Climate Change Simulations with Increasing Greenhouse Gases and Sulfate Aerosols

Research Objectives

To investigate the effect of greenhouse gas increase and sulfate aerosols on global warming.

Computational Approach

This climate model has three major components: an atmospheric spectral model, an ocean model with a 1 degree horizontal grid and 20 vertical levels, and a sea ice component including dynamics and thermodynamics.

Accomplishments

We conducted several global climate change experiments with a coupled climate model that includes the effects of increasing CO₂ and the radiative cooling effects of sulfate aerosols. The greenhouse gas increase causes general global warming; however, the sulfate aerosols cause less warming and even regional cooling, which yields climate change patterns closer to those observed.

We configured a global coupled ocean-atmosphere general circulation model without flux correction and integrated it into a set of 75-year sensitivity experiments that included increasing CO₂ concentrations and the direct and indirect efforts of anthropogenic sulfate aerosols. Sulfate aerosol forcing increased from zero to present-day estimates in the first 30 years of the integrations, while equivalent CO₂ forcing increased by 1% per year relative to the control experiment, similar to the rate of increase of observed greenhouse gas forcing over the period 1960-1990. Annual mean averages around year 30, analogous to present- day conditions, indicate better agreement with recent observed geographic and zonal mean temperature anomaly patterns in the sulfate aerosol experiments and less warming in northern summer than in winter.

Sulfate aerosols were then increased following the internationally agreed scenario, while CO₂ continued to increase at 1% per year. Averages around year 70, analogous to conditions roughly 40 years in the future, indicate warming almost everywhere in the troposphere over the globe as the CO₂ forcing overwhelms the negative radiative forcing from the sulfate aerosols. There is also a weakening of the south Asian monsoon in the sulfate aerosol experiments. Results of the sulfate aerosol experiments show qualitative agreement on the patterns of the temperature changes, both geographic and zonal, with the magnitude of the changes being a function of the size of the radiation forcing. Increased cloud reflectance and indirect sulfate aerosol experiments were also conducted.

Significance

One of the most significant issues that society has to face is climate change associated with man-made emissions of carbon dioxide and sulfate aerosols. Modern climate models are an important tool for studying possible impacts.

Publications

Washington, W. M., and G. A. Meehl. 1996. High latitude climate change in a global coupled ocean-atmosphere-sea ice model with increased atmospheric CO₂. J. Geophys. Res. 101:12,795.

Meehl, G. A., and W. M. Washington. 1996. El Niño-like climate change in a model with increased atmospheric CO₂ concentrations. Nature 382:56.

Meehl, G. A.,1997. Modification of surface fluxes from component models in global coupled models. J. Climate 10:2811.

URL

http://www.cgd.ucar.edu/ccr/

This figure shows the surface air temperature difference between a control experiment, labeled "CO₂ Only," and three experiments in which the carbon dioxide is increased by 1% per year, which takes into account roughly all of the other increases in greenhouse gases. The warming (red areas) and cooling (blue areas) correspond to estimated changes at the mid-21st century. The three alternative experiments have the additional effect of sulfate aerosols, which can cool regions where there is high industrial activity. One of the sulfate effects, termed "Direct," is generation of haze, which can reflect solar radiation. Because of uncertainties, two "Direct" experiments were conducted, one with the approximate effect and another with twice the effect. The final experiment, "Direct + Indirect," adds change in the brightness of clouds due to sulfate aerosols, which are also highly uncertain. These experiments will be repeated with improved climate models and inter-compared with observations and other climate modeling efforts.