Anthropogenic-Climate-Change by sdaferv


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									Doc 3.4.1/1 Report on ACC CLIVAR SSG-14, 19-22 April 2006

Anthropogenic Climate Change
Directions for Earth System Modelling Bryant J. McAvaney Introduction It is an obvious fact that the future of modelling efforts to further understand and predict future anthropogenic climate change is critically dependent on the development of ever more complex earth system models. A much more systems-orientated approach to the climate change problem is welcome and accompanies a much more systematic approach to exploring the myriad of processes and feedbacks that can affect projections of climate change through the 21st century. Accompanying these changes in climate systems modelling and analysis is a much broader climate-related research commitment to tackling the fundamental and difficult questions associated with the climate change issue – what constitutes “dangerous” climate change? Future Needs for ACC CLIVAR Feedback processes in the fast response component of the climate system, the atmosphere (mostly water vapour-lapse rate and clouds), are now better understood and better quantified in climate system models. However there remains considerable uncertainty associated with the representation of clouds in the modelled atmosphere with the main problems being identified with difficulties in representing boundary layer clouds in climate models (Webb et al,2006, Bony and Dufresne, 2005). Further co-operation with GEWEX/GCSS is required to help improve the atmospheric component of climate system models. Both terrestrial and marine biogechemical processes will need to be incorporated more completely into earth systems models. Although the carbon cycle in both the terrestrial and marine systems have already been incorporated into some global climate models none were utilised in the IPCC AR4 projections. Further co-operation with IGBP is needed to bring a systematic approach to carbon cycle modelling. More complete coupling between atmospheric chemistry modules and aerosol modules and between aerosol modules and the surface will also be needed to enable a more comprehensive treatment of the role of aerosols in climate change. Further co-operation with IGCP/IGAC is needed to cross fertilise ideas. The optimal resolution needed by the main modules of a coupled climate system model (atmosphere and ocean) still remains a somewhat open question but indications are such that the typical resolutions used for the IPCC AR4 projections are barely adequate. It is to be expected that the current strong interest in changes in extremes determined from climate change experiments with climate system models will continue. Of particular interest are two particular phenomena namely changes in tropical cyclones and the frequency of droughts. An even more enhanced modelling activity in both model development and model

evaluation associated with these phenomena is required in the next five years. Continued cooperation with GEWEX programmes is required. A more detailed consideration of the processes coupling between the land-atmosphere-ocean systems will become more widespread in the next five years as a more integrated approach to understanding and assessing the vulnerability and hazards to human societies is developed. This will require active participation by ACC CLIVAR in ESSP activities. The heightened awareness that the complex non-linear nature of the climate system can result in a number of „abrupt changes‟ or „surprises‟ as the climate system does/does not shift into a different climate „regime‟ has meant much greater attention is being focussed on the climate system components most susceptible to „regime change‟. Thus far, possible changes in the meridional overturning circulation in the ocean have received the greatest attention, and will continue to do so. Other potential „regime changes‟ associated with changes in biota of the land and ocean are less well studied. In the next five years we will begin to see studies in „experimental‟ versions of climate system models. ACC CLIVAR should co-operate with IGBP (through the ESSP) in this area. The coupling between aerosols and climate Climate is affected by aerosols through both their direct radiative effect and through indirect effects on cloud microphysics and on the hydrological cycle. Although these effects vary greatly depending on the type of aerosol particle, in general aerosols scatter incoming solar radiation and thus reduce the amount of direct solar radiation reaching the surface. The coupling between chemistry and climate Two aspects of atmospheric chemistry that are beginning to be incorporated into experimental climate models are: (i) simplified atmospheric chemistry especially that associated with the control of ozone and aerosols and (ii) considerations of air quality. This work (mostly conducted under the auspices of IGBP/IGAC) will begin to involve “production” type climate system models within the next five years. The cryosphere The slow manifold of the cryosphere, namely changes in land ice, may need to be included in ACC models in order to capture the potential feedback effects due to melting of say, the Greenland and Antarctic ice sheets (ACIA, 2004). The high importance of ice albedo feedback for enhancing global warming requires that greater attention be placed on sea ice modelling and evaluation (especially for the Arctic) within coupled climate models. In addition the albedo (and other) feedbacks associated with snow cover (especially in northern high latitudes) require even greater attention on the modelling and evaluation of snow cover over land. The coupling between the carbon cycle and climate Feedbacks from the carbon cycle as it responds to a changing climate can also affect response of the climate system and hence the magnitude of the warming by 2100. At present these effects are incorporated interactively only in experimental climate system models. Carbon cycle feedbacks can arise from both the land surface and the oceans and are due to both physical and biological/ecological processes. Much current research focuses on terrestrial ecology since its time scale is much faster than in the ocean and can potentially influence the

