The most comprehensive climate model to date of the continental United States predicts more extreme temperatures throughout the country and more extreme precipitation along the Gulf Coast, in the Pacific Northwest and east of the Mississippi.

Separately, a NASA (US National Aeronautics and Space Administration) study is offering new insight into how the Earth's water cycle might be influenced by global change.

These graphics illustrate some of the changes in climate predicted for the 21st century by Purdue University's Noah Diffenbaugh and his team of scientists using a computer simulation they recently completed. The simulation indicates that the entire continental United States will experience more intense heat waves, most dramatically in the desert Southwest (top figure). It also indicates that several areas, notably the Gulf Coast, will experience more storms that bring heavy precipitation (bottom figure). The computer model, which incorporates many climatic factors in unprecedented detail, suggests that these changes will be significant enough to disrupt our national economy and infrastructure. (Purdue graphic/Diffenbaugh Lab)

The climate model, run on supercomputers at Purdue University, takes into account a large number of factors that have been incompletely incorporated in past studies, such as the effects of snow reflecting solar energy back into space and of high mountain ranges blocking weather fronts from traveling across them, said Noah S. Diffenbaugh, the team's lead scientist. Diffenbaugh said a better understanding of these factors – coupled with a more powerful computer system on which to run the analysis – allowed the team to generate a far more coherent image of what weather we can expect to encounter in the continental United States for the next century. Those expectations, he said, paint a very different climate picture for most parts of the country.

"This is the most detailed projection of climate change that we have for the U.S.," said Diffenbaugh, an assistant professor of earth and atmospheric sciences in Purdue's College of Science and a member of the Purdue Climate Change Research Center. "And the changes our model predicts are large enough to substantially disrupt our economy and infrastructure."

Dr. Noah S. Diffenbaugh

The research team also includes Diffenbaugh's Purdue colleague Robert J. Trapp, as well as Jeremy S. Pal and Filippo Giorgi of the Abdus Salam International Centre for Theoretical Physics in Trieste, Italy. Their paper appears in the October 17 online edition of the journal Proceedings of the National Academy of Sciences.

Climate models are sophisticated computer codes that attempt to incorporate as many details about the complex workings of our environment as possible. Hundreds of dynamic processes, such as ocean currents, cloud formations, vegetation cover and – of particular import – the increase in atmospheric greenhouse gases, are programmed into the computers, which then attempt to discern the net effects on square-shaped plots of land that represent small pieces of the Earth's surface. The smaller these squares are, the better the resolution the model can provide.

"Just as a digital camera that creates images with more pixels can result in a better photograph, we want to make those squares as small as possible," Diffenbaugh said. "We'd also like to incorporate as much of the climate system as we can so the analysis will be realistic."

Despite the number-crunching power of the linked computers used for these simulations, a model must factor in so many changing variables that a full analysis can require months of nonstop computational effort. Diffenbaugh's team required five months to run their model on a cluster of Sun computers at the Rosen Center for Advanced Computing on Purdue's campus.

"The results were worth it, though, because this model allows us to project changes in climate with unprecedented resolution," Diffenbaugh said.

Until now, the fastest computers have been used to resolve squares 50 kilometers to a side, which can return a reasonably accurate but rather grainy "photograph" of climate change.

"We can now analyze areas that are only 25 kilometers to a side, which, for example, allows us to discern more clearly where California's central valley stops and the Sierra Nevada mountain range begins."

With their improvements over previous models, the team has been able to make several observations about the change in climate over the next century, particularly for the late century when greenhouse gas accumulation could have greater effect than, say, a decade from now.

"These projections are not necessarily about specific weather events," Diffenbaugh said. "But they do give us a good idea about what kind of weather to expect over the long run in a particular part of the country."

Some of these expectations include:

• The desert Southwest will experience more heat waves of greater intensity, combined with less summer precipitation. Water is already at a premium in the four-corners states and southern Nevada and, as years pass, even less water will be available for the region's burgeoning populations, with extreme hot events increasing in frequency by as much as 500 percent.

• The Gulf Coast will be hotter and will receive its precipitation in greater volumes over shorter time periods. "The region actually will get more rainfall than it does now, but it will not be steady," Diffenbaugh said. "We project more dry spells punctuated by heavier rainfalls. We need to perform further analyses to understand how much of this is related to tropical cyclone activity."

• In the northeastern United States – roughly the region east of Illinois and north of Kentucky – summers will be longer and hotter. "Imagine the weather during the hottest two weeks of the year," Diffenbaugh said. "The area could experience temperatures in that range lasting for periods of up to two months by century's end."

•Similarly, the continental United States will experience an overall warming trend: Temperatures now experienced during the coldest two weeks of the year will be a past memory, and winter's length will diminish as well, according to the model.

The model, Diffenbaugh said, assumes that greenhouse gases will attain a concentration more than twice their current levels, but he said he is confident that the model's performance gives as accurate a picture of the future as we can hope for at the moment.

"We checked our model's performance by analyzing the period from 1961 to 1985 for which, of course, we do not need a prediction," Diffenbaugh said. "The model performed admirably, which tells us we've got a good understanding of how to represent the physical world in terms of computer code. It’s certainly not perfect, but we'll need a computer at least 100 times as powerful as the cluster we used to really improve the accuracy. We would like to have access to such computing power in the future."

Diffenbaugh emphasized that, while the model was in no way designed to return an alarmist image of our climate's future, the picture it painted should be considered.  

