note there are “two competing mechanisms” by which global warming can change thunderstorm risk in the United States.
One effect of global warming that could reduce the probability of severe thunderstorms is a “weaker lower level global temperature gradient between equator and poles which in turn causes a weakening of the vertical wind shear.”
However, Dima and Latchman note, an “increase in vertical instability and low-level moisture would result in an increased probability of severe thunderstorms in the future, since both these factors are important for the formation and development of thunderstorms.”
Their paper was on both U.S. severe thunderstorms and tropical cyclones in a region of the Pacific encompassing a wide area, including Papua New Guinea, Fiji and Samoa.
Global warming “has a direct impact on the intensity and life cycle” of tropical cyclones “by providing more energy to the storms and allowing for a possible increase in severity and frequency,” they wrote.