Chemistry-climate interactions in the GISS GCM. Part 1: Tropospheric chemistry model description and evaluation

-"_i /Revised for Journal of Geophysical Research. , Sept. , 2000, Accepted Oct. 31,2000.Chemistry-climate interactions in the GISS GCM. Part 1:Tropospheric chemistry model description and evaluationDrew T. Shindelll, J. Lee Grenfelll, David Rind1, Colin Price2, and Volker Grewel,31 NASA Goddard Institute for Space Studies and Center for Climate Systems Research, ColumbiaUniversity, New York, New York2 Department of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, Israel3 DLR Institut fur Physik der Atmosphare, Oberpfaffenhofen, GermanyAbstract. A tropospheric chemistry module has been developed for use within the God.dardInstitute for Space Studies (GISS) general circulation model (GCM) to study interactions betweenchemistry and climate change. The model uses a simplified chemistry scheme based on CO-NOx-HOx-Ox-CH4 chemistry, and also includes a parameterization for emissions of isoprene, the mostimportant non-methane hydrocarbon. The model reproduces present day annual cycles and meandistributions of key trace gases fairly well, b_sed on extensive comparfsons with availableobservations. Examining the simulated change b_ tween present day and pre-industrial conditions,we find that the model has a similar response tc that seen in other simulations. It shows a 45%increase in the global tropospheric ozone burden, _,ithin the 25% - 57% range seen in other studies.Annual average zonal mean ozone increases by r, iore than 125% at Northern Hemisphere middlelatitudes near the surface. Comparison of model ru is that allow the ca! culated ozone to interact withthe GCM's radiation and meteorology with those that do not shows only minor differences forozone. The common usage of ozone fields that are aot calculated interactively seems to be adequateto simulate both the present day and the pre-industdal ozone distributions. However, use of coupledchemistry does alter the change in tropospheric oxidation capacity, enlarging the overall decrease inOH concentrations from the pre-industrial to tile present by about 10% (-5.3% global annualaverage in uncoupled mode, -5.9% in coupled mode). This indicates that there may be systematicbiases in the simulation of the pre-industrial to present day decrease in the oxidation capacity of thetroposphere (though a ten percent difference is well within the total uncertainty). Global annualaverage radiative forcing from pre-industrial to present day oz.me change is 0.32 W/m2. Theforcing seems to be increased by about 10% when the chemistry is coupled to the GCM. Forcingvalues greater than 0.8 W/m2 are seen over large areas of the United States, Southern Europe, NorthAfrica, the Middle East, Central Asia, and the Arctic. Radiative forcing is greater than 1.5 W/m2over parts of these areas during Northern summer. Though there are local differences, the radiativeforcing is overall in good agreement with the results of other modeling studies in both its magnitudeand spatial distribution, demonstrating that the simplified chemistry is adequate for climate studies.1. IntroductionThe buildup of both greenhouse gases and ozone precursors in the Earth's atmosphere due toanthropogenic emissions is well documented (e.g. IPCC, 1996). Since pre-industrial times, carbondioxide has increased by ~30%, methane by about 150%, and NOx is thought to have more thandoubled. Just since the 1950s, carbon monoxide levels have increased by 50% at NorthernHemisphere mid-latitudes. The direct radiative influence of the well-mixed greenhouse gases hasbeen studied intensely, and is now considered to be one of the lesser sources of uncertainty forrecent climate change (e.g. Hansen et al., 1998). Induced changes in chemistry are much less clear.These changes have an indirect radiative effect by chemically altering the abundance of radiativelyabsorbing species such as ozone and methane.The major chemical impacts of increasing emissions of greenhouse gases and ozone precursorgases such as nitrogen oxides (NOx) are to produce tropospheric ozone, and to alter tropospheric