Slivers of dust float in the upper atmosphere, scattering the sun’s rays back into space and cooling the planet in some places. In other places, the particles warm the planet.
The equivocation has meant that the particles, known as aerosols, are a significant wild card in our planet’s climate, rivaled only by clouds. So it was arguably not surprising that a study on aerosols would receive public attention.
But it was not the type of attention that the study author, Bjorn Stevens, a climatologist and director at the Max Planck Institute for Meteorology in Hamburg, Germany, was seeking. His work has been portrayed by conservative news outlets and blogs as undermining the theory of human-caused global warming. Red lights lit up. “New Climate Paper Gives Global Warming Alarmists ‘One Helluva Beating,'” Fox News declared.
In the months since the study was published, Stevens has been peppered with emails from schoolteachers and laypeople asking him, broadly speaking, whether climate change is indeed something to worry about. That brought the normally reticent scientist, who says his aim is not to convince anyone of anything, into the public sphere.
“I was touched that they’d write me and double-check that my study was being interpreted correctly,” Stevens said, speaking on a train en route to the Netherlands.
The study in question, published in Journal of Climate, is titled rather drably, “Rethinking the lower bound on aerosol radiative forcing.”
It delves into the impacts of aerosols, which are tiny pollutants of mineral dust, soot and organic matter emitted by sources such as power plants, factories and quarries. Not to be outdone, nature occasionally spews her own aerosols from volcanoes.
The particles gather at the highest reaches of the atmosphere, where the net result is that they cool the planet. In the process, they somewhat mask the warming caused by greenhouse gases. So scientists have long harbored a fear: Perhaps aerosols are cooling the planet so much that in their absence, global temperatures will rise rapidly. Such a future may play out as nations curb pollution from industries.
How big is the Earth’s umbrella?
“The fear has always been … that the warming that we do feel is the tip of the iceberg,” Stevens said.
Scientists have tried for decades to quantify the masking effect, and they have been somewhat successful. The Intergovernmental Panel on Climate Change (IPCC) says that aerosols cool the planet by between 0.1 and 1.9 watts per square meter (Wm2) (a small air conditioner cools by 60 Wm2).
Nailing down the exact number has proved difficult. Scientists just do not understand aerosols well enough — their size, shape or micro-level interactions with each other — to model them accurately using computers.
“We’ve been doing that for 20-odd years, and what you find is that you get all sorts of answers and you really don’t get anywhere,” Stevens said.
Stevens narrowed the range in his study, cutting the cooling effect to about half of IPCC’s suggestion of 1.9 Wm2.
It’s a tiny tweak with grand implications. If the maximum cooling ability of aerosols is only 1.0 Wm2, as Stevens suggests, the particles would offset only a third of warming caused by greenhouse gases. In comparison, at the IPCC’s maximum cooling value, aerosols would offset two-thirds of the warming.
The methodology Stevens used to quantify the aerosol cooling effect was so clever and provocative that a colleague termed the study an “idea paper.”
“To me, at least, those are harder to accept immediately,” Andrew Dessler, a climate scientist at Texas A&M University, said in an email.
There is no silver bullet for understanding aerosols fully since research is an accretive process. Consider clouds, for example, which are the other great unknowns in climate science. About 10 years ago, scientists did not know whether clouds were warming or cooling the planet. Now, they have more insights into their behavior.
Similarly, with many independent analyses coming out, scientists will narrow the range of aerosol’s cooling effects in the next few years, Dessler said.
You’ve got mail!
The misinterpretation of Stevens’ paper began with Nic Lewis, an independent climate scientist. In a blog post for Climate Audit, a prominent climate skeptic blog, he used Stevens’ study to suggest that as CO2 levels double in the atmosphere, global temperatures would rise by only 1.2 to 1.8 degrees Celsius. The measure is called “climate sensitivity.”
That’s less than the assumed 2 C threshold for catastrophic climatic change in parts of the world. It’s also lower than an IPCC estimate that a doubling of CO2 will raise global temperatures by 1.5 to 4.5 C.
Lewis’ blog post prompted conservative publications to crow that global warming is not a major threat. Stevens was inundated with email.
“All sorts of schoolteachers were contacting me, and they were all worried that everything they’d learned was wrong,” he said.
Soon after, he took the unusual step, for a climate scientist, of issuing a press release to correct the misconceptions. Lewis had used an extremely rudimentary, some would even say flawed, climate model to derive his estimates, Stevens said.
Narrowing climate sensitivity is challenging since the measure includes, and therefore compounds, the uncertainty inherent in aerosols, clouds and other phenomena. Different methods can give different results.
When scientists use temperature records from the 20th century to constrain sensitivity, they get low values. When they use records stretching many millenia, painstakingly assembled from trees and other proxies that contain imprints of past climates, they get values toward the higher end of the IPCC range of 1.5 to 4.5 C.
Overall, Stevens’ study skews sensitivity toward the lower end of the IPCC range, Dessler said. Other studies have found higher sensitivities, creating an urgent need to resolve these disagreements and hammer down sensitivity, he added.
Stevens said his study is something to be mulled over, but it does not call into question man-made global warming.
That’s what he said in his press release, as well.
“I continue to believe that warming of Earth’s surface temperatures from rising concentrations of greenhouse gases carries risks that society must take seriously,” he wrote, “even if we are lucky and (as my work seems to suggest) the most catastrophic warming scenarios are a bit less likely.”