Atmospheric science: The self-cleansing ability of prehistoric air.

Abstract

Isotopic data from an ice core have been used to estimate atmospheric oxidant levels during past climate transitions — pointing to relatively unexplored climate feedbacks as drivers of atmospheric composition. See Letter p.133 The abundance of tropospheric oxidants, such as ozone (O3) and the hydroxyl radical (OH), determines the lifetimes of reduced trace gases such as methane, but the response of tropospheric oxidants to climate change is poorly constrained. This paper reports isotopic data from a Greenland ice core covering the most recent glacial–interglacial cycle that demonstrate the sensitivity of tropospheric oxidants to climate change. The authors observe an increase in the O3/HOx ratio in cold climates, the opposite of current expectations, and suggest that this increase might be driven by enhanced reactive halogen chemistry, enhanced stratosphere-to-troposphere transport of O3, or a combination of the two. These processes require further consideration when assessing global methane variations during periods of abrupt climate change.