AGU journal highlights -- April 22, 2008

1. Cooling a climate disagreement

Aerosols from fossil fuel burning and other human activities cool the climate by scattering incoming solar radiation, but models and estimates appear to differ on the strength of this cooling’s global effect. Previous analysis of data from the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS) suggested that human-made aerosols exert a global direct radiative forcing of –1.9 watts per square meter in cloud-free skies, while models predict –0.7 watts per square meter. The more negative the forcing, the more incoming radiation is scattered, causing greater cooling. Bellouin et al. use updated MODIS aerosol data to estimate a global cloud-free radiative forcing of –1.3 watts per square meter. Further analysis using the Hadley Centre climate model reveals that the MODIS-based estimate is biased high by 40–50 percent mainly because it does not account for preindustrial aerosols such as those released by slash-and-burn agricultural techniques. The researchers conclude that for cloud-free conditions over oceans, the difference between modeled and observation-based estimates falls within the range of uncertainty. Over land, however, data on aerosol optical depth require refinement to further reconcile both estimates.

Title: Updated estimate of aerosol direct radiative forcing from satellite observations and comparison against the Hadley Centre climate model.

Authors: Nicolas Bellouin, Andy Jones, and Jim M. Haywood: Met Office Hadley Centre, Exeter, U.K.;

Sundar A. Christopher: Department of Atmospheric Sciences, University of Alabama, Huntsville, Alabama, U.S.A.

Source: Journal of Geophysical Research – Atmospheres (JGR-D) paper 10.1029/2007JD009385, 2008; http://www.agu.org/journals/pip/jd/2007JD009385-pip.pdf . This paper is “in press”.

2. Southern skies sensitive to ozone variation

Seasonal variation in stratospheric ozone concentration over Antarctica is large, with the greatest depletions occurring in the austral spring when the Sun returns to Antarctica. The resulting ozone hole, which is often displaced from the pole, cools the stratosphere because less ozone means less absorption of ultraviolet radiation. Although models suggest that the ozone hole also cools the troposphere over Antarctica, simulations are not entirely realistic because ozone concentrations used in models are usually averaged over latitude bands. Noting that ozone concentrations are more heterogeneous than this, Crook et al. prescribe a realistic three-dimensional distribution of ozone in a high-vertical-resolution atmospheric model and simulate the climate response to this ozone distribution. Comparing results with simulations containing averaged ozone concentrations reveals that the three-dimensional ozone yields cooler temperatures in the stratosphere and upper troposphere, with a magnitude of cooling comparable to that caused by ozone depletion itself. This result suggests that heterogeneous ozone concentrations influence Southern Hemisphere climate and that this influence will change in the future as the recovery of ozone decreases this heterogeneity.