by But in recent years, scientists have begun to define exactly how clouds will change shape and position in the rapidly warming world. The result is good news for science, but not good news for humanity.
“We have found evidence of the amplifying impact of clouds on global warming,” said Paulo Ceppi, a climate scientist at Imperial College London.
Scientists have long known that clouds have two primary influences on global climate. First, clouds are reflective: Their white surfaces reflect the sun’s rays away from the Earth, creating a cooling effect. (If the planet were suddenly devoid of these fluffy sunshades, the planet would be about five times hotter than even the most disastrous global warming projections.) But clouds also create a warming effect: Certain types of clouds insulate Earth’s radiation, keeping warm just like the carbon dioxide released by burning fossil fuels.
Which effect is stronger depends on the type of cloud. Cirrus clouds — tall, thin clouds visible in the distant atmosphere on relatively clear days — absorb and trap more radiation, warming the Earth. Stratus or stratocumulus clouds — plump, fluffy clouds that often hover over the ocean on overcast days — reflect more sunlight, cooling the Earth.
How exactly these two factors will balance out as the world warms has been uncertain. This is mainly because, while clouds may seem gigantic — when you fly through them in an airplane or view them from the ground — they form at microscopic levels, when water vapor condenses around a dust particle or droplet. As a result, they are essentially impossible to model in large standard climate models. (Clouds form at the micrometer level, while the models used by most climate scientists separate the world into blocks of hundreds of kilometers wide.)
“We have a really hard time simulating with any fidelity how clouds actually behave in the real world,” said Timothy Myers, a postdoctoral researcher at the Lawrence Livermore National Laboratory in California.
But in recent years, scientists have gained greater clarity about what will happen — and what is already happening — to clouds as the planet warms.
First, the tall, thin cirrus clouds that trap Earth’s radiation are expected to move upward in the atmosphere, towards lower temperature zones. Thanks to a complicated relationship between clouds and Earth’s radiation, that will increase the amount of radiation cirrus clouds trap in the atmosphere. “As they get higher, their greenhouse effect, or warming effect, on Earth tends to increase,” Myers said.
This result has been known for about a decade and indicates that clouds are likely to do so amplify global warming. But just in recent years, researchers have also found that the number of low-level layers or stratocumulus clouds is expected to decrease as the planet continues to warm. A study, in the journal Nature Climate Change, used satellite observations to find out how cloud formation is affected by ocean temperatures, wind speeds, humidity and other factors, and then analyzed how these factors will change with world warming.
“We concluded that as the ocean warms, low-level clouds over the oceans tend to dissipate,” said Myers, one of the study’s authors. This means there are fewer clouds that reflect sunlight and cool the earth, and the change in low-level clouds will also amplify global warming.
Another paper, in the Proceedings of the National Academy of Sciences, found a similar result, also using observational techniques. Research based on high-resolution models, which are better able to model cloud formation than large-scale, general climate models, also concluded that clouds are likely to amplify global warming.
Researchers have also begun to understand how clouds will be affected by some changes besides warming, such as the reduction of man-made aerosols in the atmosphere. Clouds form around particles floating in the atmosphere, such as aerosols; it is possible, therefore, that low-level clouds would have decreased even more if not for human-induced air pollution. According to another study published last month in the Proceedings of the National Academy of Sciences, sulfate aerosols have stimulated cloud formation, thus masking some of the global warming that has already occurred. “There is a possibility that as we clean up air pollution, we unmask global warming,” said Casey Wall, a postdoctoral researcher at the University of Oslo.
Together, these new findings have helped scientists focus on how much the planet would warm if carbon dioxide emissions into the atmosphere were to double from pre-industrial times. (Before the Industrial Revolution, the concentration of CO2 was about 280 parts per million, or ppm; now it’s as high as 412 ppm, and still rising.) Scientists once estimated that if CO2 reached 560 ppm, the temperature it would rise between 1.5 and 4 degrees Celsius: a range from a “still very livable planet” to “almost apocalyptic levels of warming”.
Much of this uncertainty has come from the cloud issue. Because clouds are already so influential on Earth’s climate, even small changes in clouds as the world warms can have large effects on future temperature change.
New cloud research indicates that lower estimates for warming are highly unlikely. Instead, recent papers estimate that CO2 levels of 560ppm would likely translate into at least 3 to 3.5 degrees of warming.
This does not mean that the world will definitely reach 3 degrees of warming: if countries continue to switch to clean energy, CO2 in the atmosphere could stabilize at a level significantly below 560ppm. But it does mean that the most optimistic estimates of how warming will play out have been taken off the table.