"The more detail we look at with these models, the more dramatic the climate's response is," he said. "Critics have complained that climate models lack sufficient spatial detail to be trusted. In terms of looking at the whole contiguous United States, we've quadrupled the spatial detail and, as a result, it appears that climate change is going to be even more dramatic than we previously thought. Of course, we can never be completely certain of the future, but it's clear that as we consider more and more detail, the picture of future climate change becomes more and more severe."

Commenting on the study, Stanford University's Stephen H. Schneider said the results confirm scientists' suspicions about the future of climate change.

"This study is the latest and most detailed simulation of climatic change in the United States," said Schneider, who is Stanford's Melvin and Joan Lane Professor for Interdisciplinary Environmental Studies. "Critics have asserted that the coarse resolution of previous studies made their sometimes dire predictions suspect, but this new result with a very high resolution grid over the United States shows potential climatic impacts at least as significant as previous results with lower resolution model. As the authors wisely note, such potential impacts certainly should not be glibly dismissed."

A NASA study is offering new insight into how the Earth's water cycle might be influenced by global change. In recent years, scientists have warned that the water cycle may be affected by temperature changes, as warmer temperatures can increase the moisture-holding capacity of air.

Image to right: This animation illustrates the stages of the water cycle, as individual water molecules travel between the oceans, water vapor in the atmosphere, water and ice on the land, and underground water.  Credit: NASA GSFC

The global water cycle involves the transfer of water molecules between the Earth's land masses, cryosphere, oceans and atmosphere. It's a gigantic system powered by the sun, fueling a continuous exchange of moisture between the oceans, atmosphere and land.

Most climate models have shown that that a warmer climate will increase global evaporation and precipitation, but the atmospheric storage of water vapor has not yet been well studied.

Image/animation to left: This animation illustrates the output of NASA's Finite-Volume General Circulation computer Model during the 5-day period just prior to the landfall of hurricane Ivan in 2004. The color represents the amount of total precipitable water (blue is low, red is high). Credit: NASA GSFC/SVS

Recently, researchers from NASA Goddard Space Flight Center, Greenbelt, Md., produced climate simulations of the early and late 20th century. They used sea surface temperature (SST) data and two computer models designed at Goddard Space Flight Center to determine how long water stays in the atmosphere. This is one way of measuring how the global water cycle might be influenced by changes in many variables, including temperature and precipitation.

Despite model differences, both simulations showed an increase in global evaporation and precipitation during this period. But, it is important to recognize that simulated atmospheric temperatures also increased during this period, raising the atmosphere's "total precipitable water" - the amount of liquid water in the atmosphere if all water vapor were suddenly condensed.

Image to right: This image of the Earth shows water vapor from the surface to the upper atmosphere (white areas). The areas with less water vapor appear deeper blue. Credit: NASA Ames

"By computing a diagnostic for the water cycle rate, which accounts for total atmospheric water vapor and the average rate of precipitation, the models show the water cycling rate is reduced as the temperature warms," said Michael Bosilovich, lead author of the study, published in the May 2005 issue of the American Meteorological Society's Journal of Climate.

When the researchers studied precipitation simulated over land and sea, they found it decreased over land as the local recycling of water vapor was reduced. Oceanic precipitation, however, had an upward trend along with increased sea surface temperatures, consistent with historical data and earlier studies.

Image above: This map of sea surface temperatures was produced using MODIS data on the Terra satellite. The red pixels show warmer surface temperatures, while yellow and green are middle values, and blue represents cold water. Credit: NASA GSFC

But, it should be noted that these contrasting land and ocean trends are not universally applicable to all regions," said Bosilovich. "For instance, the precipitation over the North American continent increases, while it decreases over the Gulf of Mexico."

The study also found that land sources of water for precipitation vary considerably within individual regions. Over time, the continental cycle of water appeared to decline, except in the central United States, where it might increase. But, further study is needed with a regional focus to accurately determine local recycling rates.

Image to right: This image shows unusual events in annual average rainfall/snowfall over (a) land and (b) oceans from 1949 to 1999 using three computer models (FVGCM, NSIPP and COLA). Chart A shows a decrease in precipitation over land in the last 50 years, with the downward slanted plots. Chart B shows an increase in precipitation over the ocean in the last 50 years, indicated by the upward slanted plots. Credit: Michael Bosilovich

"In regard to the global scale, satellite data is an essential tool in assessing the rate and intensity of the global water cycle. It helps to identify the background state of the climate, but is limited by its short duration of record and deficiencies within historical products," said Bosilovich. "This study highlights the importance of continued high quality, well-maintained observations of atmospheric water content and precipitation rates over both the land and ocean well into the future so that we can more accurately assess changes in the water cycle."

Today, NASA's Aqua and Terra satellites are providing such data by giving new, detailed information on processes that contribute to the water cycle. Ultimately these findings, coupled with data from future satellites, will be incorporated into regional and global computer models, improving both short-term weather forecasts and long-term climate forecasts. Such seasonal predictions carry significant economic implications and are also critical to water resource managers in determining water availability and management.

Image to left: Regions like the Persian Gulf in the Middle East, shown here by NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, may face increasingly severe water shortages as global climate changes. Credit: NASA GSFC

Other research programs like the NASA Energy and Water Cycle Study also use data from NASA satellites to help scientists learn more about the link between climate and the water cycle, improving their ability to predict events like floods and droughts. Co-authors of the study include Siegfried Schubert and Gregory Walker of NASA GSFC.