The Sum of the Parts

Abstract

Quantifying how emissions of any particular greenhouse gas affect the radiative forcing of climate is difficult, because of the complexity of the chemical interactions between different species and the wide range of spatial and temporal scales of atmospheric processes. Current assessments of climate change assume that a particular amount of radiative forcing cannot be attributed to any specific emissions species, and instead rely on calculations based on the atmospheric abundance of each species. Shindell et al. use a coupled chemistry-aerosol-climate model to hindcast atmospheric composition from preindustrial times to the present, caused by increased emissions of methane and the precursors of tropospheric ozone (NOx, CO, and volatile organic compounds, excluding methane). The global annual average composition response to all emission changes is nearly the same as that of the sum of the responses to individual emissions. Thus, emission figures can be used to calculate the radiative effects of these species. This emissions-based view indicates that the relative importance of various emissions is significantly different than suggested by current abundance-based assessments: Methane, in particular, is almost twice as important as previously suggested. — HJS