When politicians talk of climate “uncertainty,” they're often casting doubt on things that are well understood: Warming is happening, and humans are responsible.
When scientists talk about climate uncertainty, they're usually talking about the clouds.
Clouds are tricky because they do two things at once. All that puffy whiteness blocks solar energy from reaching the ground, bouncing it back to space, which provides a net cooling effect. But clouds also act like a blanket, capping and trapping heat in the lower troposphere, which is where people who aren't on the International Space Station live. That ambiguity makes it difficult to simulate with desirable precision how much and how fast the planet is warming, leaving a big mystery floating lazily over our heads.
Some of that uncertainty was lifted Monday, and the news isn't good. A new study in Nature analyzes almost 30 years of weather observations to show that clouds and cloudiness are changing in the way scientists would expect in a warming world. Continental storm tracks—think jet stream—are shifting poleward, leaving populous subtropical latitudes uncovered. And the clouds that are forming more often aren't the low-lying, reflective ones that cool the planet—they're the huge cottony anvils that rise high in the sky, trapping more heat.
"Cloud changes most consistently predicted by global climate models are currently occurring in nature," the authors write. "As cloud tops rise, their greenhouse effect becomes stronger."
Also, as the poleward shift continues, heat from the tropics expands further away from the equator, an unsettling development for already arid cities in places like the U.S. southwest. Less light is reflected away from the hot middle of the Earth, and increased cloud cover toward the poles creates a thickening blanket that warms the world.
A couple of things make clouds such a conundrum to climate scientists. They form through processes too small to observe, and their layering blocks satellite views of lower clouds and ground views of higher clouds. Satellite coverage goes back only to the late 1970s, with technologies designed for weather monitoring. The new study corrects the weather record to make sense for climate analysis.
“We don’t have a climate monitoring system,” said Joel Norris, a climatologist at Scripps Institution of Oceanography and the lead author of the new paper. “It’s hard to sustain a long-term climate monitoring program.” Political priorities change, budgets drop, technology evolves, and climate scientists are left having to extract consistent, long-term data from measurements taken without the long term in mind.
The corrected data should help improve their confidence in cloud studies, which currently isn’t very high. The 2013 science overview put out by the authoritative Intergovernmental Panel on Climate Change concluded that although independent sets of cloud observations are similar, “substantial ambiguity and therefore low confidence [remain] in the observations of global-scale cloud variability and trends.”
“This is an exciting and comprehensive study,” said Kate Marvel, a scientist at NASA’s Goddard Institute for Space Studies. “But other studies have found that these things are happening, and this work provides more evidence of these effects.”
Next up for the study's authors is to untangle further how natural events, such as volcanoes, and greenhouse-led warming contribute to the cloud changes. Ultimately, greater understanding of how clouds behave will help strengthen projections of the rate and trajectory of global warming.