Clouds over Estrange, Sweden in January, 2011.
Credit: Peggy Achtert, distributed by the EGU under a Creative Commons License
This summer, a widely derided study claiming to overturn the scientific consensus on clouds and climate change kicked off a mini-whirlwind in the climate science community. This wasn't because the findings were revolutionary, but rather because of the public ruckus that arose around the study's publication. By the time the dust settled weeks later, the editor of the journal that carried the original study resigned, saying the paper should not have been published.
The paper, which had been published in the journal Remote Sensing, suggested clouds, rather than carbon dioxide, are causing global warming.
Call it the Cloud Wars. In the largely political debate over global warming, the role of clouds in the climate system is a perennial topic of argument. Basic research — such as a recent early investigation of the effect of cosmic rays on cloud formation — gets taken out of context, used to support arguments far beyond its scope. Climate blogs blow up with angry back-and-forth banter. As soon as it simmers down, another controversial paper restarts the cycle yet again.
Even for scientists more interested in data modeling than environmental policy, clouds can be a source of aggravation. They come and go without leaving much of a trace, meaning there's no long-term record of their existence like the record that Antarctic ice cores provide for carbon dioxide. Depending on where clouds are, how tall they get, and even what they look like on a microscopic level, these fluffy billows of mist can either trap heat or bounce it back to space. And no one knows exactly how clouds will respond as global temperatures creep upward. [Read: Top 10 Surprising Results of Global Warming]
Nonetheless, researchers say, scientists are getting better at understanding how clouds play into the climate system. No matter clouds' role, researchers say, they aren't likely to save the Earth from the warming effect of greenhouse gases.
"There's no evidence that clouds provide anything other than a neutral or positive feedback," said Brian Soden, a professor of meteorology and physical oceanography at the University of Miami. Positive feedback means that as the atmosphere heats up, clouds behave in ways that trap more heat, exacerbating the warming.
Clouds are the reason why climate scientists can't say for certain how much temperatures would go up for a given amount of carbon dioxide. If carbon dioxide were to double, the Intergovernmental Panel on Climate Change (IPCC) estimates that global temperatures would go up by between 3 to 7 degrees Fahrenheit (2 and 4.5 degrees Celsius). At the low end of that estimate are models that find little to no positive feedback from clouds. At the high end are models that suggest cloud changes in a warming world will lead to even more warming.
Scientists can't yet narrow the range of warming any finer than the IPCC estimates, largely because clouds refuse to behave in an easy-to-understand manner. Water vapor alone is simple: Warmer air can hold more moisture, and humid air traps more heat, so in a warming world, water vapor is only going to make things worse. But clouds can go either way. Their whiteness reflects the sun's energy back into space, causing a cooling effect. At the same time, clouds serve as a "blanket" that holds heat in (which is why cloudy nights tend to be warmer than clear ones).
"So you have these two big offsetting terms," said Andrew Dessler, a climate scientist at Texas A&M University. "You have to really know those terms very accurately to know the net effect of clouds."
Complicating matters, different clouds have different warming and cooling effects. Researchers now understand that clouds higher in the atmosphere tend to trap extra heat, leading to more warming. Lower clouds, however, remain a bit of a mystery. That's especially true for the very low clouds that hug the coastlines in places like California and the southern coasts of Africa, said Stephen Klein, an atmospheric scientist at Lawrence Livermore National Laboratory in California. [Images: Photos Reveal Amazing Clouds]
"These clouds in particular have different responses in different models," Klein told LiveScience.
Researchers are tackling the low-cloud problem in a couple of ways, Klein said. First, they're looking to satellite data on cloud activity to try to observe what clouds have been doing over the 30 to 40 years that data are available.
Second, researchers are zooming in on their models to get at the nitty-gritty of low clouds. Right now, global climate models capture Earth that at a resolution equivalent to a blurry photograph. Now, scientists have ramped up that resolution by about 1,000 times for small areas.
