Reblog: A New Tweak for Global Warming Predictions

Posted: November 13, 2012 in Uncategorized

The New York Times‘ Green Blog writer Rachel Nuwer explores changes in the prediction about global warming.

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.”

While the study does not yield a new estimate of future warming, it adds another brick to the edifice of mainstream climate science. “This is a really interesting and provocative idea, which may well be right, but time will tell if this is really a key breakthrough,” said Andrew Dessler, a climate scientist at Texas A&M University who was not involved in the work.

“Science papers are frequently like cooking chili — you don’t really know how it’ll turn out until you put it in the fridge for a day or two,” he said.

Raymond Pierrehumbert, the Louis Block Professor in geophysical science at the University of Chicago, described the paper as “another useful data point on the spectrum of estimates of cloud sensitivity,” though not “an absolute game changer.”

He pointed out, for example, that the correlation between water vapor and clouds was drawn from short-term seasonal fluctuations. Researchers cannot be certain that those same correlations would hold true in response to a doubling of carbon dioxide in the atmosphere in the future, Dr. Pierrehumbert added.

“The whole problem is really the nature of the observations,” he said. “We don’t have long enough satellite records of cloud observations to really do this kind of study by directly looking at which models get the low clouds right, so we try to indirectly run around the inadequacies of the satellite record.”

Dr. Trenberth worries that observations of earth from space will only become more scarce in the future as financing for satellite programs falls short, impeding future research that could help refine climate change predictions.

Regardless of scientists’ ability to accurately predict the extent of future changes, Dr. Pierrehumbert pointed out that the planet was not helpless. “The only way to guard ourselves against the risk of really strong climate change is just to emit less CO2, because the one thing we know for sure is the less we emit, the less the maximum climate change is going to be,” he said.

Evidence of the human influence on climate is with us already, Dr. Trenberth noted, in terms of a general trend of more severe storms, extreme droughts, heat waves and wildfires.

“What our study suggests is that, yes, we should pay special attention to these early signs, because as we go further into the future, the effects are apt to be even bigger still,” he said.

The full article may be accessed at http://green.blogs.nytimes.com/2012/11/08/a-new-tweak-for-global-warming-predictions/.

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