Seminar

Abstract

The short-lived climate forcers (SLCFs) ozone, methane and aerosol particulates and their indirect effects on cloud properties significantly impact regional and global climate but in complex ways involving both warming and cooling mechanisms. Selective reduction of the warming SLCFs is currently receiving attention as a way of mitigating near-term warming, reducing the rate of warming (important for adaptation of ecosystems) and simultaneously improving air quality. To date, the feedbacks from anthropogenic land cover change have not been considered in assessments of historical and future SLCFs. Lack of quantitative information of these changing interactions, which perturb emissions of reactive carbon from vegetation, deposition rates of pollution to ecosystems and the underlying surface albedo, represents a major uncertainty in the ability to assess the climate and air quality benefits of reductions in the SLCFs. Here, a global chemistry-climate model (NASA ModelE-Y) with a new interactive vegetation biophysics module that incorporates photosynthesis-dependent isoprenoid emissions is applied to quantify the effects of historical (1850 - 2005) cropland expansion on the SLCFs. The resultant biogenic organic aerosol direct radiative forcing entirely counteracts the biophysical (albedo) forcing (+0.16 versus -0.09 W/m2). The land cover change alone implies a 15% longer methane lifetime in 1850 than 2005 while overall net effects on ozone radiative forcing are small. The analyses are extended to integrate the effects of future anthropogenic land cover change into projected aggressive air pollution emission abatement scenarios.