"They do a very good job of simulating these clouds that are very difficult for global models," Klein said. "So you can try to use them as a benchmark to assess how well the climate model is doing."
A combination of observation and modeling has turned up other important progress on the cloud question. In a warming world, climate scientists have found, clouds have a tendency to shift toward the poles, leaving the sunny midlatitudes relatively clear. That's not a good thing, Dessler told LiveScience.
"You take a cloud and you move it to a higher latitude where there's less sun, so it reflects less light to space, so it cools less," he said.
And one other piece of bad news: Warming clouds tend to shift higher in the atmosphere, where they trap more heat while reflecting no more sun than they would have lower down. That creates another positive feedback in the warming cycle.
While there is still a lot of work needed to pin down the interaction of clouds and climate, climate experts say the uncertainty shouldn't be misconstrued as evidence that manmade climate change isn't occurring. The scientific debate is over the amount of change, not the fact that it's happening.
Outside the scientific arena, however, uncertainty over clouds is often presented in a very different light. In August, researchers at the CERN particle physics laboratory in Switzerland released a study in which they simulated the formation of aerosols, the fine particles around which clouds coalesce. The study found that stimulating an artificial atmosphere with a particle beam boosts the formation of aerosols. That leaves open the possibility that cosmic rays, the space particles that rain down on Earth's atmosphere, could influence cloud formation, and as a result, climate.
The lead researcher on the study, CERN physicist Jasper Kirkby, told LiveScience at the time that the results shouldn't be overinterpreted: "It's part of the jigsaw puzzle" of climate change, Kirkby said, adding that the finding "in no way disproves the other pieces."
Not all aerosols form clouds, Kirkby said, so it remains to be seen whether the very small aerosols he and his team created in the lab would grow large enough to seed clouds in the atmosphere. Simulations also need to encompass the lower atmosphere layers, he said, where cloud formation is most common.
All this caution was lost, however, on the skeptical blog "Watts Up With That?" which ran with the headline "CERN Experiment Confirms Cosmic Rays Influence Climate Change." (Later, an update revised that headline to "CERN Experiment Confirms Cosmic Rays Influence Cloud Seeds.")
Clashing cloud studies
Likewise, the cloud paper that spurred one journal editor to resign received major media coverage after a Forbes op-ed written by a fellow from the libertarian Heartland Institute ran under the headline, "New NASA Data Blows Gaping Hole in Global Warming Alarmism."
The research argued that rather than acting as a feedback in the climate system, clouds might actually cause changes in climate on their own (clouds would change via "chaos" in the atmosphere, study researcher Roy Spencer of the University of Alabama, Huntsville, told LiveScience at the time). Spencer also told LiveScience that claims that his work disproves man-caused climate change were overblown, though he is skeptical that humans cause climate change.
Other researchers, however, have criticized not just the media coverage but also Spencer's work as flawed. Climate scientists pointed to the fact that the study was based on a model that didn't include El Niño cycles or other relevant ocean cycles. The study also failed to address previous research that had debunked similar claims; it was the failure to catch this fact during peer review that led the editor of the journal Remote Sensing to resign, saying he took responsibility for publishing a paper that "should not have been published."
Within days of the editor's resignation, Dessler published a study refuting Spencer's claims in the journal Geophysical Research Letters.
"I said, 'Let's quantitatively measure how much energy the clouds are trapping and how much energy it takes to change the climate, and see if the clouds are trapping enough energy to change the climate,'" Dessler said. "The answer is, they're not."
The changes Spencer saw in his model are explained by El Niño/La Niña cycles, Dessler said, not caused by clouds.
According to Dessler, the brouhaha over the paper illustrates the problems with translating scientific certainties and uncertainties into a polarized political environment.
"Every month, dozens, if not hundreds, of papers are published that are in agreement with the mainstream theory of climate science," he said. "But every year, one or two skeptical papers get published, and these are then trumpeted by sympathetic media outlets as if they'd discovered the wheel. It therefore appears to the general public that there's a debate."