SAN DIEGO -- Tell us what you don't know.
That's the message military and national security experts gathered here want to send to climate scientists.
While political leaders on Capitol Hill seek definitive answers about how quickly the world's climate will change, military and national security experts say they're used to making decisions with limited information.
But as they turn their attention to the geopolitical implications of climate change, they're pressing scientists to help them understand the risk and uncertainty inherent in forecasts of future environmental shifts.
"Are we going to wait for perfect data? No. Not only the Department of Defense but any successful organization doesn't wait for perfection," said Rear Adm. David Titley, who heads the Navy's Task Force Climate Change. "But we need to understand, how certain are you? And what does that mean?"
"We have to know what's plausible," said retired Air Force Maj. Gen. Richard Engel, who headed a recent National Intelligence Council assessment of the national security risks posed by climate change.
Titley and Engel were among the military, national security and climate experts who met this week at a conference organized by the Scripps Institution of Oceanography's new Center for Environment and National Security to discuss the kinds what kinds of forecasts will be needed for foreign policy planning.
Much of the discussion centered on the difference between risk and uncertainty.
Getting a better fix on sea level rise 'is a big deal'
Military and national security experts said climate forecasters often focus on averages, or the most likely scenario, without determining the probability of an extreme climate shift.
Jay Gulledge, senior scientist at the Pew Center for Global Climate Change and a fellow with the Center for a New American Security, pointed to estimates of sea level rise.
The last report of the Intergovernmental Panel on Climate Change, issued in 2007, predicted the sea would rise between 7 and 23 inches by 2100 -- but issued a giant caveat. The IPCC cautioned that an additional rise could come from rapid and unpredictable melting of massive ice sheets in Greenland and Antarctica, which it didn't attempt to estimate.
Since then, scientists have raced to improve their understanding of the processes that contribute to sea level rise. Many studies now suggest that seas will rise 1 to 2 meters by 2100.
But as Gulledge noted, there's a big difference between 1 meter of sea level rise -- roughly 3 feet -- and 2 meters. "We're at a point now where we can't say we're not going to get 2 meters of sea level rise by 2100, though it's perhaps not likely," he said.
That's a problem for national security planners, who normally look at a wide range of future scenarios -- including many that are unlikely, though potentially devastating.
But climate scientists said that making those kinds of determinations is difficult and in many cases is hampered by the amount of computing power available to run climate models.
"We've just barely gotten to the stage where we can make these projections," said Bruce Cornuelle, a physical oceanographer at Scripps. "The problem of putting probabilities on them is much harder."
For example, the Met Office, the United Kingdom's weather service, recently updated its climate change projections for Britain, indicating the probability of different scenarios.
Defense experts negotiate with modelers for probabilities
But doing so required compromise, said Vicky Pope, head of climate change advice at the Met Office's Hadley Centre.
"We had to use a low-resolution [climate] model," she said. "We were constrained by the computer model we had. And what that doesn't do is give you the outliers" -- extreme, but low probability, events.
Still, climate scientists said their models have improved greatly since the last report by the Intergovernmental Panel on Climate Change was released in 2007. At the Hadley Center, they're running a new model -- for the next IPCC report -- that covers changes in the atmosphere, the ocean, the carbon cycle, chemistry and land use.
Observations of key aspects of climate change are also improving. Scripps glaciologist Helen Fricker noted that scientists now understand how melting ice sheets in Greenland and Antarctica and glaciers on land contribute to sea level rise -- something that was an open question just a few years ago.
That, she said, factors into understanding what the future may bring.
Meanwhile, the Defense Department is beginning to negotiate directly with climate modelers to get the future forecasts it needs.
According to Titley, the Navy and Air Force are in talks with the National Oceanic and Atmospheric Administration to develop next-generation climate models that will incorporate knowledge of the social sciences, agriculture, and marine ecosystems -- "not just understanding that temperature is going up 'X' degrees."
Supporters of the proposed effort include NOAA Administrator Jane Lubchenco and White House science adviser John Holdren, Titley said.
"We are putting this together," he said. "This is going to be a big deal for us."
Correction: Maj. Gen. Engel is retired from the Air Force; a previous version misstated his service branch.