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

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 Inter- governmental Panel on Climate Change （IPCC） expressed ＂substantial confidence＂ that near-term global warming would occur twice as fast as subsequent observation. Given rising CO2 concentration, few models predicted no wann- ing 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 indi- cates 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 feed- backs 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 dis- crepancies are taken into account, the impact of anthro- pogenic 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

Richardson et al. （Sci Bull, 2015. doi：10.1007/ sl1434-015-0806-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-in- stantaneous response at odds with ocean warming and other observations. However, the simple model, informed by its authors＇ choice of parameters, usually hindcasts observed temperature change more closely than the general-circu- lation models, and finds high climate sensitivity implausi- ble. With IPCC＇s choice of parameters, the model is further validated in that it duly replicates IPCC＇s sensitivity interval. Also, fast climate system response is consistent with near-zero or net-negative temperature feedback. Given the large uncertainties in the initial conditions and evolutionary processes determinative of climate sensitivity, subject to obvious caveats a simple sensitivity-focused model need not, and the present model does not, exhibit significantly less predictive skill than the general-circula- tion models.