Future impact of anthropogenic sulfate aerosol on

Authors: Irene FischerBruns Dorothea F Banse Johann Feichter

Publish Date: 2008/09/16

Volume: 32, Issue: 4, Pages: 511-524

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Abstract

We examine the simulated future change of the North Atlantic winter climate influenced by anthropogenic greenhouses gases and sulfate aerosol Two simulations performed with the climate model ECHAM4/OPYC3 are investigated a simulation forced by greenhouse gases and a simulation forced by greenhouse gases and sulfate aerosol Only the direct aerosol effect on the clearsky radiative fluxes is considered The sulfate aerosol has a significant impact on temperature radiative quantities precipitation and atmospheric dynamics Generally we find a similar but weaker future climate response if sulfate aerosol is considered additionally Due to the induced negative topoftheatmosphere radiative forcing the future warming is attenuated We find no significant future trends in North Atlantic Oscillation NAO index in both simulations However the aerosol seems to have a balancing effect on the occurence of extreme NAO events The simulated correlation patterns of the NAO index with temperature and precipitation respectively agree well with observations up to the present The extent of the regions influenced by the NAO tends to be reduced under strong greenhouse gas forcing If sulfate is included and the warming is smaller this tendency is reversed Also the future decrease in baroclinicity is smaller due to the aerosols’ cooling effect and the poleward shift in track density is partly offset Our findings imply that in simulations where aerosol cooling is neglected the magnitude of the future warming over the North Atlantic region is overestimated and correlation patterns differ from those based on the future simulation including aerosolsThe increase of anthropogenic greenhouse gases in the earth’s atmosphere is the main forcing responsible for the global warming expected for the next decades On the other hand aerosols as forcing agents of climate change are known to be important as shown by analyses of satellitebased measurements eg regarding the clearsky direct radiative forcing eg Bellouin et al 2005 Since aerosols experience a large temporal and spatial variability they are considered as one of the largest sources of uncertainty in climate projections Their typical lifetime in the atmosphere is on the order of several days and depends strongly on their physical and chemical characteristics as well as the frequency of precipitation events One key aerosol is sulfate of anthropogenic origin the only one considered in this model study Sulfate SO4 is an oxidation product of sulfur dioxide SO2 and a good indicator of industrial pollution Sulfur dioxide is emitted mainly by fossil fuel burning which accounts for about 72 of the total emissions anthropogenic and natural whereas biomass burning contributes only about 2 Forster et al 2007 It is oxidized to sulfuric acid gas which condenses quickly on existing particles or forms new sulfate particles The radiative forcing due to the sulfate aerosol’s backscattering of sunlight is known to modify the anthropogenic greenhouse effect significantly namely to oppose global warming This fact has been demonstrated by introducing sulfate aerosol into climate models eg Mitchell and Johns 1997 Reader and Boer 1998 Carnell and Senior 1998 Roeckner et al 1999 As SO2 emissions have harmful impacts on human health and natural environment as they cause for instance acid rain many developed countries have been reducing these emissions from power stations Thus it can be anticipated that this compensating effect will be weaker in the futureThe effects of sulfate aerosol may be separated into the ‘direct effect’ in regions without clouds and ‘indirect effects’ in cloudy regions The direct aerosol effect on climate is due to scattering and absorption of sunlight In case of scattering particles the amount of sunlight that reaches the earth’s surface is reduced due to the presence of the aerosol Indirect effects are for instance the influence of the aerosol particles on cloud microphysical properties such as reflectivity lifetime and precipitation rates An evaluation of the indirect effects and their consequences is beyond the scope of this paper In this study we concentrate on the possible future climatic changes in the North Atlantic region with special emphasis on the direct radiative impact of sulfate aerosol from anthropogenic origin The direct aerosol forcing represents only a part of the overall aerosol effect therefore our work represents also only a part of the entire picture of the aerosol’s role in our climate We investigate a simulation forced by greenhouse gases and a simulation forced by both greenhouse gases and sulfate aerosol Ulbrich and Christoph 1999 and Knippertz et al 2000 also analyzed the same simulation forced by greenhouse gases used in this study with respect to NAO and cyclone tracks However these studies do not take into account the additional effects of sulfate aerosol To our knowledge such an isolation of the aerosol forcing effects on the climate change in a specific region has not been attempted previouslyDetails about the model and a description of the experimental design are given in Sect 2 In Sect 3 we describe the method to calculate