A New Tweak for Global Warming Predictions

http://green.blogs.nytimes.com/2012/11/08/a-new-tweak-for-global-warming-predictions/?ref=weather

Correction Appended

Josh Haner/The New York TimesTo bypass the challenges of predicting how clouds will change, scientists used relative humidity as a stand-in of sorts.

While scientists express confidence that the earth will continue to warm in response to an increase in atmospheric carbon dioxide levels, fine-tuning those projections has been a challenge. The majority of estimates fall between a rise of 2 to 4.5 degrees Celsius (3.6 degrees to 8.1 degrees Fahrenheit), which leaves quite a wide range of uncertainty.

While the numbers may seem relatively trivial to the layman, they represent a vast range of potential impacts on society in terms of sea-level rise, heat waves and extreme weather.

A new paper in Friday’s issue of the journal Science adds to that discussion, suggesting that future warming may fall on the high side of climate projections.

“There’s been a lot of uncertainty and quite a range of this quantity called climate sensitivity in the climate models,” said one of the authors, Kevin Trenberth, a senior scientist in the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo. “If you take our results at face value, it certainly indicates that the climate change will be at the higher side of what’s been put forth previously, and that’s not good news.”

To arrive at that conclusion, Dr. Trenberth and his co-author, John Fasullo, set out to assess which of 16 leading climate models most accurately portrayed the earth’s current climate, especially in regard to clouds.

Clouds significantly influence the earth’s temperature, but predicting how those ever-shifting masses will change is notoriously difficult.

To get around that challenge, the researchers focused on the relationship between relative humidity — a measure of moisture in the atmosphere — and clouds. A strong, observable relationship exists between the two: when relative humidity is high, condensation occurs and clouds form. Models, or computer projections of future climate trends, can indirectly represent clouds by taking into account relative humidity measurements, which are readily available and bypass much of the complexity that bogs down cloud dynamics.

The authors compared how well the current climate models that they reproduced observed satellite data of relative humidity in the tropics and subtropics, where monsoon seasons result in annual cycles of cloudy and clear skies. The models that best represented the real-world monsoon processes, they found, tended to be those situated on the higher end of the projections of warming.

This result suggests that relative humidity is a necessary metric for models to perform accurately, Dr. Trenberth said, but not a sufficient criterion in itself for predicting the degree of future change.

Like a grading system, researchers could use this new finding to assess how closely existing models reflect reality. “The results we found do not guarantee that the more sensitive models are correct,” he said. “But we now know that those less sensitive models are certainly not correct.”