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Expecting the unexpected

A large reduction of mountain snow pack in Washington's Cascade Mountains would significantly affect water supply for irrigation and hydropower generation in the region
Click here for a high resolution photograph.

Discovering unexpected impacts from climate change is something researcher Ruby Leung is getting used to.

In a recent study, Leung looked at how climate change affects temperatures, expecting to find an increase in extreme hot days as global temperatures rise. Instead, she discovered that it is possible to have not only more extreme hot days in summer but also more extreme cold days in winter.

"We play a role in developing the models, but they are so complicated that sometimes the answer looks different from what we expected," said the Pacific Northwest National Laboratory scientist. Leung is part of a team of PNNL scientists who are working to predict regional climate change and its impact on water resources.

The rise in the potential number of extreme cold days in the winter can be explained by changes in cloud cover, Leung said. For example, in the winter, really cold days are usually clear days. At night, when the sky is clear, the radiation from the day escapes. It is possible that changes in cloud cover related to global warming will result in more frost days in the winter as well as the expected increase in the number of hot days in the summer.

Leung's experience illustrates an issue that climate researchers deal with every day—uncertainty. "There is uncertainty related to the global model because we have to represent all these physical processes and we don't fully understand them yet."

Some of the phenomena, like wind, can be explained using very basic equations. But many physical processes are so complicated and have such a large range of scale that they simply can't be described with a single equation. So when climate researchers try to project into the future, there is a lot of uncertainty.

One way to think of atmospheric processes, Leung explained, is as a huge continuum, starting locally and moving to a global scale with all the pieces continually interacting. For example, there may be local turbulence related to local winds, but this is constantly interacting with large-scale circulation that results from the difference between ocean and land.

For physical climate researchers like Leung who depend on the accuracy of computer models to understand and predict how different climate systems will interact, reducing uncertainty is a constant goal.

"The type of uncertainty I deal with is related to scaling," Leung said. "I try to get down from the larger cells that global models represent to the local scale, which holds more interest for people because it affects their daily activities."

Regional models allow Leung to "zoom in" on a specific region like North America because they are more precise and have higher resolution than global models and, therefore, can be used to better understand how global climate change predictions will affect specific areas. For example, knowing that there will be warming or drying trends over a large area, Leung may try to predict changes that might happen on the local scale.

Water resources are also an important issue in the western United States, and there is concern about how greenhouse warming will affect water resources in the future, especially with increasing population, an additional stressor to the water system.

In an early study of western states, Leung and PNNL hydrologists found that expected global temperature increases of two to three degrees Celsius would decrease snow pack in the mountains 50 to 70 percent by about 2050.

"This was quite surprising in the late '90s," Leung said. "Now other studies have been done, and our results are consistent with what has been observed in the past and other predictions for the future."



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