Table of Contents Research News The NERSC Center Appendices

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

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)

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.

“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 latitude north 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

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.

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)

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.

Improving hurricane defenses

After the devastation caused by hurricanes Katrina and Rita, the Federal Emergency Management Agency (FEMA) asked the U.S. Army Corps of Engineers to run a series of simulations estimating hurricane-induced storm surge elevations to help improve hurricane defenses along the Gulf Coast. To assist in this effort, the DOE Office of Science allocated 800,000 processor hours of supercomputing time at NERSC to this project.

“NERSC … has a well-earned reputation for providing highly reliable systems, fast turnaround on critical projects, and dedicated support for users,” said Secretary of Energy Samuel Bodman when announcing the allocation. “Because these simulations could literally affect the lives of millions of Americans, we want to ensure that our colleagues in the Corps of Engineers have access to supercomputers which are up to the task.”

As hurricanes move from the ocean toward land, the force of the storm causes the seawater to rise as it surges inland. The Corps of Engineers used its DOE supercomputer allocations to create revised models for predicting the effects of 100-year storm-surges—the worst-case scenario based on 100 years of hurricane data—along the Louisiana, Mississippi, and Texas coast lines (Figures 3 and 4). In particular, simulations were generated for the critical five-parish area of Louisiana surrounding New Orleans and the Lower Mississippi River.

These revised models of the effects known as “storm-surge elevations” are serving as the basis of design for levee repairs and improvements currently being designed and constructed by the Corps of Engineers in the wake of Hurricane Katrina’s destruction in the New Orleans Metro Area.

Figure 3. Overview simulation showing elevated storm surges along the Gulf Coast. (Click on images to enlarge.)

Additionally, Gulf Coast Recovery Maps were generated for Southern Louisiana based on FEMA’s revised analysis of the frequency of hurricanes and estimates of the resulting waves. These maps are being used on an advisory basis by communities currently rebuilding from the 2005 storms.

Figure 4. Simulation detail showing highest surge elevation (in red) striking Biloxi, Miss. New Orleans is the dark blue crescent to the lower left of Biloxi.

Having access to the NERSC supercomputers allowed the Corps of Engineers to create more detailed models of the effects of Hurricane Rita and other storms along the Gulf Coast. Increased detail gave the Corps of Engineers and FEMA more information about the local effects of such storms.

For example, storm surge elevations are greatly influenced by local features such as roads and elevated railroads. Representing these details in the model greatly improves the degree to which computed elevations match observed storm surge high-water marks and allows the Corps to make better recommendations to protect against such surges.

The Corps of Engineers team also ran hurricane simulations on the DoD Major Shared Resource computers at the Engineering Research and Development Center (ERDC). Due to the tremendous computational requirements of these hurricane protection projects and urgent timelines, only by working together and using both DOE and DoD resources was the Corps able to provide high-quality engineering solutions.

As a result of the computer runs, the Corps determined that the applications produced incorrect results at topographic boundaries in some instances, and codes were modified to improve the accuracy of the results. For example, the runs at NERSC have improved the Corps’ ability to model the effects of vegetation and land use on storm surges which propagate far inland, as Hurricane Rita did on Sept. 24, 2005.

This article written by: David Hosansky, NCAR; Jon Bashor and John Hules, Berkeley Lab.

 

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).