Extreme Weather

Scripps Climate Experts Address Extreme Weather In The 21st Century

Dynamical models that simulate climate change scenarios in the coming century suggest an increasing likelihood of extreme weather events. The types of events that could become more frequent and intense range from droughts and heat waves to hurricanes and rains with the potential to cause floods.

Several Scripps Institution of Oceanography at UC San Diego researchers study extreme weather events using the historical record and present-day data as a means of improving forecasting tools. Their discoveries could lead to a better understanding of the relationships between climate and phenomena ranging from wildfires to infectious disease. This work also considers the influence of climate on society and human influence on climate.

In several recent studies of extreme weather events, Scripps climate researcher Alexander Gershunov has characterized the nature of heat waves and made inferences about their probability and changing features in coming decades.

In collaboration with European colleagues, Gershunov recently conducted an analysis of the 2003 heat wave in Europe that killed an estimated 35,000 people and caused widespread environmental and economic disruption. The study indicated that global warming is causing heat waves to be more spatially extensive and thus more likely to have widespread consequences. These anthropogenic trends are expected to continue, and to also continue to be modulated by natural climate variability.

In the past 20 years, greater-than-usual precipitation in the central and eastern United States has mitigated heat wave activity there, keeping it below what had been forecast by climate models. Greater amounts of rainfall and snow increase soil moisture. Wetter soil requires more energy to heat than dry soil and the evaporation process has a cooling effect on soil similar to the effect of sweating on the body.

The study had predicted that an end to or break in this wet spell would result in heightened heat wave activity over the region. That prediction was borne out in summer 2006. Natural decadal fluctuations in precipitation and soil moisture modulate the global warming trend in regional heat wave activity. According to the study, a drought can now result in heat waves in excess of those observed during the 1930s Dust Bowl.

Gershunov and colleagues projected in their analysis that unprecedented heat waves should become more frequent in the central and eastern United States if precipitation trends reverse and lead to drier conditions in the future. Because of generally dry summer soil conditions in the western United States, the correlation between precipitation and its influence on extreme heat events is less pronounced there than in the rest of the country.

In another study, Gershunov and fellow Scripps climate scientist Dan Cayan defined and analyzed heat wave activity in day and nighttime temperatures over California and focused on a July 2006 event that was notable for its unprecedented magnitude, especially in nighttime temperatures. An increasing frequency and magnitude (i.e. intensity, duration and spatial extent) of nocturnal heat waves, which typically accompany daytime heat waves in summer, is clearly visible in the instrumental record. Impacts of this trend can range from increased energy demand at night to effects on sleep patterns, stress and exacerbation of illnesses.

The analysis drew a link between the unusually intense, extensive and long-lasting heat and statewide excessive atmospheric moisture that was unrelated to precipitation. This work underscores the necessity of considering synoptic weather patterns in understanding long-term climatic trends in regional weather and climate extremes.

Other Scripps projects include the development of a data-driven theoretical approach that reliably models the probability of daily precipitation extremes and another that uses climate information to enhance the predictability of infectious disease outbreaks. These projects involve collaborations with international colleagues and are among several that aim at an improved, more detailed understanding of regional climatic change that should result in regional applications in California and other key regions around the world.