Global warming: Why clouds may be less helpful than models forecast

As global surface temperatures continue to rise as a result of human industrial activity, climate models may be overestimating the ability of clouds to curb climate change, new research has found.

The study, published Friday in the journal Science, used newly available satellite technology to analyze clouds and determine the proportion of ice versus liquid water.

This ratio has important implications for climate modeling, as it impacts the amount of solar radiation being thrown back into space.

"The broader implication of this work is that for the same amount of carbon dioxide in the atmosphere, we'll see greater global warming than currently predicted," explains the study's lead researcher, Trude Storelvmo, a Yale assistant professor of geology and geophysics, in a telephone interview with The Christian Science Monitor, "so for global policy it means more fossil fuels need to stay in the ground."

Clouds sit on a spectrum, in terms of their water composition, with the coldest being full of ice and the warmest full of water; this study focused on clouds that lie somewhere between the two extremes, known as mixed-phase clouds.

The more liquid water a cloud contains, the better it is at reflecting solar radiation back into space, helping to keep the Earth cool by preventing the sun's rays from ever hitting the surface. So, as our atmospheric temperatures rise, and the ice in mixed-phase clouds melts, those clouds become more reflective, in what represents a negative feedback loop for global temperature.

In other words, the more ice the clouds have in them, the stronger the buffer they are against global warming. What this study found was that climate models have overestimated the amount of ice in our clouds, meaning there is less available to melt and offset temperature rises.

"It’s really in the last couple of years that this problem has been identified in the models," says Dr. Storelvmo, "and it's because of the new instruments available on CALIPSO."

CALIPSO is a NASA satellite, launched in 2006, that carries out climate observations. It uses lidar to fire a laser toward Earth and, based on the back-scattering of light, it can determine the proportion of liquid water and ice in the cloud cover.

While Storelvmo acknowledges that this hardware has been orbiting our planet for a decade, she explains that it takes time to see the patterns emerge and then to determine the studies to be undertaken, on top of the time and computer power it takes to carry out the research.

The actual numbers are up for debate, but there does seem to be fairly broad acceptance in the scientific community as to the fundamentals of the work’s conclusions, as Michael Mann, distinguished professor of atmospheric science at Pennsylvania State University, explains to the Monitor.

"I find the paper reasonably compelling that shortcomings in how certain key cloud processes are treated could well be leading to an overestimation of the ability of cloud feedbacks to ameliorate global warming," says Dr. Mann in an e-mail interview. "There is indeed other recent work that makes a similar case."

But he describes the study as more of a " 'proof-of-concept' than a precise estimate of the impact of the effects studied," pointing out that the authors themselves are hesitant to commit to any definite figures in relation to temperature rises.

Yet the study does give a range.

The Intergovernmental Panel on Climate Change (IPCC) in 2013 estimated climate sensitivity – the increase in global temperature as a result of a doubling in atmospheric carbon dioxide – to be within a range of 2 to 4.7 degrees Celsius.

Storelvmo and her colleagues push that upper bound to as high as 5.3 degrees Celsius, based on their new findings.

There is, however, more work to be done: Storelvmo wants to broaden the impact of the research by using it to consider a wider range of climate models, as well as digging into the reasons underlying the flaws it has exposed.

As Mann of Penn State explains, these findings present "an even greater challenge for efforts to reduce carbon emissions fast enough to avoid breaching the dangerous 2 degree C (3.5 F) warming limit," a temperature rise that has become widely accepted as the barrier the planet must not breach.