Black carbon aerosol is carbon dioxide’s darker accomplice. Not as well known as the atmospheric molecule that has defined our global climate crisis, black carbon still plays a role in amplifying warming trends, particularly in the Arctic. But how did it get there? Experimental studies continue to find more of this light-absorbing pollutant in the Arctic than global climate models predict, keeping its origin a mystery.
A new article  in Scientific Reports provides a partial answer by improving the resolution of climate models and illuminating how small-scale phenomena can play a huge role in global weather patterns. The conclusions get us closer to understanding just how our fossil fuel industry is forever changing the planet beyond 66 degrees N.
Two possibilities, zero answers
The incomplete combustion of fossil fuels, biomass, or agricultural waste creates black carbon. These tiny bits of leftover carbon are classified as particulate matter smaller than 2.5 microns in diameter (PM2.5) and are associated with a host of negative health effects like heart or lung disease. Developing countries like India or China are struggling mightily to manage local black carbon levels that are rapidly rising.
Unfortunately, degrading human health is not the only danger. According to the EPA, black carbon is the most effective PM2.5 at absorbing solar energy, capturing a million times more sunlight per unit mass compared to carbon dioxide. Now, imagine this pollutant finding a home on the snow plains of the Arctic. Snow normally reflects sunlight efficiently to provide negative feedback countering the effects of global warming. This is one of the Arctic’s sole defenses left to stop severe melting as temperatures rise. But as black carbon accumulates in the Arctic and replaces snow-white with carbon-black, reflective surfaces transform into light-absorbing hothouses, amplifying warming trends and pushing the Arctic closer to its watery fate.
If black carbon normally arises from human combustive processes, how does it end up blackening the Arctic? This is the question researchers are still trying to answer. Computer climate models are one of the best tools to understand climatic processes. Unfortunately, all of them underestimate black carbon levels in the Arctic compared to on-the-ground, experimental measurements.
Researchers expect two possibilities could account for the experiment-model discrepancy: 1) there are unknown sources of black carbon in the Arctic that we haven’t found and thus haven’t included in current climate models; or 2) the models aren’t accurately capturing how black carbon moves along atmospheric currents from mid-latitudes to the pole. There is some evidence to support the latter. Low-pressure systems begin at mid-latitudes and follow the polar front to the Arctic. This provides a potential pathway for black carbon to arise in industrial centers in the Northern Hemisphere (U.S., Europe, China) and surf all the way to the top of the planet. But global climate models have predicted that the polar front drops all the black carbon through precipitation far before the Arctic Circle.
A finer look
Any computational climate model must divide the planet into a grid of finite-size regions, defining the resolution of the model. The size of these grid regions effectively sets a limit to what can be seen: models are blind to any behavior changing on length scales smaller than the grid size. Previous models have used a resolution of 50 kilometers, but it is well known that a weather system like the polar front contains small vortices and quick temperature changes on a scale much smaller than this. Knowing this, Sato et al. improved their model resolution by an order of magnitude, decreasing each grid region to a width of 3.5 kilometers. Results using this model provide the first answers to how so much black carbon could accumulate in the Arctic.
The figure below shows results from the same simulation done at three levels of resolution – 3.5 km, 14 km, and 56 km (top, middle, and bottom rows, respectively). Comparing the simulations illuminates two major features driving black carbon transport to the Arctic that previous models could not capture: 1) small-scale vortices and 2) cloud formation.
The impact of vortices can be seen in the left part of the figure. The gray-scale images on the left visualize the polar front by mapping liquid water mass across a region near the southern tip of the Arctic. This type of graphic essentially describes the direction of wind currents in the front. The 56-km model only has sufficient resolution to describe a broad, candy-cane-like structure at the center of the weather system. In contrast, the 3.5-km resolution model predicts smaller-scale vortices within the larger, curling system. The researchers predict that these vortices impact how much black carbon can be transported over long distances, however this idea must be explored further in future work.
Second, and more importantly, the 3.5-km resolution model improved the description of cloud formation. The 56-km model smeared cloud formation across most of the front, thus predicting more rain that would move black carbon out of the atmosphere and onto the ground before reaching the Arctic. In contrast, the 3.5-km model predicted much sparser cloud formation and less precipitation, allowing more black carbon to reach the pole before being released by the clouds.
As a result of these two factors, the finer-resolution model predicts much more black carbon at latitudes near 60 degrees N. This can be seen in the righthand side of the figure, in which green regions indicate much more black carbon than blue ones. We now have an answer: industrial operations in the Northern Hemisphere, burning coal, oil, and biomass, are likely channeling black carbon to the Arctic via the polar front.
Not done yet
Despite this incredible step forward in understanding, the finer resolution model still underestimates experimental black carbon levels in the Arctic. The authors admit that, even at this fine of a resolution, the model is probably missing even more detailed features controlling cloud formation and precipitation. The only answer is better supercomputing power to support models with sub-kilometer resolution.
But this is science – every new answer provides a glimpse of how a little bit better technology could give us even more comprehensive understanding. The trend – that we are closing the gap between experimental and modeled results – is the important and hopeful part. Once we understand how black carbon arrives at the Arctic, we can finally begin the hard work of trying to stop the process.
 Sato Y et al. “Unrealistically pristine air in the Arctic produced by current global scale models.” Scientific Reports, 6:26561, 2016.
Figure of model results courtesy of Reference 1
Sato, Y., Miura, H., Yashiro, H., Goto, D., Takemura, T., Tomita, H., & Nakajima, T. (2016). Unrealistically pristine air in the Arctic produced by current global scale models Scientific Reports, 6 DOI: 10.1038/srep26561