Global climate change Research
Research into the cooling impact of aerosols sends climate contrarians into a tailspinBy Gayathri Vaidyanathan and ClimateWire | April 23, 2015
The particles, known as aerosols, are a significant wild card in our planet's climate, rivaled only by clouds.
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.