the aerosol effect our significance test and the cyclone tracking method Section 4 presents the results for the direct aerosol effect upon temperature radiative quantities precipitation NAO baroclinicity and cyclone track density The paper is summarized and concluded in Sect 5We analyze data of two transient model simulations performed with the ECHAM4/OPYC3 coupled atmosphere–ocean general circulation model AOGCM of the Max Planck Institute for Meteorology MPIM Roeckner et al 1999 It consists of the atmospheric component ECHAM4 a spectral transform model at T42 resolution employing 19 vertical levels and the oceanic component OPYC3 an updated version of the OPYC model developed by Oberhuber 1993 which uses isopycnal coordinates at 11 vertical levels Both model components are coupled quasisynchronously exchanging daily averaged quantities once a day The mixed layer temperature and the seaice variables are received by the atmospheric component without any adjustment For the oceanic component solar radiation river discharge wind stress and friction velocity are passed without adjustments as well Restricted to heat and freshwater only nonseasonal constant flux adjustments were employed to prevent the model from drifting to an unrealistic climate state The adjustments were estimated from a 100year coupled model spin up Details on the coupling technique can be found in Bacher et al 1998ECHAM4/OPYC3 is an AOGCM that has been used in climate modeling studies by several research groups worldwide Work published earlier evaluated the performance of the atmospheric model and the coupled model eg Roeckner et al 1996a b Bacher et al 1998 Roeckner et al 1999 Despite the restriction of the flux correction to heat and freshwater only the annual and seasonal climate of the coupled model agrees well with that produced by the atmospheric model component alone when it is forced with observed sea surface temperatures SSTs The coupled model captures many features of the observed interannual SST variability in the tropical Pacific mainly related to El Niño events The results of the AOGCM show also adequate agreement with observations regarding the weakening of the westerlies across the North Atlantic in El Niño/Southern Oscillation ENSO winters as well as regarding the weak tendency for colder than normal winters in Europe Stephenson and Pavan 2003 compared the NAO simulated by the coupled model to observed data together with the results of 16 other AOGCMs see Sect 45 Comparisons of modeled data with reanalysis data and observations that are made for this study will be shown in Sects 45–47We investigate two transient climate simulations Roeckner et al 1999 One of the simulations is forced with timedependent anthropogenic greenhouse gases alone referred to as simulation GHG in the following The second simulation additionally includes the backscattering of solar radiation due to the presence of sulfate aerosol originating from anthropogenic sulfur emissions GSD Natural biogenic and volcanic sulfur emissions are neglected The GHG simulation has been performed for the period 1860–2100 the GSD simulation for 1860–2050 Since GSD is available only until 2050 we do not consider the last 50 years of GHG The simulations allow for a separation of the aerosol effect from the anthropogenic greenhouse effect by building differences between the variables of both data sets see Sect 31 A third simulation GSDIO which has also been published by Roeckner et al 1999 accounts for the influences of greenhouse gases and additionally ozone and considers both the direct and the indirect sulfate aerosol effects The reason why the GSDIO simulation is not evaluated here is that a separation of the anthropogenic greenhouse gas effect and the aerosol effect would be impeded by the consideration of ozoneIn the simulations GHG and GSD changes in concentrations of the major greenhouse gases CO2 CH4 N2O and certain halocarbons are prescribed from 1860 to 1990 as observed From 1991 to 2050 these concentrations are assumed as projected into the future according to the IPCC IS92a forcing scenario Houghton et al 1992 In simulation GSD the tropospheric sulfur cycle is fully coupled with the meteorology of the atmospheric component ECHAM4 Feichter et al 1996 1997 The sulfate mass mixing ratio computed by the model is transformed into particle number concentrations under the assumption of a lognormal size distribution The wavelength dependent optical properties are determined according to Mie Theory Transport and deposition processes are calculated interactively The simulated trend in sulfate deposition in GSD since the end of the nineteenth century is broadly consistent with ice core measurements Roeckner et al 1999 The assumed future emission of sulfur dioxide also based on scenario IS92a is higher than those for the six SRES emission scenarios Nakicenovic and Swart 2000 The reason is that due to local air quality concerns in the industrialized countries scenarios published since 1995 assume sulfur controls of different degrees for the future This fact had not been considered at that time when the IS92a scenario was designed The projected future increase in SO2 emissions by the developing countries is clearly visible in our simulations in a shift in sulfate burden towards the equator as will be shown in Sect 41

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