The way exhaust-aerosol particles from ships spur cloud development in "ship tracks" that trail these vessels, and which reflect radiant energy back to space, has been used to understand the effects of atmospheric aerosol particles on cloud reflectivity more broadly. But, according to a new study, ship track-derived data cannot be generalized to estimate anthropogenic aerosols' role in climatological forcing, say the authors; it overstates anthropogenic aerosols' cooling effect. The findings highlight an important bias in current aerosol-cloud forcing estimations as they're currently used to project climate change. By reflecting incoming solar radiation back into space, stratocumulus clouds cool the planet and play an important role in mediating Earth's atmospheric temperatures. Effects of atmospheric aerosol particles on cloud formation and their overall reflectivity represents a major source of uncertainty in understanding and forecasting human impacts on the climate system. Data from studies involving the long, linear stratocumulus clouds formed by the rising exhaust from ships - ship tracks - have been used to estimate the radiative impact of anthropogenic aerosols. Here, Franziska Glassmeier and colleagues argue that this approach cannot be used to estimate the cooling effect of anthropogenic aerosols in the atmosphere. By comparing detailed numerical simulations with satellite analysis, Glassmeier et al. show how data derived from ship-track studies - particularly the effect of aerosols on non-precipitating stratocumulus clouds - leads to an overestimation of their cooling effect by up to 200%. According to the authors, the warming effect of decreasing stratocumulus clouds must be accounted for to constrain anthropogenic aerosol's role in cloud-mediated radiative forcing. Based on their results, they write, "it seems desirable to identify alternatives to ship-track studies that allow for a direct observation of aerosol effects."
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Journal
Science