Can we trust climate models?

Abstract

What are the predictions of climate models, should we believe them, and are they falsifiable? Probably the most iconic and influential result arising from climate models is the prediction that, dependent on the rate of increase of CO 2 emissions, global and annual mean temperature will rise by around 2-4 ∘ C over the 21st century. We argue that this result is indeed credible, as are the supplementary predictions that the land will on average warm by around 50% more than the oceans, high latitudes more than the tropics, and that the hydrological cycle will generally intensify. Beyond these and similar broad statements, however, we presently find little evidence of trustworthy predictions at fine spatial scale and annual to decadal timescale from climate models. How to cite this article:WIREs Clim Change 2014Change , 5:435-440. doi: 10.1002 INTRODUCTION C limate models are the primary tool by which we create knowledge about the future impact of human activities on the global climate system. Possibly the most iconic and influential result arising from climate models is the prediction that, dependent on the rate of increase of CO 2 emissions, global and annual mean temperature will (with high probability) rise by around 2-4 ∘ C over the 21st century (see Ref 1, Fig SPM.5). Supplementary predictions of a similar status are that the land will on average warm by around 50% more than the oceans, high latitudes more than the tropics, and nights more than days. Besides surface temperature changes, the hydrological cycle is expected to generally intensify by a few percentage points per degree Celsius of warming, and the stratosphere will cool. These results are broadly agreed upon by all global climate models (GCMs) which have contributed to the Climate Model * Correspondence to: jules@blueskiesresearch.org.uk Uncertainty Estimation Research Group, RIGC/JAMSTEC, Yokohama, Japan Conflict of interest: The authors have declared no conflicts of interest for this article.Inter-comparison Project (CMIP) experiments 2 over the past several decades, although the magnitudes of the expected changes remain somewhat uncertain. Many more detailed predictions can, in principle, be made, for example on a regional basis, but uncertainty tends to increase substantially as the spatial scale decreases. For example, even the sign of the change in precipitation is uncertain in many areas over the coming decades.But should we believe any or all of these predictions? And if so, which ones, and why? These are the fundamental questions which we hope to address in this article. We start by exploring the origins of the models and considering the nature of the knowledge that they impart. We overview the strengths and weaknesses of the models and then consider to what extent these models may be falsifiable or considered trustworthy. THE ORIGINS OF CLIMATE MODELSScientific models are simplifications of nature built to gain better understanding of how nature works. They are of course, trivially false in the sense that all models are approximations to ...

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