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The Concept of Climate Sensitivity: History and Development

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The Concept of Climate Sensitivity: History and Development 1 The concept of climate sensitivity: history and development Natalia Andronova, Michael Schlesinger, Suraje Dessai, Mike Hulme and Bin Li 1.1 Introduction Between the 1960s and 1980s various types of deterministic models were used to estimate climate sensitivity, leading to a The climate sensitivity concept (CSC) has more than a century wide range of results. However, it was a mixture of modeling of history. It is closely related to the concept of “climate results and expert assessment – the Charney report of 1979 forcing” or “radiative forcing,” which was fully presented and (NAS, 1979) – that established the range of 1.5 C to 4.5 C discussed by successive IPCC Assessment Reports (e.g. see that was later reported in all three IPCC Assessment Reports Chapter 6 Houghton et al., 2001). According to CSC, a change (IPCC, 1990, 1996, 2001). in the equilibrium global Currently the primary reason for the substantial range in near-surface air temperature (NST) of the Earth, 1T, due to an model-based estimates of climate sensitivity is widely external disturbance of the Earth’s energy balance (radiative believed to be differences in their treatment of feedbacks forcing), can be linearly related to a change in the net radiation (Schlesinger, 1985, 1988, 1989; Cess et al., 1996; Colman, at some level in the atmosphere, 1F. Thus, 2003) – particularly cloud feedbacks. But systematic com- parisons have not been made to confirm that this is true for the 1T ¼ ‚1F; ð1:1Þ current generation of models. Within international climate where ‚ is the climate sensitivity, which characterizes the modeling projects, the development of new models, together ability of the climate system to amplify or reduce the initial with both formal and informal model comparison exercises temperature change initiated by the external forcing. The cli- that are currently being conducted by various groups, suggests mate sensitivity has been estimated using Eq. (1.1) most fre- that a renewed focus on the reasons for different model-based quently from the NST change, 1T2x, resulting from the estimates of climate sensitivity may be particularly useful at radiative forcing due to a doubling of atmospheric carbon this time. dioxide concentration from pre-industrial levels, 1F2x: The probability density and cumulative distribution func- tions for 1T obtained recently using the instrumental tem- 1T 2x ‚ ¼ 2x : ð : Þ peratures and estimated forcing from the mid-nineteenth 1 1 2 F2x century to the present by four groups (Andronova and Schle- Thus 1T2x has become a surrogate for ‚ and has played a singer, 2001; Forest et al., 2002; Gregory et al., 2002; Knutti central role throughout the history of IPCC in interpreting the et al., 2002), using different methods, indicate that the IPCC 1 output of numerical models, in evaluating future climate range for the climate sensitivity, 1.5 C T2x 4.5 C, is too changes from various scenarios, and in attributing the causes narrow. This is consistent with the observation that experts of observed temperature changes. routinely underestimate uncertainty (Kahneman et al., 1982; Human-induced Climate Change: An Interdisciplinary Assessment, ed. Michael Schlesinger, Haroon Kheshgi, Joel Smith, Francisco de la Chesnaye, John M. Reilly, Tom Wilson and Charles Kolstad. Published by Cambridge University Press. Ó Cambridge University Press 2007. 6 Andronova et al. Shlyakhter, 1994). The four independent estimates indicate obtained from climate simulations with time-dependent for- that there is no likelihood that 1T2x < 0.6 C (this is a parti- cing, F(t). If the radiative forcing in Eq. (1.3) were made cularly robust finding), but there is a 5% likelihood that time-independent, then dHðtÞ=dt would approach zero 1T2x 9 C. with increasing time as the climate system approaches equi- As we show here, some recent studies suggest that new librium and ‚eff(t) would approach ‚eq. Time series of the insights into the likely range of climate sensitivity may be effective climate sensitivity demonstrate how feedbacks of the possible through comparisons of models and observational climate system evolve with time. data – contemporary, historical and paleoclimatic. Other The transient climate sensitivity (after Murphy and Mitch- recent studies raise issues regarding the applicability of ell, 1995) is given by response/forcing relationships in the climate system – such as 1 ð Þ the degree of predictability of climate and the relevance of T t2x ‚trans ¼ ð1:4Þ climate predictability for estimates of climate sensitivity, and F2x the degree to which forcings such as those due to where 1T(t ) is the change in NST when the CO con- solar variability, well-mixed greenhouse