How climate change is altering Earth’s cloud cover

Nicknamed the wild cards of climate change, clouds have evaded most scientists’ attempts to nail down their long-term patterns. But one team just found some answers.

An innovative study published Monday in Nature pieces together vaporous cloud data, providing the first credible demonstration that the cloud changes scientists expected from climate change are really happening and could speed up global warming.

An analysis of satellite data between 1983 and 2009 reveals that cloud tops are reaching higher into the atmosphere and that cloudy storm tracks are shifting toward Earth’s poles, confirming the predictions of climate change models. Both these trends suggest that clouds will be exacerbating – rather than mitigating – climate change.

“For the last 3 ICC [Intergovernmental Panel on Climate Change] reports, when it comes to the cloud section there’s always very low confidence and high uncertainty. So now I think we’ve moved the needle on that,” Joel Norris, lead author of the study and a climate professor at the University of California San Diego's Scripps Institution of Oceanography, tells The Christian Science Monitor in a phone interview Monday.

Clouds have remained a dominant source of uncertainty in our understanding of changes in the climate system for decades because satellites are designed to predict short-term weather trends, and previous studies had failed to successfully account for inconsistencies in data collection because of factors such as satellite degradation, shifts in their orbit, and replacement with more sensitive equipment.

Dr. Norris collaborated with scientists from UC Riverside, Lawrence Livermore National Laboratory, and Colorado State University to correct for incongruities between satellite data and to confirm that two of the main cloud change patterns predicted by virtually all climate change models are indeed present.

“If the highest clouds get higher [as has been observed], that’s effectively increasing the thickness of this cloud blanket so it’s going to reduce how much thermal radiation gets out to space,” Norris tells the Monitor. And small changes could have a big impact.

“The reason the earth is warming up due to greenhouse gases is just a 0.2 percent difference between what comes in and what goes out in terms of solar radiation coming in and thermal radiation going out,” Norris adds.

Why has a warmer world allowed clouds to reach higher into the atmosphere, thickening the cloud blanket and further encouraging warming? Clouds occur in the layer of the atmosphere called the troposphere, Norris explains, beneath the relatively warmer stratosphere, which acts as cap on clouds that lack the buoyancy to rise up into warmer air. An increase in greenhouse gases heats the troposphere, making it warmer below, and cools the stratosphere, making it cooler above. Since the capping layer is no longer as effective as it used to be, clouds can rise higher.

The second confirmed pattern was the shift of cloudy storm tracks poleward, causing a decline in cloud coverage in middle latitudes. This expansion of the tropics beyond its current 20 to 30 degrees latitude has also been predicted by earlier studies.

When the 27 years of satellite data was compared with thousands of models, those that predicted cloud patterns based on natural variability alone fell flat, but global climate model simulations that account for changes in greenhouse gases and aerosols were a compelling match.

Global warming since the 1980s has two causes that the study wasn’t able to distinguish between: an increase in greenhouse gases and reactive warming following cooling brought on by two volcanic eruptions at the beginning of the studied time frame.

“Recovery from the volcanic activity – that’s also global warming, just from a different source. We don’t have enough information now to know how much comes from one and how much from the other,” Norris says, adding, “We think in the long run the greenhouse gases is going to win out in the end, even if a big eruption does occur.”

Leo Donner, an expert in atmospheric sciences at the National Oceanic and Atmospheric Administration, who was not involved in the study, says the research provides an important confirmation that basic cloud processes are operating as scientists believed they would.

“It gives us some confidence, the fact that these basic processes are lining up as expected, that we can build on those in the future and attempt to get a better quantitative understanding of how the clouds amplify greenhouse gas warming,” Dr. Donner tells the Monitor in a phone interview Monday.

“The big question is not so much are the basic changes that are being confirmed by the study occurring, but how large? What is their magnitude?” Donner asks.

That's exactly the question Norris hopes to explore next. He wants to quantify the magnitude of cloud changes and sort out the importance of greenhouse gases versus volcanic activity in driving them. By providing support for areas where scientists have reached broad consensus, this study provides a basis for exploring some of the areas where climate change models disagree, such as whether low-level subtropical clouds will increase or decrease with global warming.