Black carbon in Antarctica

Snow-covered surfaces in Antarctica are among the cleanest and most reflective on Earth. Yet even on the remote continent, light-absorbing particles from fossil-fuel emissions embed themselves in snow and reduce the amount of solar radiation reflected back to space. Humans’ contribution to snow–albedo feedback is a key component in climate models, but it remains poorly constrained because many current remote sensing systems and algorithms only roughly estimate the physical characteristics of exposed snow.

Now scientists led by Kimberly Casey of NASA’s Cryospheric Sciences Laboratory and the US Geological Survey’s Land Remote Sensing Program have directly measured the composition and spectral response of Antarctic snow, which let them better determine the effect of pollution on snow radiative energy balance. At seven sites around the Amundsen–Scott South Pole Station during the southern summer of 2014–15, Casey and her team measured surface reflectance with a portable spectrometer that detects solar radiation from visible to short-wave IR. The researchers determined grain sizes and element concentrations in collected snow samples using analytical chemistry, microscopy, and spectroscopy techniques. The data show that high concentrations of trace elements and black carbon result in radiative forcing up to 70 W/m², nearly two orders of magnitude above that of clean snow.

Casey and coworkers found that even snow that appears as white as its surroundings to the naked eye (see photo) can be significantly less reflective than truly pure snow. By the time particles have collected on snow and ice at visibly noticeable levels, their radiation effect has likely reduced the albedo by at least 25%. Casey hopes the new results demonstrate the importance of quantifying pollutant impacts on snow albedo reduction and improving climate models’ accounting of how blackened snow affects reflectance. (K. A. Casey et al., J. Geophys. Res. Atm., in press, doi:10.1002/2016JD026418 .)