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Two climate scientists behind a significant paper on the accuracy of climate models have forcefully rejected a potentially serious criticism of their methods made by another climate scientist.

Jochem Marotzke of the Max Planck Institut and Piers Forster of the University of Leeds have posted a blog article refuting the criticisms of their methodology made by independent climate scientist Nic Lewis.

The Marotzke and Forster paper, published in the prestigious journal Nature, addresses the apparent failure of computer climate models to predict the so called pause in global warming and the implication that the warming effect of greenhouse gases may be less than is widely assumed.

This failure of computer climate models to predict the pause is a key criticism of climate science made by climate sceptics and has prompted a number of climate scientists to suggest that computer models may be systematically overestimating the warming impact of greenhouse gases. Indeed, Lewis was co-author of an important paper published last year arguing that climate sensitivity was less than that assumed in the climate models used by the Intergovernmental Panel on Climate Change (IPCC). 

The Marotzke and Forster paper, entitled Forcing, feedback and internal variability in global temperature trends, concludes that unpredictable and chaotic random fluctuations in the climate system are to blame for the failure to predict the pause rather than short comings in the climate models themselves and – crucially – that there is no evidence from recent observations to support the suggestion that the models systematically overestimate the response of the climate system to greenhouse gas warming.

But this conclusion has been challenged by Lewis who told that the methodology of the Nature paper was “not valid mathematically and not valid statistically”.

Lewis has set out his criticisms in a highly technical and mathematical article posted on the Climate Audit blog site run by climate sceptic Steve McIntyre. Lewis argues that the methods described in the paper use “circular logic” and wrote: “I was slightly taken aback by the paper, as I would have expected either one of the authors or a peer reviewer to have spotted the major flaws in its methodology”.

But this view is rejected by the authors of the Nature study. The variations in the results of the climate model simulations described in the paper are “totally dominated by the internal variability of the climate system”, co-author Jochem Marotzke told “We totally reject the accusations of circularity“. 

Lewis argues that the main flaw in the Marotzke and Forster research concerns the use of an equation describing energy balance in the climate system that links the so called radiative forcing acting to warm the climate with the sensitivity of the climate to changes in greenhouse gas levels. This equation is central to the logic used by Marotzke and Forster.

Lewis states that the values used for two key quantities in this equation, concerning the sensitivity of the climate and the rate of heat uptake by the climate, were themselves generated by computer climate models that perform calculations using these same two variables. Lewis writes: “This circularity fundamentally invalidates the regression model assumptions. Accordingly, reliance should not be placed on any of the results in the Nature paper.”

In a strongly worded rejection posted on the Climate Lab Book blogsite run by climate scientist Ed Hawkins, Marotzke and Forster state bluntly: “This allegation is incorrect”.

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