Is Earth's climate really changing? Climate researchers have answered this question with a resounding "Yes!" It is widely accepted that Earth's temperature will rise two to three degrees Celsius over the next 50 years if current fossil fuel emissions continue to grow at their present rate. The next question is how this warming trend will affect the climate—particularly water resources. Researchers at Pacific Northwest National Laboratory are working to answer this and other questions related to physical climate change by developing a better understanding of clouds, which can be used to improve global climate models. "Climate change will affect us first and foremost in water resources," said Tom Ackerman, who is the chief scientist of the Atmospheric Radiation Measurement Program, ARM. "To understand how we will be affected and to move forward, we must be able to understand clouds." ARM was created by the U.S. Department of Energy to help resolve scientific uncertainties related to global climate change by focusing specifically on the role of clouds and their influences on radiative feedback processes in the atmosphere. ARM is a multi-lab program comprising seven national laboratories that supports scientific research at universities, national laboratories and research companies. "We work on the physical side of climate change," Ackerman said, "but the work we do is linked with the work being done at PNNL's Joint Global Climate Research Institute in Maryland. Simulations of future climate change, which we carry out with our models, are dependent on scenarios of energy use and power generation developed by the researchers at JGCRI." For example, if the price of gasoline goes up to $3 per gallon in the United States, Americans might burn less fossil fuels. Physical climate models run at PNNL take into account changes in the amount of fossil fuel expected to be burned over the next 50 years. Ackerman and his team run climate models that span the globe. These models have become increasingly complex over the past 20 years and now include atmospheric, land and ocean models. "But, when we look at global models, the weakest point is always the clouds," Ackerman said. "Clouds are like the thermostat of the water system. They reflect radiation from the sun and absorb warm radiation that is trying to leave the earth. The warmer the temperature, the more water vapor the atmosphere can contain." Water is intimately tied to the way that solar and infrared energy drive the climate. Water vapor and clouds control the amount of energy that earth takes in and gives off. And, because water vapor concentration increases in a warmer atmosphere, the water cycle tends to amplify warming created by greenhouse gases, such as carbon dioxide and methane, produced by human activity. "ARM focuses on water via clouds because water is physically important to the climate system, is a critical piece of physical climate models, and is the most difficult to represent well with models," Ackerman said. "We have a lot of experience in the physical understanding of clouds and cloud processes. Now we are moving into the computer modeling arena," Ackerman said. PNNL's climate researchers use the supercomputer at the Environmental Molecular Sciences Laboratory, a DOE user facility at PNNL, to run complicated climate models. Using data gathered from ARM over the past 10 years, Ackerman and his team are building more complex global climate models that explicitly include cloud and water processes. "It is important for us to think about the climate problem in different ways," he said. "We want to take advantage of all the information we've gathered from ARM over the past 10 years." The next generation of questions Ackerman and his colleagues will be answering includes determining the degree to which carbon emissions will affect global temperature and the changes that will take place in particular regions. "Every time we answer a question," Ackerman said, "we are asked a harder question."