The best evidence is that the climate is now being driven by amplifying feedbacks (see Study: Water-vapor feedback is “strong and positive,” so we face “warming of several degrees Celsius”), most notably:

In spite of all evidence to the contrary, the deniers/delayers/inactivists, led by MIT’s Richard Lindzen, have argued that negative feedbacks dominate the climate system.  In particular, they have asserted that clouds are a negative feedback.  A major new study in Science from  “Observational and Model Evidence for Positive Low-Level Cloud Feedback” (subs. req’d) is thus a potentially huge – and worrisome – piece of research.

I’m in an all-day meeting, so I’m mainly going to reprint the study abstract, the accompanying Science news story, “Clouds Appear to Be Big, Bad Player in Global Warming” (subs. req’d), and the press release from the University of Miami’s Rosenstiel School of Marine and Atmospheric Sciences, who led the study (with the Scripps Institution of Oceanography, my old stomping ground snorkeling reef).

It is worth noting that the one climate model the researchers found was “particularly realistic” in modeling the cloud feedback, the Hadley Center’s HadGEM1, finds, “When carbon dioxide is doubled, the model warms the world by 4.4°C; the median of the models for a doubling is 3.1°C.”  Considering that we are headed toward more than a tripling of CO2 concentrations this century, that is very, very worrisome.

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Figure 1

FIGURE: Leaky clouds. Decades-long records show that when sea surface temperature (SST) warms, cloud cover-especially from low clouds (bottom)-decreases (blues, top), letting in more sunlight.

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Let’s start with the PR:

Strong Evidence That Cloud Changes May Exacerbate Global Warming

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Virginia Key, Fla. – The role of clouds in climate change has been a major question for decades. As the earth warms under increasing greenhouse gases, it is not known whether clouds will dissipate, letting in more of the sun’s heat energy and making the earth warm even faster, or whether cloud cover will increase, blocking the Sun’s rays and actually slowing down global warming.

In a study published in the July 24 issue of Science, researchers Amy Clement and Robert Burgman from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and Joel Norris from Scripps Institution of Oceanography at UC San Diego begin to unravel this mystery. Using observational data collected over the last 50 years and complex climate models, the team has established that low-level stratiform clouds appear to dissipate as the ocean warms, indicating that changes in these clouds may enhance the warming of the planet.

Because of inconsistencies in historical observations, trends in cloudiness have been difficult to identify. The team broke through this cloud conundrum by removing errors from cloud records and using multiple data sources for the northeast Pacific Ocean, one of the most well-studied areas of low-level stratiform clouds in the world. The result of their analysis was a surprising degree of agreement between two multi-decade datasets that were not only independent of each other, but that employed fundamentally different measurement methods. One set consisted of collected visual observations from ships over the last 50 years, and the other was based on data collected from weather satellites.

“The agreement we found between the surface-based observations and the satellite data was almost shocking,” said Clement, a professor of meteorology and physical oceanography at the University of Miami, and winner of the American Geophysical Union’s 2007 Macelwane Award for her groundbreaking work on climate change. “These are subtle changes that take place over decades. It is extremely encouraging that a satellite passing miles above the earth would document the same thing as sailors looking up at a cloudy sky from the deck of a ship.”

What was not so encouraging, however, was the fact that most of the state-of-the-art climate models from modeling centers around the world do not reproduce this cloud behavior. Only one, the Hadley Centre model from the U.K. Met Office, was able to reproduce the observations. “We have a long way to go in getting the models right, but the Hadley Centre model results can help point us in the right direction,” said co-author Burgman, a research scientist at the University of Miami.

Together, the observations and the Hadley Centre model results provide evidence that low-level stratiform clouds, which currently shield the earth from the sun’s radiation, may dissipate in warming climates, allowing the oceans to further heat up, which would then cause more cloud dissipation.

“This is somewhat of a vicious cycle potentially exacerbating global warming,” said Clement. “But these findings provide a new way of looking at clouds changes. This can help to improve the simulation of clouds in climate models, which will lead to more accurate projections of future climate changes. “

One key finding in the study is that it is not the warming of the ocean alone that reduces cloudiness – a weakening of the trade winds also appears to play a critical role. All models predict a warming ocean, but if they don’t have the correct relationship between clouds and atmospheric circulation, they won’t produce a realistic cloud response.

“I am optimistic that there will be major progress in understanding global cloud changes during the next several years,” said Norris. “The representation of clouds in models is improving, and observational records are being reprocessed to remove spurious variability associated with satellite changes and other problems.”

Both Clement and Norris, who is a professor of atmospheric and climate science at Scripps, have received National Science Foundation Faculty Early Career Development awards for their work on climate change. The National Oceanic and Atmospheric Administration Climate Program Office also provided support for this research.

