​A cooling blanket of smoke

Smoke from large forest fires is puzzling to climate researchers. Do the soot particles cool or heat the earth? Remote sensing expert Dr. Martin de Graaf believes he is about to find out.

Enormous quantities of smoke are released into the atmosphere during the dry monsoon season from June to November as a result of the many forest and savanna fires in Africa. De Graaf determined the amount of solar radiation that is absorbed by this smoke. Together with colleagues from the Royal Netherlands Meteorological Institute (KNMI) he used a novel detection technique with a satellite instrument aboard ESA’s Envisat satellite.

The findings, which were published in November last year in Geophysical Research Letters, are an important step forward in climate research. Radiation processes between clouds and smoke can now be more effectively incorporated into climate models.

“Smoke scatters much more sunlight than it absorbs,” said De Graaf, who works at TU Delft’s Geosciences and Remote Sensing Group (Faculty of Civil Engineering and Geosciences) and at KNMI. “Therefore, smoke but also other aerosols like smog, are believed to cool the climate on average, by reducing the amount of sunlight that can be absorbed by the surface below the aerosol layer.”

Yet there is another mechanism that is still poorly understood that could cause soot to cool the planet. By heating up the air. Does this sound peculiar? Well, that might be part of the reason why climate models until now hardly took this effect into account.

Normally smoke scatters much more sunlight than it absorbs, but things change dramatically above the southeastern Atlantic Ocean where there is a permanent layer of clouds which reflect sunlight very efficiently. When smoke floats over these clouds, a substantial amount of light is absorbed by the smoke particles which is otherwise reflected back into space. This absorbed solar radiation heats the air. By doing so, it also functions like a blanket preventing water vapor from rising higher up into the atmosphere. Thicker layers of clouds mean that less sunlight reaches the ocean which, in turn, warms less. 

So by heating the air, the smoke particles might actually cool the planet even more then already thought. The implications for climate research are important. “Fires around the world can have various effects on climate,” said De Graaf, who in 2005 mapped the smoke from Indonesian fires. Indonesia is also afflicted every year by huge fires. Rainforest is burned to make way for palm oil plantations. During the notorious 2013 Southeast Asian haze, caused by large-scale burning in many parts of Sumatra and Borneo, people needed facemasks all the way up to Singapore.

De Graaf hasn’t proved yet that this extra cooling effect of smoke really occurs. “The balance between net warming or cooling is determined by both the way clouds are formed and how, where and when smoke occurs, which can be very different around the globe. In 2016 I hope I will be able to calculate the net effect of smoke over the southeast Atlantic Ocean. I will then be doing soot particle counts with an airplane above the clouds off the coast of Namibia. Those data combined with satellite data should provide us with the answer. If we can understand the processes over the southeast Atlantic, we may be able to improve climate models and find the net global effect. It will change the way we think about aerosols.”

Martin de Graaf et. al., Aerosol direct radiative effect of smoke over clouds over the southeast Atlantic Ocean from 2006 to 2009, Geophysical Research Letters 41, November 2014