On any given day, clouds spread across about two-thirds of the globe. They control global surface temperature more than any other single influencing factor, including greenhouse gases. And even though they are one of the most critical parts of the global climate system, clouds are still the greatest source of uncertainty when it comes to projecting how much Earth will heat up in the future.
But that's starting to change as scientists peer deep inside clouds to study changes at the microscopic level and use new satellites that capture the big picture. Depending how high and thick they are, and how much water and ice they contain, they either cool or heat the planet by reflecting incoming solar radiation back to space or trapping it at the surface, like a blanket.
For now, the overall effect of clouds is to cool the planet. But two new studies published recently in the journal Nature Geoscience suggest that clouds are likely to change in ways that will intensify global warming.
One of the research projects, led by scientists at the Swiss Federal Institute of Technology in Zürich, found that, when tiny soot particles combine with ozone or sulfuric acid, the result can be changes in clouds—both low-lying and high altitude—that cause more heating. The second study, by Norwegian climate scientists, suggests that changes to clouds, particularly over the Southern Ocean, could end up causing the planet to warm by more than the 9 degrees Fahrenheit expected if atmospheric carbon dioxide doubles from the pre-industrial level.
A Cloud/Climate Feedback Loop
Scientists say it's important to understand that the climate and clouds affect each other in an amplifying loop. Global warming changes how and where clouds form and persist, and how much ice or water is in them. And as those properties change, the shifts will, in turn, affect the climate. The two new studies suggest that in the long-term, this loop will probably end up intensifying the global warming effect of greenhouse gases.
Clouds have such a big effect on the climate system that, if their extent or reflective properties were to change by 20 percent, it would have more of an impact than all the greenhouse gases released by human activity, said Andrew Gettelman, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado, who was not involved in the two new studies but does related research.
If the overall cooling effect of clouds declined by just 10 percent on a global scale, it would warm the planet as much as a doubling of carbon dioxide in the atmosphere. And if all clouds suddenly disappeared, it would add as much heat as if a 25-watt light bulb were switched on above each square meter of the planet's surface. That's why it's urgent to understand how clouds will change as the climate keeps warming, Gettelman said.
"If you warm the planet, you change the environment the clouds live in. Some of that is broadly known and easy to estimate," he said. "The problem is trying to describe cloud physics going on at the micron scale. That's the diameter of human hair." Those microphysical processes are critically important to understanding how clouds will change regionally and globally, but it's been hard to extrapolate the details and detect large-scale trends.
"We only have reliable, global satellite observations of clouds for perhaps 20 years or so," Gettelman said. "There are variations in cloud cover due to El Niño and other factors. But it's hard to pick out a long term trend from the variability." Research like the study on soot particles helps show what's going inside the clouds, and new satellite instruments that measure clouds globally help scale up that information to show the climate impacts, he added.
A few studies have evaluated data from the new satellites, and the results are not good news for the climate. The findings suggest that reflective, cooling cloud cover in the tropics decreased by 2 to 6 percent since 1983, mainly over the oceans, said ETH Zürich cloud researcher Ulrike Lohmann, who led the new study on how soot particles interact with clouds. That's significant for the climate, she said, because those regions receive intense sunlight all year round, so losing reflective clouds will accelerate ocean warming.
Because clouds play such a critical role regulating global surface temperature, she said, while it's important to understand how they are changing, the remaining questions don't justify delaying climate action.
"The easy answer is, we know enough about climate change to realize that it's important to cut greenhouse gas emissions to reach the targets of the Paris climate agreement," Lohmann said. Part of the agreement articulates the need to remove carbon dioxide from the atmosphere before warming reaches 2 degrees Celsius—3.6 degrees Fahrenheit—a threshold for catastrophic climate change.
"The sooner we approach 2 degrees Celsius, the sooner we need to take CO2 out. Any uncertainty in climate projections, or new information about how clouds affect warming, shifts the horizon for that," she said. "Because clouds affect the radiation budget so much, they are a big control. If you change the reflection by 10 percent, it has an effect that is a similar order of magnitude as doubling of CO2."
A Bad Combo for the Climate: Soot and Carbon Dioxide
Basic principles of chemistry and physics can show how that reflectivity can be changed by natural processes or from external influences like soot.
Lohman said, "We want to understand what's driving cloud formation. For the longest time, we didn't think soot particles affect clouds." But when her team mimicked cloud formation in a lab, they were able to see how soot particles changed when combined with ozone or sulfuric acid (another polluting byproduct of fossil fuel burning), and how that affected cloud reflectivity.
Once they quantified those changes, they plugged the numbers into a climate model and found that, when atmospheric CO2 doubled, the chemically altered soot particles caused a reduction of low-level clouds that reflect sunlight, and enhanced the thickness of high-altitude clouds that trap heat, leading to more warming.
Changing clouds also have implications for plant life, because photosynthesis operates differently in direct sunlight under clear skies from the way it does in scattered sunlight in cloudy conditions. And changes to clouds often also imply changes to precipitation.
And, Lohmann added, the findings of the study are also important for nascent geoengineering plans that call for spreading tiny sulfur-based particles widely in the atmosphere to cool the climate by artificially creating the reflective properties of clouds. Whether such plans could work, or whether there is a risk of unintended consequences, depends in part on how those particles change once they are in the atmosphere.
I would imagine that, 20 years from now, geoengineering might be the most important part of this," she said.
Clouds That Can No Longer Cool
The other recent cloud study, led by University of Oslo scientists, showed that global warming will eventually cut the volume of ice particles in massive cloud banks over the Southern Ocean, around Antarctica.
"The ice in those clouds makes them brighter and more reflective, but if we allow enough warming, there won't be any more ice," said University of Oslo climate researcher Trude Storelvmo, co-author of the study, published Oct. 26 in the journal Nature Geoscience.
The findings suggest that, if the average global temperature increases by 5 to 7 degrees Fahrenheit, the clouds over the Southern Ocean would lose their cooling effect and further raise the global temperature by an additional 2 to 3 degrees.
Figuring out exactly how sensitive the climate is to rising greenhouse gas concentrations depends in large part on knowing what happens to clouds, and the computer models have to use correct information to end up with accurate projections.
"We have to start at the right point to be able to simulate how it will change over time, and we haven't had good measurements on this particular cloud property," Storelvmo said. "But now we have these wonderful new measurements that have really changed things," she continued, describing the lidar and radar readings that can show the ratio of water to ice inside the clouds much more accurately than just 10 years ago.
"In this model, we saw that, when we had warming of 3 to 4 degrees Celsius (5.4 to 7.2 degrees Fahrenheit), we had no more ice," she said. At that point, the clouds lose their cooling effect.
Storelvmo added, "We can't be sure that's when it's going to happen. We don't want to be overconfident of how much warming it takes, but we wanted to bring home the point that we are in a state now where we have this cooling effect is helping us. At a certain point we lose that."