Black carbon in the atmosphere infographic

Black carbon is the second most important anthropogenic contribution to climate warming

Black carbon in the atmosphere absorbs and scatters sunlight.  Absorption caused by black carbon heats the atmosphere.   As an aerosol black carbon scatters sunlight which reduces the light that reaches the Earth’s surface.  The major sources of black carbon emissions are from burning fuel especially on- and off-road diesel engines and solid fuels for heating and cooking, and open burning of savanna and forest.  In addition when black carbon is deposited on snow and ice it decreases the reflectivity of these surfaces.  This causes more solar radiation to be absorbed which warms snow and ice packs.  Black carbon also interacts with clouds in complex, little understood ways.  There is large uncertainty in estimates of black carbon emissions and their impact on the climate, but it is very highly probable that its net effect is to warm the climate.  Its contribution to warming the climate is estimated to lie between carbon dioxide and methane making it the second most important anthropogenic source of global warming.

Introduction

Black carbon is an aerosol that is formed during combustion of carbon-based fuels.  It is black which means that it strongly absorbs visible light from the sun.  It does not stay in the atmosphere very long – on average only about a week – because it is removed by rain and and through other processes.  But as an aerosol it can be carried by winds to remote regions including polar regions where it can be deposited on snow and ice packs.

The largest global sources of black carbon are diesel engines, solid fuels including wood and coal burned for cooking and heating, and open burning of forests and savannas.  The difficulty in quantifying emissions from such diverse sources creates large uncertainty in estimating total global black carbon emissions and in assessing its impact on the climate.

Estimating black carbon emissions

Attempting to estimate black carbon emissions by accounting for the many sources of black carbon cannot do better than suggesting that that probably between 2 and 29 megatonnes of black carbon enters the atmosphere per year.  The largest contribution is believed to be from energy-related combustion including diesel engines and solid fuels for heating and cooking with the remainder from open burning of forests, grasslands, and agricultural residues.  Remote sensing observations suggest that anthropogenic sources are responsible for 14 megatonnes of black carbon entering the atmosphere per year.

Impact of black carbon on the climate

Black carbon in the atmosphere absorbs and scatters sunlight.  Absorption heats the atmosphere where black carbon is present.   As an aerosol it also reduces the sunlight that reaches the Earth’s surface.  The largest contribution to direct heating of the atmosphere are emissions from fuel sources, followed by solid fuel for heating and cooking and open burning. The direct absorption of sunlight by black carbon deposited on snow and ice produces warming which affects snow and ice packs themselves, leading to additional climate changes and ultimately to earlier onset of melt and less highly reflective snow and ice.   Processes involving the interaction of black carbon with clouds are little understood and contribute significant uncertainty to the estimate of the total contribution of black carbon to atmospheric warming.

Effect of black carbon on climate Contribution to warming (watts per square meter) Uncertainty (watts per square meter)
Direct +0.71 (+0.09 to +1.26)
Cloud semi-direct and indirect +0.23 (-0.47 to +1.0)
Deposited on snow and sea-ice +0.13 (+0.04 to 0.33)

Total contribution to climate warming

According to the IPCC it is very highly probable that the net effect of black carbon on the climate is warming.  The best estimate of the warming caused by black carbon through all mechanism is +1.1 watts per square meter with very large uncertainty bounds – it could be as low as  +0.17 and as high as +2.1 watts per square meter.  For comparison, in 2005 warming caused by emissions of the two most significant long-lived greenhouse gases, carbon dioxide (CO2) and methane (CH4),  were +1.56 and +0.86 watts be square meter,  respectively.  This suggests that black carbon may be second only to carbon dioxide as an anthropogenic source of climate warming,

Sources

Bounding the role of black carbon in the climate system: A scientific assessment, Bond, T. C., et al. (2013), J. Geophys. Res. Atmos., 118, 5380–5552, doi:10.1002/jgrd.50171.