Models overestimate 60 year decadal trends

Watts Up With That?

Guest essay by Clive Best

Marotzke & Forster(2015) found that 60 year trends in global surface temperatures are dominated by underlying climate physics. However, the  data show that climate models overestimate such 60 year decadel trends after 1940.

Comparison of 60y trends in observations and models (see text for details).

The recent paper in Nature by Jochem Maritzke & Piers Forster ‘Forcing, feedback and internal variability in global temperature trends’ has gained much attention because it makes the claim that climate models are just fine and do not overstimate warming despite the observed 17 year hiatus since 1998. They attempt to show this by demonstrating that 15y trends in the Hadcrut4 data can be expected in CMIP5 models through quasi random internal variability, whereas any 60y trends are deterministic (anthropogenic). They identify ‘deterministic’ and ‘internal variability’ in the models through a multi-regression analysis with their known forcings as input.

Delta{T} = frac{Delta{F}}{(alpha + kappa)} + epsilon

where Delta{F} is the forcing, alpha is a climate feedback and kappa is fraction of ocean heat uptake and epsilon is…

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