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| Global cloud patterns (NASA Goddard Space Flight Center) |
And moreover, it has shifted in such a way — by expanding subtropical dry zones, located between around 20 and 30 degrees latitude in both hemispheres, and by raising cloud tops — as to make global warming worse.
"As global warming occurs, there's the expectation that the storm track will shift closer to the pole and the dry areas of the subtropics will expand poleward," said Joel Norris, a climate scientist at the Scripps Institution of Oceanography at the University of California, San Diego, and the study's lead author. The work was conducted with scientists at Scripps, the University of California at Riverside, Lawrence Livermore National Laboratory and Colorado State University.
[Why uncertainty about climate change is definitely not out friend] The study observed this change, but a northward shifting of storm tracks was not the only effect. The tops of clouds are also now reaching higher into the atmosphere, Norris explained. "An increase of CO2 leads to cooling of the stratosphere, so it's cooling down, the troposphere underneath is warming up, and so that means, as the clouds rise up they can rise up higher than they did before," Norris adds.
That these things would happen in theory, based on our understanding of the physics of the atmosphere, has long been expected. The physical reasons for the expectation get complicated fast, involving factors like the atmospheric "Rossby radius of deformation," and how the Earth's rotation bends the path of winds — the so-called Coriolis force, Norris explains. But all of this has long been an expectation based on runs of sophisticated climate simulations that embed within their coding the fundamental equations that govern the behavior of the atmosphere.
However, the study painstakingly pieced together images from weather satellites between the years 1983 and 2009 — correcting for the numerous known quirks of these satellites that have also made their measurements of atmospheric temperatures a messy affair — to line up pre-existing theory with observations.
"We're seeing what the climate models think the pattern of cloud change would be," Norris said.
Here's a graphic the researchers provided with the study, showing the changes, and how they match theoretical expectations as encoded in climate models:
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| Locations where the majority of climate models and the majority of satellite records agree on how cloudiness changed from the 1980s to the 2000s, relative to the global mean change. (Joel Norris) |
Note that it is not like some parts of the world don't have any clouds any more. Still, the changes are significant in the context of how radiation originating from the sun enters, and ultimately departs from, the Earth's system.
Not just one but both of these changes to clouds are "positive feedbacks" to climate change — tending to make warming worse.
Moving cloud tracks toward the poles enhances warming because at higher latitudes, less solar radiation strikes the Earth — so white clouds are reflecting less of it away from the planet than they would if they were closer to the tropics and the Equator, Norris said. Meanwhile, he continued, higher cloud tops in effect thicken the total column of cloud, and that means more trapping of infrared or heat radiation that would otherwise exit to space.
"We now have a thicker blanket, which is also a warming effect," Norris said.
Fortunately, these are not new or previously unknown positive feedbacks — they are already contained within the calculations used to derive the climate's "sensitivity" to greenhouse gases and thus to project how bad warming could get. So this is more a reaffirmation of the existing theory (which was bad enough already) than a discovery of new perils.
However, there are other debates happening right now about other possible cloud changes that would tend to worsen warming beyond current expectations, if they are indeed happening. But that remains to be fully resolved by the scientific community.
It is important to note that the current study, observational in nature, detected changes in clouds, rather than firmly pinpointing their causes or documenting the consequences of these changes. Indeed, the study notes that in addition to climate warming, a "recovery" of the atmosphere from high levels of atmospheric aerosols following the enormous volcanic eruptions of El Chichón in 1982 and Mount Pinatubo in 1991 also seems to be a contributor. Those aerosols also had a cooling effect that the globe is rebounding from.
These cloud changes are, of course, hardly without consequence — the growth of so-called dry zones or drylands, as the planet warms, has been long predicted and indeed, observed by climate scientists. Places from California to Southern Africa could experience more dry conditions going forward as cloud belts shift. "The global dryland expansions will increase the population affected by water scarcity and land degradations," a recent study noted.
The new research lines up with this prior line of thought from the cloud angle. "Every observational record exhibits a decline in cloud amount or albedo [reflectivity] at mid-latitudes in both hemispheres that is nearly always statistically significant," the study notes.
Clouds are "perhaps the largest uncertainty in our understanding of climate change," the paper observes. Maybe that just became a little bit less uncertain.
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