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Lewis and Crok: Climate less sensitive to CO2 than models suggest

By Judith Curry | Climate Etc. | March 5, 2014

Nic Lewis and Marcel Crok have published a new report on climate sensitivity.

The title of the report is “A sensitive matter: How the IPCC buried evidence showing good news about global warming.”  The report is published by the GWPF. The long version of the report is found [here]; a short version is found [here].

From the press release issued by the GWPF:

A new report published by the Global Warming Policy Foundation shows that the best observational evidence indicates our climate is considerably less sensitive to greenhouse gases than climate models are estimating.

The clues for this and the relevant scientific papers are all referred to in the recently published Fifth Assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). However, this important conclusion was not drawn in the full IPCC report – it is only mentioned as a possibility – and is ignored in the IPCC’s Summary for Policymakers (SPM).

For over thirty years climate scientists have presented a range for climate sensitivity (ECS) that has hardly changed. It was 1.5-4.5°C in 1979 and this range is still the same today in AR5. The new report suggests that the inclusion of recent evidence, reflected in AR5, justifies a lower observationally-based temperature range of 1.25–3.0°C, with a best estimate of 1.75°C, for a doubling of CO2. By contrast, the climate models used for projections in AR5 indicate a range of 2-4.5°C, with an average of 3.2°C.

This is one of the key findings of the new report Oversensitive: how the IPCC hid the good news on global warming, written by independent UK climate scientist Nic Lewis and Dutch science writer Marcel Crok. Lewis and Crok were both expert reviewers of the IPCC report, and Lewis was an author of two relevant papers cited in it.

In recent years it has become possible to make good empirical estimates of climate sensitivity from observational data such as temperature and ocean heat records. These estimates, published in leading scientific journals, point to climate sensitivity per doubling of CO2 most likely being under 2°C for long-term warming, with a best estimate of only 1.3-1.4°C for warming over a seventy year period.

“The observational evidence strongly suggest that climate models display too much sensitivity to carbon dioxide concentrations and in almost all cases exaggerate the likely path of global warming,” says Nic Lewis.

These lower, observationally-based estimates for both long-term climate sensitivity and the seventy-year response suggest that considerably less global warming and sea level rise is to be expected in the 21st century than most climate model projections currently imply.

“We estimate that on the IPCC’s second highest emissions scenario warming would still be around the international target of 2°C in 2081-2100,” Lewis says.

I was asked to review this article prior to publication, and then was subsequently asked to write the foreword.  The text of my foreword:

The sensitivity of our climate to increasing concentrations of carbon dioxide is at the heart of the scientific debate on anthropogenic climate change, and also the public debate on the appropriate policy response to increasing carbon dioxide in the atmosphere. Climate sensitivity and estimates of its uncertainty are key inputs into the economic models that drive cost-benefit analyses and estimates of the social cost of carbon.

The complexity and nuances of the issue of climate sensitivity to increasing carbon dioxide are not easily discerned from reading the Summary for Policy Makers of the assessment reports undertaken by the Intergovernmental Panel on Climate Change (IPCC). Further, the more detailed discussion of climate sensitivity in the text of the fullWorking Group I reports lacks context or an explanation that is easily understood by anyone not actively reading the published literature.

This report by Nic Lewis andMarcel Crok addresses this gap between the IPCC assessments and the primary scientific literature, providing an overview of the different methods for estimating climate sensitivity and a historical perspective on IPCC’s assessments of climate sensitivity. The report also provides an independent assessment of the different methods for estimating climate sensitivity and a critique of the IPCC AR4 and AR5 assessments of climate sensitivity.

It emphasizes the point that evidence for low climate sensitivity is piling up. I find this report to be a useful contribution to scientific debate on this topic, as well as an important contribution to the public dialogue and debate on the subject of climate change policy.

I agreed to review this report and write this Foreword since I hold both authors of this report in high regard. I have followed with interest Nic Lewis’ emergence as an independent climate scientist and his success in publishing papers in major peer-reviewed journals on the topic of climate sensitivity, and I have endeavored to support and publicize his research. I have interacted with Marcel Crok over the years and appreciate his insightful analyses, most recently as a participant in climatedialogue.org.

The collaboration of these two authors in writing this report has resulted in a technically sound, well-organized and readily comprehensible report on the scientific issues surrounding climate sensitivity and the deliberations of the IPCC on this topic.

While writing this Foreword, I considered the very few options available for publishing a report such as this paper by Lewis and Crok. I am appreciative of the GWPF for publishing and publicizing this report. Public accountability of governmental and intergovernmental climate science and policy analysis is enhanced by independent assessments of their conclusions and arguments.

JC comments:  I did think twice about writing a foreword for a GWPF publication.  I try to stay away from organizations with political perspectives on global warming.  That said, GWPF has done some commendable things, notably pushing for inquiries into the Climategate affair.  And there really are very few options for publishing a report like this.

I think it is important to put forward alternative assessments of the key elements of the climate change debate — alternative to reports issued by the IPCC, the UK MetOffice, and the RS/NAS.

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