Anthropogenic Climate Change for 1860 to 2100 Simulated with the Hadcm3 Model Under Updated Emissions Scenarios
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Climate Dynamics (2003) 20: 583–612 DOI 10.1007/s00382-002-0296-y T.C. Johns Æ J.M. Gregory Æ W.J. Ingram C.E. Johnson Æ A. Jones Æ J.A. Lowe J.F.B. Mitchell Æ D.L. Roberts Æ D.M.H. Sexton D.S. Stevenson Æ S.F.B. Tett Æ M.J. Woodage Anthropogenic climate change for 1860 to 2100 simulated with the HadCM3 model under updated emissions scenarios Received: 29 April 2002 / Accepted: 31 October 2002 / Published online: 18 February 2003 Ó Springer-Verlag 2003 Abstract In this study we examine the anthropogenically considered, but marked drying in the mid-USA and forced climate response over the historical period, 1860 southern Europe and significantly wetter conditions for to present, and projected response to 2100, using South Asia, in June–July–August, are robust and sig- updated emissions scenarios and an improved coupled nificant. model (HadCM3) that does not use flux adjustments. We concentrate on four new Special Report on Emission Scenarios (SRES) namely (A1FI, A2, B2, B1) prepared for the Intergovernmental Panel on Climate Change 1 Introduction Third Assessment Report, considered more self-consis- tent in their socio-economic and emissions structure, There is growing expectation that increases in the con- and therefore more policy relevant, than older scenarios centrations of greenhouse gases arising from human ac- like IS92a. We include an interactive model representa- tivity will lead to substantial changes in climate in the tion of the anthropogenic sulfur cycle and both direct twenty first century. Indeed, there is already evidence and indirect forcings from sulfate aerosols, but omit the that anthropogenic emissions of greenhouse gases second indirect forcing effect through cloud lifetimes. (GHGs) have altered the large-scale patterns of tem- The modelled first indirect forcing effect through cloud perature over the twentieth century (Santer et al. 1996; droplet size is near the centre of the IPCC uncertainty Tett et al. 1999; Barnett et al. 1999; Stott et al. 2001; range. We also model variations in tropospheric and Mitchell et al. 2001), although natural factors, including stratospheric ozone. Greenhouse gas-forced climate changes in solar output and episodic volcanic emissions, change response in B2 resembles patterns in IS92a but is may also have contributed. Future climate change will smaller. Sulfate aerosol and ozone forcing substantially probably be dominated by the response to anthropogenic modulates the response, cooling the land, particularly forcing factors. Large uncertainties remain, however, in northern mid-latitudes, and altering the monsoon the expected anthropogenic climate response, partly structure. By 2100, global mean warming in SRES sce- through model uncertainty in the projected climate sen- narios ranges from 2.6 to 5.3 K above 1900 and pre- sitivity to increasing GHGs and partly as a result of both cipitation rises by 1%/K through the twenty first century physical and modelling uncertainties in the amplitude (1.4%/K omitting aerosol changes). Large-scale patterns and geographical patterns of non-GHG forcings which of response broadly resemble those in an earlier model modulate this GHG-forced response (particularly from (HadCM2), but with important regional differences, sulfate aerosol indirect cooling effects). particularly in the tropics. Some divergence in future The main motivation for the present study is to response occurs across scenarios for the regions provide up-to-date estimates of global climate change suitable for deriving impacts assessments on regional T.C. Johns (&) Æ J.M. Gregory Æ W.J. Ingram Æ C.E. Johnson scales with self-consistent scenario assumptions. Even A. Jones Æ J.A. Lowe Æ J.F.B. Mitchell Æ D.L. Roberts though some of the regional details may be unreliable, D.M.H. Sexton Æ S.F.B. Tett Æ M.J. Woodage this approach at least gives a picture of possible conse- Met Office, Hadley Centre for Climate Prediction and Research, London Road, Bracknell, RG12 2SY, UK quences of alternative future emissions scenarios, de- E-mail: tim.johns@metoffice.com rived in a physically consistent way, which can be interpreted for policymakers in the context of negotia- D.S. Stevenson Department of Meteorology, University of Edinburgh King’s tions to limit emissions so as to prevent ’dangerous’ Buildings, Edinburgh EH9 3JZ, Scotland, UK, climate change. 