gases, and aerosols 2x 2 centration increases to double the pre-industrial value in a may produce different responses. transient climate simulation, particularly one in which the CO In Section 1.2 we briefly review the history of the climate 2 concentration increases at 1% per year. Because the thermal sensitivity concept. Section 1.3 presents some recent devel- inertia of the ocean 1T(t ) < T , ‚ < ‚ . Calculations of opment of this concept. Section 1.4 discusses some of its 2x 2x trans eq the transient climate sensitivity are primarily used for com- possible future developments. Concluding remarks are given parison among coupled atmosphere/ocean general circulation in Section 1.5. models because these models take thousands of years to equilibrate. However, because the actual climate system is 1.2 History of the climate sensitivity always facing external stresses from multiple simultaneous concept (CSC) forcings, at any particular time the climate system is facing transient sensitivity. Under slowly evolving external forcings, The concept of climate sensitivity and the development of this the transient sensitivity may be approximated by ‚ . concept are directly related to empirical or model estimations eq Model-based estimation of climate sensitivity varies con- that established a linear relationship between radiative forcing siderably from model to model because of different para- and near-surface air temperature. CSC originated from the meterizations of physical processes that are not explicitly concept of the greenhouse effect, introduced by Arrhenius at resolved in the respective models, such as clouds. For exam- the end of the nineteenth century (Arrhenius, 1896). Arrhenius ple, energy balance models (EBMs), typically one- or two- defined the greenhouse effect in terms of 1T . Almost a 2x dimensional, generally predict only the NST and frequently century later, Budyko (1972) and Sellers (1969) repeated only in terms of its globally averaged value. The value of Arrhenius’s calculations using more comprehensive energy 1T obtained by EBMs ranges from 0.24 C (Newell and balance models (North, 1981), and strongly supported the 2x Dopplick, 1979) to 9.6 C (Mo¨ller, 1963). concept of the greenhouse effect. As a result, the climate Radiative-convective models (RCMs) determine the vertical sensitivity concept was promulgated. distribution of atmospheric temperature and the surface tem- Until the third IPCC report (IPCC, 2001), the concept of perature, virtually always in terms of their globally averaged climate sensitivity was based on calculations of the equili- values. As their name indicates, radiative-convective models brium NST change. However, the third IPCC report (Cubasch, include the physical processes of radiative transfer and con- 2001) defined three measures of climate sensitivity, with the vection. The value of 1T obtained by RCMs ranges from differences between the measures arising from the different 2x 0.48 C (Somerville and Remer, 1984) to 4.2 C (Wang and types of simulations performed with the climate model. Stone, 1980). The equilibrium climate sensitivity ‚ is given by Eq. (1.1) eq Atmospheric general circulation models (AGCMs) simulate and is estimated from climate simulations in which the radiative the vertical and geographical distributions of temperature, forcing does not vary with time after an initial change, such as surface pressure, wind velocity, water vapor, vertical velocity, for an instantaneous CO doubling. Most estimations were made 2 geopotential height, ground temperature and moisture, and the for ‚ , which we will review below. eq geographical distribution of snow and ice on the ground. The effective climate sensitivity (after Murphy, 1995) is AGCMs have been coupled with different ocean and sea-ice given by models. These include a swamp ocean which has zero heat 1TðtÞ capacity but infinite water, a mixed-layer ocean which has a ‚ ðtÞ¼ ð1:3Þ eff FðtÞdHðtÞ=dðtÞ prescribed depth, and a full dynamic ocean GCM, which is the oceanic counterpart of the AGCM. Sea-ice models range from where dHðtÞ=dt is the change in heat storage of the climate freezing a swamp ocean (where the temperature would system – essentially the heat taken up or lost by the ocean, otherwise drop below the freezing temperature of seawater) The concept of climate sensitivity 7 that has only thermodynamics and no transport by wind or Tol and de Vos (1998) (TdV) made one of the first inverse currents, to a fully dynamic-thermodynamic sea-ice model. In estimations of the probability density function (pdf) of climate all cases the atmosphere/ocean model has a finite horizontal sensitivity. They used a simple statistical model and Bayesian and vertical resolution. The value of 1T2x obtained by updating in combination with expert opinion and observational AGCMs ranges from 1.3 C (Washington and Meehl, 1983) to
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