magnitude of the climate change on decadal time scales. There are large uncertainties associated with two major feedbacks firstly the enhanced loss of carbon from soils as temperature increases and secondly changes in the frequency and area of ecological disturbance (most notably fire) that emit large pulse of carbon into the atmosphere. It is anticipated that research into both these effects incorporated into “production” climate system models will become more widespread in the next five years. Coupling between land and atmosphere Associated with consideration of the carbon cycle (as mentioned above) and in support of atmospheric chemistry greater attention will need to be given to: (1) exchange of reactive and conservative compounds (2) feedbacks between land biota, aerosols and atmospheric composition (3) dynamics of the land surface-vegetation-water-atmosphere system (4) material and energy transfers in the soil-canopy-boundary-layer system. Coupling between ocean and atmosphere In addition to consideration of the standard processes that exchange water, heat and momentum across the ocean-atmosphere boundary increased attention will be focussed on the air-sea fluxes of all radiatively active gases (not only carbon dioxide). Biogeochemical interactions and feedbacks between ocean and atmosphere will continue to be investigated. ACC CLIVAR should continue to co-operate with IGBP in these areas. Coupling between land and ocean Detailed consideration of the coupling of the land and ocean, especially consideration of coastal system, has generally been lacking in climate systems models. The implications of land-use change on river catchments and the coastal zone interaction will begin to be investigated over the next five years with higher resolution „experimental‟ climate system models. Monitoring these activities within the IGBP framework by ACC CLIVAR will be important. Predictability of Climate System Palmer (2005) has consistently pointed out the requirement for both studies of predictability and ensemble prediction in climate system science. Ensemble prediction techniques applied to climate change have become to be established (Murphy et al 2004; Stainforth et al, 2005). To date most of the perturbations have only been applied to the „fast physics‟ of the atmospheric component of an earth system model, as yet no account has been taken of uncertainties in bio-geochemical feedbacks. The fundamental argument for an ensemble approach is that no matter how much the resolution of models is increased there will always be unresolved scales requiring parametrisation. Thus we can never be certain that increasing resolution will necessarily lead to a reduction in uncertainty in projections of climate change. Palmer (2005) proposes a hugely computer intensive approach to deciding between high and low climate sensitivity models which may become possible in the near future. Concluding Remarks Research since the Third Assessment Report of the IPCC continues to challenge the well publicised IPCC projection of 1 1.4 to 5.8 increases in global mean temperature by 2100. New understanding of aerosol effects, carbon cycle dynamics and cryosphere changes is pushing the consensus projected temperature increase towards the upper end of the IPCC range.

However hand in hand with this emerging understanding is the fact that there remain many uncertainties associated with these amplifying effects so that the magnitude, rate and timing of the temperature increase remain very uncertain. A major challenge to ACC CLIVAR and the scientific community in general over the next five years is to determine how important these amplifying effects are and hence the degree of realism to be associated with the preliminary projections of significantly enhanced warming. The role of the ACC component CLIVAR within the broader ESSP concept of WCRP/IGBP should be to foster a more systematic, and more complete, approach to studying the role of feedbacks with the climate system. In conjunction with this effort, enhanced studies of the predictability (as distinct from projections/predictions) of climate change, especially in climate extremes. A higher profile of ACC CLIVAR in the WCRP COPES strategy promoting the use of ensemble prediction techniques in climate systems modelling is appropriate. A rapid convergence of ACC CLIVAR modelling activities within programmes in the IGBP and WCRP framework is foreseen over the coming five years. It is imperative that ACC CLIVAR be an important presence in the Earth Systems Science Partnership (ESSP) of IGBP/WCRP/IHD. While a full Earth Systems Model envisaged by ESSP may not become a reality within five years, modelling groups preparing potential modelling input for the fifth IPCC Assessment in about 2012 will have only until about the end of 2008 to configure their “production” climate system models to incorporate some (at least) of the factors discussed in this paper. References Palmer, T., 2005, Global warming in a nonlinear climate – Can we be sure? Europhysics News, March/April 2005, 42-46. ACIA: Arctic Climate Impact Assessment Report, 2004. Arctic Council and International Arctic Science Committee. Brasseur, G, and W. Steffen, 2005, IGCP: A second decade of global change research, EOS, Milly, P.C.D., Dunne, K.A. and Vecchia, A.V., 2005, Global pattern of trends in streamflow and water availability in a changing climate. Nature 438, 347-350. Murphy, J.M., Sexton, D.M.H., Barnett, D.N., Jones G.S., Webb, and M.J. et al, 2004: Quantifying Uncertainties in climate change using a large ensemble of global climate model predictions. Nature 430, 768-772. Stainforth, D.A., T. Aina, C. Christensen, M. Collins, D.J. Frame, J.A. Kettleborough, S. Knight, A. Martin, J. Murphy, C. Piani, D. Sexton, L.A. Smith, R.A. Spicer, A.J. Thorpe, M.R. Allen, 2005, Uncertainty in Predictions of the Climate Response to Rising Levels of Greenhouse Gases. Nature, 433, 403-406. Bony S, Dufresne J-L (2005) Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models. Geophys Res Lett 32:L2080610.1029/2005GL023851 M.J. Webb, C.A. Senior, D.M.H. Sexton, W.J. Ingram, K.D Williams, M.A. Ringer, B.J. McAvaney, R. Colman, B.J. Soden, R. Gudgel, T. Knutson, S. Emori, T. Ogura, Y. Tsushima,

N. Andronova, B. Li, I. Musat, S. Bony, and K.E. Taylor, 2006, "On the contribution of local feedback mechanisms to the range of climate sensitivity in two GCM ensembles”, Climate Dynamics (accepted)

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