584 Johns et al.: Anthropogenic climate change for 1860 to 2100 simulated with the HadCM3 model Some questions that arise are whether GHG-induced warming (determined by global average emissions) will 2 Model description dominate, or whether local sulfate aerosol forcing (much The HadCM3 coupled model (Gordon et al. 2000) is used in the more dependent on local emissions) will be a significant simulations. HadCM3 was developed from the earlier HadCM2 factor over the twenty first century and, if the latter, model (Johns et al. 1997), but various improvements to the atmo- what differences in the regional responses might be ex- sphere and ocean components mean that it needs no artificial flux pected under different emissions scenarios? Could non- adjustments to prevent excessive climate drift. The atmosphere and linear climate-chemistry feedbacks become increasingly ocean exchange information once per day, heat and water fluxes being conserved exactly. Momentum fluxes are interpolated be- important to consider in future, with an implication of tween atmosphere and ocean grids so are not conserved precisely, added importance attached to the details of alternative but this non-conservation is not thought to have a significant effect. emissions pathways that result in similar time-integrated emissions? To tackle such questions it is important that there should be consistency and some plausibility about 2.1 Atmosphere the emissions scenarios used to force climate models. The atmospheric component of the model, HadAM3 (Pope et al. The IPCC SRES (‘Special Report on Emissions Sce- 2000), has 19 levels with a horizontal resolution of 2.5° latitude by narios’, Nakicenovic et al. 2000) marker scenarios de- 3.75° longitude, comparable to a spectral resolution of T42. veloped in conjunction with the IPCC Third Assessment A new radiation scheme is included with six spectral bands in Report, have been developed specifically to provide the shortwave and eight in the longwave. The radiative effects of minor greenhouse gases as well as CO2, water vapour and ozone more self-consistent and up-to-date scenarios for climate are explicitly represented (Edwards and Slingo 1996). A simple modelling than hitherto. We have therefore primarily parametrization of background aerosol (Cusack et al. 1998) is also adopted these in the present study, though we also refer included. back to the older IS92a scenario which has already been A new land surface scheme (Cox et al. 1999) includes a repre- used extensively in climate impact studies (e.g. US sentation of the freezing and melting of soil moisture, as well as surface runoff and soil drainage; the formulation of evapotranspi- National Assessment Report 2000; Hulme et al. 1999; ration includes the dependence of stomatal resistance on temper- Hulme and Jenkins 1998). ature, vapour pressure and CO2 concentration. The surface albedo Given emission scenarios of GHGs, sulfur and other is a function of snow depth, vegetation type and also of tempera- anthropogenic precursor gases, understanding the cli- ture over snow and ice. A penetrative convective scheme (Gregory and Rowntree 1990) mate sensitivity to the resulting anthropogenic forcings is used, modified to include an explicit downdraught and the direct and the interactions between climate change, atmo- impact of convection on momentum (Gregory et al. 1997). Para- spheric chemistry feedbacks and transport processes is metrizations of orographic and gravity wave drag have been revised still a major challenge. Coupled climate models in- to model the effects of anisotropic orography, high drag states, flow blocking and trapped lee waves (Milton and Wilson 1996; Gregory cluding representations of the relevant physical, dy- et al. 1998). The large-scale precipitation and cloud scheme is namical and chemical processes are an essential tool, formulated in terms of an explicit cloud water variable following not just for making predictions but for increasing un- Smith (1990). The effective radius of cloud droplets is a function of derstanding of the feedbacks and sensitivities. Here we cloud water content and droplet number concentration (Martin use a state-of-the-art coupled climate model (HadCM3, et al. 1994). Gordon et al. 2000) incorporating an explicit sulfur cycle to predict anthropogenic sulfate burdens from emissions and including the anthropogenic forcing ef- 2.2 Ocean and sea ice fects thought to be most relevant at the moment, ap- The oceanic component of the model has 20 levels with a horizontal plied in a way as consistent with the emissions scenario resolution of 1.25·1.25°. At this resolution it is possible to rep- as possible. The results therefore represent plausible resent important details in oceanic current structures (Wood et al. estimates of global climate change consequent upon the 1999). Horizontal mixing of tracers uses a version of the adiabatic socio-economic assumptions underlying the emissions diffusion scheme of Gent and McWilliams (1990) with a variable scenarios. thickness diffusion parametrization (Wright 1997; Visbeck et al. The structure is as follows: in the next section 1997). There is no explicit horizontal diffusion of tracers. The we document the version of the coupled model used in along-isopycnal diffusivity of tracers is 1000 m2/s and horizontal the study and the physical basis for calculating the momentum viscosity varies with latitude between 3000 and 6000 m2/s at the poles and equator respectively. anthropogenic forcings. In Sect. 3 we briefly outline the Near-surface vertical mixing is parametrized by a Kraus-Turner performance of the model in a long control integration mixed layer scheme for tracers (Kraus and Turner 1967), and a with no anthropogenic forcing imposed.