Climate Impact of Soot Is Exaggerated, Study Says

By Arthur Neslen, The Guardian

The global warming effect of ‘black carbon,’ or soot, has been greatly exaggerated due to mistaken assumptions about the atmospheric altitude at which its particles are concentrated, according to a new study.

Emissions should be the focus of climate efforts, rather than soot, say team in a Nature paper.
Credit: Adam Cohen/flickr

Soot plumes belch from chimneys, stoves and forest fires, causing numerous health ailments and, it was thought, a contribution to climate change second only to carbon dioxide.

But when recent observations about the atmospheric height of soot particles were used, a model simulation by the Center for International Climate and Environmental Research-Oslo (Cicero), published in the journal Nature Communications, found that its warming impacts were roughly halved.

“Soot located at high altitudes has a stronger climate effect than soot closer to the ground,” said Øivind Hodnebrog, the study’s first author. “Most of today’s climate models have too much soot in the upper part of the atmosphere and as a consequence, estimates of its climate effect could be overestimated.”

Uncertainty surrounds the exact influence of black carbon on global warming, partly because of the difficulties involved in estimating how much of it is there is.

Soot particles are also enigmatic substances, with warming and cooling properties that depend on the atmospheric conditions they encounter as they drift upwards.

In the upper troposphere at tropical and middle latitudes, they have the potential to absorb and emit heat and solar radiation. But if they do not rise that far, they may stabilize lower-lying clouds that block the sunlight, so reducing temperatures.

“We found a quite strong negative [warming] effect, a cooling effect which partly counteracts the warming caused by the black carbon itself,” Hodnebrog told the Guardian. “Overall the net effect is still positive but the warming is much less.”

The Cicero team used contemporary observations taken from the Pacific and Arctic – regions relatively free of the short-term particle fluctuations caused by regional industrial emissions – which indicated that the cloud-forming effect of black carbon had been overlooked.

The global warming effect of ‘black carbon’has been greatly exaggerated due to mistaken assumptions about the atmospheric altitude at which its particles are concentrated.
Credit: UN Photo/Kibae Park/flickr

Other models had “severely overestimated” the presence of black carbon in the upper atmosphere, possibly because of an over-estimation of their lifespan, they concluded.

The Cicero researchers factored in data showing that the particles are short-lived and relatively localized phenomena, with a lifetime of just a few days before rain washes them out of the clouds they have drifted into.

The implications for efforts to combat climate change could be profound. “Soot has been highlighted as a candidate for strong emissions reductions, to achieve a rapid limitation of anthropogenic climate change,” said Bjørn H. Samset, another Cicero researcher. “Our results show that this potential is not as large as many believe.”

The new study’s findings are qualified with a caveat that they stem from a single climate model and “generaliz ation of these results should therefore be done with some caution”.

Even so, “the bottom line is that efforts to counter global warming should be focused more on CO2 than black carbon,” Hodnebrog said, “because that way you can be sure that you are reducing global warming.”

Reprinted with permission byThe Guardian.