Why models run hot: results from an irreducibly simple climate model

Abstract

An irreducibly simple climate-sensitivity model is designed to empower even non-specialists to research the question how much global warming we may cause. In 1990, the First Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) expressed ''substantial confidence'' that near-term global warming would occur twice as fast as subsequent observation. Given rising CO 2 concentration, few models predicted no warming since 2001. Between the pre-final and published drafts of the Fifth Assessment Report, IPCC cut its near-term warming projection substantially, substituting ''expert assessment'' for models' near-term predictions. Yet its long-range predictions remain unaltered. The model indicates that IPCC's reduction of the feedback sum from 1.9 to 1.5 W m -2 K -1 mandates a reduction from 3.2 to 2.2 K in its central climate-sensitivity estimate; that, since feedbacks are likely to be net-negative, a better estimate is 1.0 K; that there is no unrealized global warming in the pipeline; that global warming this century will be \1 K;and that combustion of all recoverable fossil fuels will cause \2.2 K global warming to equilibrium. Resolving the discrepancies between the methodology adopted by IPCC in its Fourth and Fifth Assessment Reports that are highlighted in the present paper is vital. Once those discrepancies are taken into account, the impact of anthropogenic global warming over the next century, and even as far as equilibrium many millennia hence, may be no more than one-third to one-half of IPCC's current projections.

Keeping it simple: the value of an irreducibly simple climate model

“…Outputs from the general-circulation models cited in IPCC’s five Assessment Reports FAR, SAR, TAR, AR4 and AR5 [1–5] were examined in [6] using a simple model calibrated against IPCC’s central climate-sensitivity estimates in AR4-5 and were found to overestimate observed air temperature trends. Reasons for this hot running were discussed.…”

“…In [6], it was concluded that once due allowance is made for these and other shortcomings in the general-circulation models, the likely global warming in response to a doubling of CO 2 concentration is not 3.3 K but 1 K or less and that even if all available fossil fuels are combusted <2.2 K warming will result.…”

“…The irreducibly simple model presented in [6], encapsulated in (1), determines the surface temperature response Δ T t to anthropogenic radiative forcings and consequent temperature feedbacks over any given period of years t :where is the fraction of total anthropogenic forcing represented by CO 2 over t years, and its reciprocal allows for non-CO 2 forcings as well as the CO 2 forcing; Δ F t is the radiative forcing in response to a change in atmospheric CO 2 concentration over t years, which is the product of a constant k and the proportionate change ( C t / C 0 ) in CO 2 concentration over the period, where C 0 is the unperturbed concentration ([36]; TAR, ch. 6.1); r t is the transience fraction, which is the fraction of equilibrium sensitivity expected to be attained over t years, allowing any delayed-response profile to be modeled as a time series; λ ∞ is the equilibrium climate-sensitivity parameter, which is the product of the Planck or zero-feedback sensitivity parameter λ 0 (AR4, p. 631 fn.)…”

“…doi:10.1007/s11434-015-0806-z) suggest that the irreducibly simple climate model described in Monckton of Brenchley et al (Sci Bull 60:122–135, 2015. doi:10.1007/s11434-014-0699-2) was not validated against observations, relying instead on synthetic test data based on underestimated global warming, illogical parameter choice and near-instantaneous response at odds with ocean warming and other observations.…”

A mental picture of the greenhouse effect

“…An interesting question is whether reviving this old physics-model in the context of modern data can convince the scientifically literate who still do not acknowledge an AGW. This conceptual model also explains why some contrarian views based on IR flux are flawed (Monckton et al 2015): They do not account for convection. There is a cause for optimism if the reason for different views is 'agnotological' in terms of forgotten ideas (Proctor and Schiebinger 2008).…”