Here is the study abstract:

Feedbacks involving low-level clouds remain a primary cause of uncertainty in global climate model projections. This issue was addressed by examining changes in low-level clouds over the Northeast Pacific in observations and climate models. Decadal fluctuations were identified in multiple, independent cloud data sets, and changes in cloud cover appeared to be linked to changes in both local temperature structure and large-scale circulation. This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales. The observed relationships between cloud cover and regional meteorological conditions provide a more complete way of testing the realism of the cloud simulation in current-generation climate models. The only model that passed this test simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.

Finally, here is the accompanying Science news story by Richard Kerr:

Clouds Appear to Be Big, Bad Player in Global Warming

Climate researchers have long viewed clouds’ reaction to greenhouse warming as the key to understanding the world’s climatic fate. As rising carbon dioxide strengthens the greenhouse, will some clouds thicken and spread, shading the planet and tempering the warming? Or will they thin and shrink, letting in more sunshine to amplify the warming? The first reliable analysis of cloud behavior over past decades suggests-but falls short of proving-that clouds are strongly amplifying the warming. If that’s true, then almost all climate models have got it wrong.

The new study “confirms with observations that low clouds are critical for the climate system’s response,” says climate modeler Gerald Meehl of the National Center for Atmospheric Research in Boulder, Colorado. But “it’s really a challenge for models” to simulate that response, he adds. If real-world cloud amplification works the way the study indicates, researchers say, global warming could be even worse than the typical model predicts.

Clouds have been a climate conundrum in part because no one has been keeping an eye on them the way the weatherman has been recording temperature for more than a century. On page 460, climate researcher Amy Clement of the University of Miami in Florida and colleagues consider the two best, long-term records of cloud behavior over a rectangle of ocean that nearly spans the subtropics between Hawaii and Mexico. Other researchers had compiled one of the records from eyeball estimates of cloud cover made by mariners who passed through the region from 1952 to 2006. The other record, which runs from 1984 to 2005, came from satellite measurements, which Clement and her colleagues adjusted to account for calibration shifts from one satellite to the next.   Figure 1  Leaky clouds. Decades-long records show that when sea surface temperature (SST) warms, cloud cover-especially from low clouds (bottom)-decreases (blues, top), letting in more sunlight.

Between them, the observations recorded the two major climate shifts that roiled the North Pacific during the periods they covered. In a warming episode that started around 1976, the ship-based data showed that cloud cover-especially low-altitude cloud layers-decreased in the study area as ocean temperatures rose and atmospheric pressure fell. One interpretation, the researchers say, is that the warming ocean was transferring heat to the overlying atmosphere, thinning out the low-lying clouds to let in more sunlight that further warmed the ocean. That’s a positive or amplifying feedback. During a cooling event in the late 1990s, both data sets recorded just the opposite changes-exactly what would happen if the same amplifying process were operating in reverse. “All of the elements of a positive feedback are there,” Clement says.

Even so, positive low-cloud feedback was only a supposition until the group looked at another sort of satellite measurement of the second natural climate shift. That showed that when decreasing cloud cover let the sun leak through, the additional solar heating was large enough to account for much of the ocean warming. A positive feedback operating in the decades-long climate shifts “is real,” Clement concludes. And other studies link cloud changes in the northeastern tropical Pacific to atmospheric changes across the Pacific.

But is such a feedback actually working to amplify global warming? To get some indication, Clement and her colleagues checked the archives of a study in which the international Coupled Model Intercomparison Project compared the results of 18 global climate models run under standardized conditions. Clement and her colleagues tested whether each model was properly simulating each element of the positive cloud feedback they had found in the northeastern Pacific.

When the results were in, only two models showed low clouds producing a positive feedback as observed. One of them stood out from the pack. The HadGEM1 model from the U.K. Met Office’s Hadley Center in Exeter produced patterns of warming and circulation changes during greenhouse warming that resembled those of all 18 models averaged together-the best guide available. The group also concluded that HadGEM1’s simulation of meteorological processes in the lowermost kilometer or two of the atmosphere-where the key low-lying clouds reside-is particularly realistic.

As it happens, the HadGEM1 model is among the most sensitive of the 18 models to added greenhouse gases. When carbon dioxide is doubled, the model warms the world by 4.4°C; the median of the models for a doubling is 3.1°C. That gap raises a red flag for Clement. “We tend to focus on the middle of the range of model projections and ignore the extremes,” she says. “I think it does suggest serious consideration should be given to the upper end of the range.”

Climate researchers agree that Clement and her colleagues may be on to something. “There’s been a gradual recognition that this rather boring type of [low-level] cloud is important in the climate system,” says climate researcher David Randall of Colorado State University, Fort Collins. “They make a good case that in [decadal] variability there is a positive feedback. The leap is that the same feedback would operate in global climate change.” The study tends to support an important role for marine low clouds in amplifying global warming, he says, but it doesn’t prove it.  One clear contribution of the study, Randall says, is to point the way toward more reliable climate models. The paper “is definitely a reasonable approach to deciding which models to pay the most attention to,” he says. In its previous international assessments, Randall notes, the Intergovernmental Panel on Climate Change assumed that all models are created equal. “I think we have to get away from that.”

The time to act was yesterday.