Cloudy Forecast: Growing Uncertainty Over Cloud Behaviour
The biggest source of climate uncertainty is white and fluffy. In recent years, the uncertainty has actually grown.
UP IN THE AIR Clouds can both warm and cool the planet. How clouds will respond to global warming is the largest source of uncertainty in climate change predictions.
From space, clouds appear to perform an intricate and never-ending ballet. Thin streaks dance at the poles, vast storms plow across the jet streams, spinning cyclones get tossed up in the tropics and deep convecting monsters churn near the equator. Clouds whip and curl and billow, materializing seemingly out of nowhere and dissipating just as mysteriously.
The mystery deepens when scientists try to understand how clouds influence climate. Clouds lead a sort of double life, both trapping and deflecting planet-warming energy. Their molecules, like all water in the atmosphere, contribute to the greenhouse effect by lapping up infrared radiation emitted by Earth and redirecting some of that energy back toward the planet’s surface. But clouds’ white tops also reflect, collectively, almost a quarter of the solar radiation that reaches them, in effect shading the planet.
All told, clouds cool through reflection far more than they warm through the greenhouse effect. Without them, Earth’s surface would be, on average, about 5 degrees Celsius warmer. “Clouds are really at the heart of the climate system,” says Sandrine Bony, a climate scientist at the Université Pierre et Marie Curie in Paris.
That clouds both warm and cool is established. But how the global balance between those two effects will shift as the climate heats up is not. Even seemingly minor shifts in clouds’ behavior could substantially dampen or accelerate global warming.
Early predictions suggested that clouds might work to counteract rising temperatures: As oceans absorb more heat, they add more water vapor to the air. This, the thinking went, would create more sunlight-reflecting clouds, which would help cool the planet. In climate speak, this is known as a negative feedback. Research over the last two decades suggests, however, that the cloud feedback is more complicated and likely to result not in cooling but in added warming.
But no one knows how much additional warming, if any, to expect. The United Nations Intergovernmental Panel on Climate Change, which represents the collective knowledge of the world’s climate scientists, considers cloud feedbacks the top source of uncertainty in climate change prediction. This uncertainty is reflected in the reports that the panel releases every five to seven years. In its 2007 report, the panel estimated that if the concentration of carbon dioxide in the atmosphere were to double from its preindustrial level — a likely outcome by the end of this century — global average temperature would rise between 2 and 4.5 degrees Celsius. The panel’s latest report, officially published January 30, estimates a temperature rise of 1.5 to 4.5 degrees with carbon dioxide doubling. In other words, seven years later, the uncertainty has actually grown.
Researchers nevertheless insist that they understand clouds much better than they did in 2007. “We’ve moved from the unknown unknowns to the known unknowns,” says Leo Donner, a scientist who develops climate simulations at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. “I would argue that in fact there has been very significant progress, though [it’s] correct that the bottom line is still not changed.”
Scientists like Donner are increasingly convinced that cloud feedbacks are not going to diminish greenhouse gas warming. But to really put this issue to bed, they say they need more sophisticated cloud observations extending over decades. Whether researchers will get this crucial data record is far from certain.
Getting clouds’ numbers
Until recently, researchers had no way to monitor clouds on a global scale. Ground-based observatories could see only the bottoms of clouds. And while scientists could send balloons and airplanes through individual clouds to gather more complete profiles, these methods provided only local snapshots. Things improved in 1999 when NASA launched the first of its two MODIS instruments, which circle the planet and look down on cloud tops. But these data, too, are limited: Clouds and sea ice are nearly indistinguishable from above, and a several-kilometer-thick cloud can hide large variations in its interior.
In 2006, NASA launched the satellites CloudSat and CALIPSO. These sister orbiters fly in close formation and send out beams — radar for CloudSat and lidar (the laser version of radar) for CALIPSO — that penetrate deep into clouds and bounce off water droplets and airborne particles called aerosols before returning to the satellites. Zipping around the planet roughly every hour and a half, the satellites send down a continuous trove of data that scientists can access almost in real time.
illustration of NASA’s CloudSat: Data collected by a new generation of cloud-gazing satellites such as NASA’s CloudSat (shown in an artist’s conception) are helping researchers pin down the basic properties of clouds and fine-tune climate simulations. JPL/NASA
CloudSat and CALIPSO are “a revolution in observing technology,” says Ulrike Lohmann, a cloud physicist at the Swiss Federal Institute of Technology in Zurich. Previously, scientists didn’t always know even basic things such as clouds’ altitudes, volume and how often they produce rain. Researchers have been surprised to learn how much of the water in clouds is frozen, says Lohmann. “The amount of ice in the atmosphere seems to exceed the amount of liquid almost everywhere.”