Strong Regional Gradients in Shortwave Greenhouse Gas Radiative Forcing and Implications for Radiative Transfer Parameterizations

The shortwave radiative forcing of greenhouse gases including carbon dioxide and methane is larger than the well-known longwave effects of many less important anthropogenic forcing agents routinely included in radiative forcing assessments from the IPCC. Comprehensive shortwave radiative forcing calculations have previously not been undertaken due to their extreme computational expense. However, by leveraging supercomputing resources, we have completed the first global, spatially-resolved calculations of forcing at the surface, tropopause, and top-of-atmosphere by both carbon dioxide and methane. This calculation reveals that greenhouse gas exhibits strong spatial gradients associated primarily with low-latitude surfaces that have high near-infrared albedo, and secondarily with cloud coverage. The reason for this albedo dependence is that surfaces that are highly reflective in the near-infrared lead to more absorption of reflected shortwave photons by these gases. Localized annual-mean forcing from pre-industrial to present-day carbon dioxide and methane increases can be as large ten times the global annualized shortwave forcing.

Furthermore, we show that our estimates of shortwave forcing by anthropogenic greenhouse gases are robust and, by leveraging results from planetary observations, largely invariant with respect to future developments in the spectroscopy of carbon dioxide and methane. However, our findings point to the need to ensure that shortwave radiative transfer parameterizations are informed by and properly formulated with realistic near-infrared surface albedo data.