A Warmer, Stormier World

By the end of this century, bigger hurricanes, longer heat waves, and more extreme weather will be evident

The verdict is in from the Intergovernmental Panel on Climate Change (IPCC). The summary of its Fourth Assessment Report says the world is already committed to centuries of warming, shifting weather patterns, and rising seas from the human production of greenhouse gases, but warming can be substantially blunted with prompt action.

Those conclusions were based on the research of thousands of scientists worldwide, including the climate simulations created by Warren Washington and his colleagues at the National Center for Atmospheric Research (NCAR) and elsewhere using CCSM3, a climate code whose development was funded primarily by the National Science Foundation (NSF) and the Department of Energy (DOE). These simulations investigate the response of the Earth’s climate to future emissions scenarios that represent different policy choices for energy use and global development.

Data produced by these simulations are freely available to the research and education community via the DOE Earth System Grid. Among the recent studies based on these and other simulations are two that forecast more severe storms and more extreme weather in general.

Going to the extremes

Many previous studies have looked at how average temperature or rainfall might change in the next century as greenhouse gases increase. However, a new study titled “Going to the extremes: An intercomparison of model-simulated historical and future changes in extreme events”[1] looks more specifically at how weather extremes could change.

Figure 1. A thunderstorm cloud passes over the plains east of Denver. The number of days with heavy precipitation is expected to increase in the northern tier of U.S. states. (Photo by Carlye Calvin, ©UCAR, used with permission) (Click image for larger view)

“It’s the extremes, not the averages, that cause the most damage to society and to many ecosystems,” said NCAR scientist Claudia Tebaldi, lead author for the report. “We now have the first model-based consensus on how the risk of dangerous heat waves, intense rains, and other kinds of extreme weather will change in the next century.”

Tebaldi and colleagues based their work on simulations from nine different climate models, including CCSM3, for the periods 1980–1999 and 2080–2099. The simulations were created on supercomputers at NERSC and other research centers in France, Japan, Russia, and the United States. Each model simulated the 2080–2099 interval three times, varying the extent to which greenhouse gases accumulate in the atmosphere. These three scenarios were used to account for uncertainty over how fast society may act to reduce emissions of carbon dioxide and other greenhouse gases over coming decades.

From the model output, the scientists computed ten different indices of climate extremes, with five related to temperature and five to moisture. For instance, a frost days index measures how many days per year temperatures dip below 32 degrees Fahrenheit, while a dry days index measures the length of each year’s longest consecutive string of days without rain or snow. Because the impact of a given index can be stronger in one climatic zone than another, the authors expressed the results in terms of statistical significance at each location.

For all three greenhouse-gas scenarios, the models agree that by 2080–2099:

• The number of extremely warm nights and the length of heat waves will increase significantly over nearly all land areas across the globe. During heat waves, very warm nights are often associated with fatalities because people and buildings have less chance to cool down overnight.
• Most areas above about 40 degrees north latitude will see a significant jump in the number of days with heavy precipitation (days with more than 0.4 inches). This includes the northern tier of U.S. states, Canada, and most of Europe.
• Dry spells could lengthen significantly across the western United States, southern Europe, eastern Brazil, and several other areas. Dry spells are one of several factors in producing and intensifying droughts.
• The average growing season could increase significantly across most of North America and Eurasia.

Most of these trends are significantly weaker for the lowest-emission scenario than for the moderate and high-emission scenarios. Thus, the authors add, lowering the output of greenhouse gases over the next century should reduce the risk that the most severe changes will occur.

Breeding bigger hurricanes

Rising ocean temperatures in key hurricane breeding grounds of the Atlantic and Pacific oceans are due primarily to human-caused increases in greenhouse gas concentrations, according to a study published in the September 11, 2006 issue of the Proceedings of the National Academy of Sciences (PNAS).[2]

Figure 2. Hurricane Ioke passes by the Hawaiian Islands on August 21, 2006, with 132-mile-per-hour winds in this satellite image. The storm, renamed Typhoon Ioke as it moved west across the International Date Line, later intensified to become the most powerful central Pacific storm on record. (Image: Hal Pierce, SSAI/NASA GSFC) (Click image for larger view)

Using 22 different computer models of the climate system, including CCSM3, Benjamin Santer and six other atmospheric scientists from Lawrence Livermore National Laboratory, together with Tom Wigley, Gerald Meehl, and Warren Washington from NCAR, and collaborators from eight other research centers, have shown that the warming sea surface temperatures (SSTs) of the tropical Atlantic and Pacific oceans over the last century are linked to human activities.

“We’ve used virtually all the world’s climate models to study the causes of SST changes in hurricane formation regions,” Santer said.

Research published during the past year has uncovered evidence of a link between rising ocean temperatures and increases in hurricane intensity. This has raised concerns about the causes of the rising temperatures, particularly in parts of the Atlantic and Pacific where hurricanes and other tropical cyclones form.

Previous efforts to understand the causes of changes in SSTs have focused on temperature changes averaged over very large ocean areas, such as the entire Atlantic or Pacific basins. The new research specifically targets SST changes in much smaller hurricane formation regions.

“The important conclusion is that the observed SST increases in these hurricane breeding grounds cannot be explained by natural processes alone,” said Wigley. “The best explanation for these changes has to include a large human influence.”

Hurricanes are complex phenomena that are influenced by a variety of physical factors, such as SSTs, wind shear, water vapor, and atmospheric stability. The increasing SSTs in the Atlantic and Pacific hurricane formation regions are not the sole determinant of hurricane intensity, but they are likely to be one of the most significant influences.

“It is important to note that we expect global temperatures and SSTs to increase even more rapidly over the next century,” Wigley said. According to Santer, “In a post-Katrina world, we need to do the best job we possibly can to understand the complex influences on hurricane intensity, and how our actions are changing those influences.”

Other institutions contributing to this study include the University of California, Merced; Lawrence Berkeley National Laboratory; Scripps Institution of Oceanography; the University of Hamburg; the University of East Anglia; Manchester Metropolitan University; NASA’s Goddard Institute for Space Studies; and NOAA’s National Climatic Data Center.

[1] Claudia Tebaldi, Katharine Hayhoe, Julie M. Arblaster, and Gerald A. Meehl, “Going to the extremes: An intercomparison of model-simulated historical and future changes in extreme events,” Climatic Change 79, 185 (2006). Funding: BER, NSF, EPA.

[2] B. D. Santer, T. M. L. Wigley, P. J. Gleckler, C. Bonfils, M. F. Wehner, K. AchutaRao, T. P. Barnett, J. S. Boyle, W. Brüggemann, M. Fiorino, N. Gillett, J. E. Hansen, P. D. Jones, S. A. Klein, G. A. Meehl, S. C. B. Raper, R. W. Reynolds, K. E. Taylor, and W. M. Washington, “Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions,” PNAS 103, 13905 (2006).