Princeton Study Tosses Cold Water On Method Used For Measuring Climate Change

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Chris White Tech Reporter
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Climate scientists grossly underestimate the effect cloud cover has on global temperatures, according to researchers at Princeton University’s environmental institute.

Climate models factor in too much of the sun’s daily heat into their projections about what the Earth’s land temperatures could look like long-term. Inaccuracies in accounting for daily cloud cycle distorts the effectiveness of a tool scientists use to measure climate change, according to a study published in the journal Nature Communications.

“It’s important to get the right result for the right reason,” co-author Amilcare Porporato, a professor of environmental engineering at the Princeton Environmental Institute, told Nature Communications. While the findings do not invalidate projections outright, he added, they do toss a wrench in modern methods scientists use to determine future land temperatures.

“These errors can trickle down into other changes, such as projecting fewer and weaker storms,” he said. “We hope that our results are useful for improving how clouds are modeled, which would improve the calibration of climate models and make the results much more reliable.”

Porporato’s research found that inaccurately measuring cloud cycle results in the sun pounding the Earth with an extra 1-2 watts of energy per square meter. Scientists argue carbon dioxide pumped into the atmosphere through the last 100 years produced approximately an extra 3.7 watts of energy per square meter.

“The error here is half of that, so in that sense it becomes substantial,” said Porporato, who co-authored the study with Jun Yin, a postdoctoral research associate in civil and environmental engineering. Yin said climatologists generally do a good job measuring average cloud coverage, but miss important peaks in actual cloud coverage.

Yin and Porporato used satellite images from 1986-2005 to compare the averages they came up with to those from nine climate models, most of which erroneously found the thickest coverage occurring in the morning over the land rather than in the early afternoon. “A small difference in timing can have a big radiative impact,” said Yin, who analyzed cloud coverage at three-hour intervals.

“Climate scientists have the clouds, but they miss the timing,” Porporato said. “There’s a strong sensitivity between the daily cloud cycle and temperature. It’s like a person putting on a blanket at night or using a parasol during the day. If you miss that, it makes a huge difference.”

Climate models usually focus on more generalized measures, such as air pollution and seasons, to determine the effect a cloud’s so-called Diurnal cycle has on the climate, Gabriel Katul, a professor of micrometeorology at Duke University, told Natural Communications.

“There are practical reasons why data-model comparisons were conducted in a manner that masked the diurnal variation in clouds,” said Katul, who did not work on the study but is knowledgeable about the science tying cloud cycle to climate forecasts. “Diurnal variation was somewhat masked by the fact that much of the climate-model performance was reported over longer-term and larger-scale averages.”

Yin and Porporato are not the only ones to put a crimp in the measurements climatologists use to forecast future land temperatures, and ocean water temperatures. Researchers at the University of California, San Diego, for instance, conducted a study earlier this year finding that ocean temperatures were warming much slower than initially thought.

Each layer of water in the ocean has vastly different temperatures, so determining the average temperature is nearly impossible without glossing over important data, according to Geoscientist Jeff Severinghaus, an academic at Scripps Institution of Oceanography. Instead, he measured the ratio of noble gases such as argon, krypton, and xenon in the atmosphere, which are in direct relation to the ocean’s temperature.

The ratio of these gases allows for a much more effective and exact calculation of average global ocean temperature, according to Severinghaus and his team of researchers at Scripps. They discovered that xenon and krypton are well preserved in ice cores and can, therefore, provide temperature information that scientists can use to study many other aspects of the earth’s oceans.


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