Climate simulations show that interactions between particles of black carbon and convective and cloud processes in the atmosphere must be considered when assessing the full climatic effects of these light-absorbing particulates.
Black carbon1, often referred to as soot, is emitted during the incomplete combustion of fossil fuels, biofuels or wood. In contrast to other particulates emitted into the atmosphere by human activities, black carbon absorbs sunlight efficiently. This absorption leads to local heating of the atmosphere, warming the planet. Black carbon has received particular interest recently2 in the context of changes in climate policy. It remains in the atmosphere for only a few days, so cutting black-carbon emissions may be a viable way to reduce global warming over the next few decades, alongside measures to mitigate changes in the emissions of carbon dioxide.Such actions need to be supported by a good understanding and prediction of the climate role of black carbon. Writing in the Journal of Climate, Sand et al.3 report climate simulations that provide insights into these issues. The results highlight challenges for upcoming international initiatives aimed at better understanding how the climate responds to changes in the composition of the atmosphere.
Sand and colleagues used a numerical simulator of Earth's atmosphere and ocean to compare the effects on climate of artificially large increases in the emission of carbon dioxide and black carbon. They find that, although the increases were designed to exert similar perturbations in Earth's energy budget (the net flow of incoming and outgoing energy), changes in the planet's surface temperature and rainfall are considerably weaker in the simulation with elevated concentrations of black carbon.
This result confirms the importance of rapid responses in the atmosphere to changes in black carbon. These responses manifest themselves as warming at height and changes in cloud properties that lead to a net decrease in mid- and high-level cloud (Fig. 1). Moreover, they act to offset the initial artificially large perturbation, mainly because the warming and cloud loss at altitude effectively radiate energy to space, before the surface climate is able to respond. However, the magnitude of the rapid responses reported by Sand et al. — roughly seven times stronger than those to carbon dioxide — will come as a surprise to many climate scientists.
The researchers also highlight another result, which has implications for numerical simulations of climate change. By using a pair of experiments, both of which explore the climate impacts of black carbon and differ only in whether black-carbon changes can also adjust to atmospheric-circulation responses, Sand et al. demonstrate the role of the two-way black carbon–atmosphere interactions in driving the full climate response. Their findings are unexpected because these interactions seem to be the dominant cause of the climate response to changes in black carbon. The change in global surface temperature varies by a factor of two between the two experiments, with considerably larger differences at altitude. Indeed, many rainfall responses appear only when feedbacks of black carbon-to-atmosphere-to-black carbon are included.
The authors point out that the feedback loop of black carbon to itself through changes in climate may be particularly strong in their simulations because their model contains an unusually active atmospheric convection. Moreover, this strength may be exacerbated further by the artificially large perturbation imposed. Experiments with other numerical models may find weaker responses. Nevertheless, the large differences in the climate impacts of black carbon, when its two-way interaction with meteorology is also included, may make it harder to determine black carbon's full climate impact.
The various groups of climate scientists each focus on specific aspects of the climate system to better understand the effects of atmospheric changes. For those who work on atmospheric particulates, such as black carbon, an important aim is to quantify the particulates' impact on Earth's energy budget. A largely separate community studies atmospheric feedbacks, such as convection and clouds. Plans are already under way to design climate-model experiments under the Coupled Model Intercomparison Project, Phase 6 (ref. 4), which will provide improved knowledge of future climate responses and feed results to the next assessment report of the Intergovernmental Panel on Climate Change. Contributions to several of these experiments will either prescribe a fixed meteorology to explore the impacts on Earth's energy balance, or use fixed concentrations of atmospheric particulates to explore atmospheric feedbacks.
Such a pragmatic approach enables groups to concentrate resources on particular aspects of the climate-change problem and gain better insight into the processes involved. However, Sand and co-workers' findings suggest that when it comes to understanding the full climate impact of black carbon, it will be crucial to account for both how black carbon influences atmospheric circulation and also how these changes feed back on the atmospheric distribution of black carbon. This highlights the risk of simplified or idealized approaches, which may produce misleading conclusions about the total climate impact of changes in black-carbon concentrations.Footnote 1
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