Satellite measurements of the clear-sky greenhouse effect from tropospheric ozone

Radiative forcing from anthropogenic ozone in the troposphere is an important factor in climate change, with an average value of 0.35 W m ² according to the Intergovernmental Panel for Climate Change (IPCC). IPCC model results range from 0.25 to 0.65 W m², owing to uncertainties in the estimates of pre-industrial concentrations of tropospheric ozone, and in the present spatial and temporal distributions of tropospheric ozone, which are much more variable than those of longer-lived greenhouse gases such as carbon dioxide. Here, we analyse spectrally resolved measurements of infrared radiance from the Tropospheric Emission Spectrometer on board the NASA Aura satellite, as well as corresponding estimates of atmospheric ozone and water vapour, to obtain the reduction in clear-sky outgoing long-wave radiation due to ozone in the upper troposphere over the oceans. Accounting for sea surface temperature, we calculate an average reduction in clear-sky outgoing long-wave radiation for the year 2006 of 0.48±0.14 W m² between 45°S and 45° N. This estimate of the clear-sky greenhouse effect from tropospheric ozone provides a critical observational constraint for ozone radiative forcing used in climate model predictions.