The Arctic is warming about twice as fast as the rest of the world, mainly due to the global increase in the carbon dioxide concentration in the atmosphere. But carbon is responsible for the accelerated warming not only in the gaseous compound CO2, but also as so-called black carbon. Scientists have now been able to show in a new study that the influence of “black carbon” on climate change in the Arctic has been greatly underestimated until now. The study was published in the journal Atmospheric Chemistry and Physics.
Particulate black carbon is formed during the incomplete combustion of fossil fuels, biofuels and biomass and is a component of aerosols. On the one hand, black carbon contributes to atmospheric warming because the particles absorb solar radiation. On the other hand, it is deposited on the earth’s surface and thus also on ice and snow, reducing their reflectivity and causing them to melt more quickly. Since there are hardly any sources of black carbon in the Arctic, it is assumed that most of it comes from regions outside the Arctic.
So far, estimates of the relative contribution of the various sources to Arctic black carbon have been subject to large uncertainties. Therefore, as part of the Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project (PAMARCMiP) led by the Alfred Wegener Institute (AWI) in Germany, the international research team with scientists from Japan, Germany, and France conducted measurements in the atmosphere at altitudes of up to five kilometers in the Arctic in March and April 2018. The vertical measurement profiles were obtained using the AWI research aircraft Polar 5 and Station Nord (81.6°N, 16.7°W) as a base of operations.
To determine the reasons behind annual fluctuations, the researchers compared the measurements from spring 2018 with earlier measurements from 2008, 2010, and 2015, each of which were also taken in the spring. They measured mass concentrations of black carbon ranging from 7 to 23 nanograms per cubic meter in 2018, which are comparable to those in 2010. In contrast, the values in 2008 and 2015 were significantly higher at all altitudes up to five kilometres.
The scientists found that variations in black carbon concentrations correlated with the frequency of wildfires north of 50° North, as determined from images taken by the MODIS satellite. Black carbon emissions from forest fires in regions of western and eastern Eurasia were transported with air masses to the Arctic, where they were likely responsible for the observed increase in black carbon levels during the Arctic spring, according to the researchers.
The scientists report that they could occasionally see a layer of pollution in the atmosphere during the 2018 airborne measurements, presumably due to biomass burning in the mid-latitudes.
The research team also investigated the extent to which current model simulations can reproduce the observed annual variations in black carbon concentrations. The advantage of the models is that they can estimate contributions from anthropogenic black carbon sources and from biomass burning separately. These reflected observations in 2010 and 2018, when biomass burning was low, relatively well. However, in 2008 and 2015, when many wildfires occurred, they showed much smaller values than the observations. These results suggest that current models generally reproduce the contribution of anthropogenic black carbon well, while underestimating the contribution of black carbon from biomass burning by a factor of three.
The authors assume that the warming of the atmosphere by black carbon in the Arctic is greatest in spring, when its mass concentration is highest and the incident solar radiation increases. The amount of black carbon in the Arctic in spring is also important because slight changes in the timing of snow and ice melt can affect the radiative budget in the Arctic. Increasing warming also increases the likelihood of forest fires in the mid and higher latitudes, which in turn increases black carbon concentrations in the Arctic. The current study has shown that the amount of Arctic black carbon can have a greater impact on radiation patterns in the Arctic than previously thought.
Julia Hager, PolarJournal
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