Bjorn Lomborg has a new paper published in the Global Policy journal, titled: Impact of Current Climate Proposals. (hattip Bishop Hill and WUWT)

From the Abstract

This article investigates the temperature reduction impact of major climate policy proposals implemented by 2030, using the standard MAGICC climate model. Even optimistically assuming that promised emission cuts are maintained throughout the century, the impacts are generally small. ………… All climate policies by the US, China, the EU and the rest of the world, implemented from the early 2000s to 2030 and sustained through the century will likely reduce global temperature rise about 0.17°C in 2100. These impact estimates are robust to different calibrations of climate sensitivity, carbon cycling and different climate scenarios. Current climate policy promises will do little to stabilize the climate and their impact will be undetectable for many decades.

That is pretty clear. COP21 in Paris is a waste of time.

An alternative estimate is provided in a paper by Boyd, Turner and Ward (BTW) of the LSE Grantham Institute, published at the end of October.

They state

The most optimistic estimate of global emissions in 2030 resulting from the INDCs is about halfway between hypothetical ‘business as usual’ and a pathway that is consistent with the 2°C limit

The MAGICC climate model used by both Lomborg & the IPCC predicts warming of about 4.7°C under BAU, implying up to a 1.35°C difference from the INDCs, compared to the 0.17°C maximum calculated by Lomborg, 8 times the amount. Lomborg says this is contingent on no carbon leakage (exporting industry from policy to non-policy countries), whilst citing studies showing that it could offset 10-40%, or even over 100% of the emissions reduction. So the difference between sceptic Lomborg and the mighty LSE Grantham Institute is even greater than 8 times. Yet Lomborg refers extensively to the August Edition of BTW. So why the difference? There is no explicit indication in BTW of how they arrive at their halfway conclusion. nor a comparison by Lomborg.

Two other estimates are from the UNFCCC, and Climate Action Tracker. Both estimate the INDCs will constrain warming to 2.7°C, or about 2.0°C below the MAGICC BAU scenario. They both make assumptions about massive reductions in emissions post 2030 that are not in the INDCs. But at least the UNFCCC and CAT have graphs that show the projection through to 2100. Not so with BTW.

This is where the eminent brain surgeons and Nobel-Prize winning rocket scientists among the readership will need to concentrate to achieve the penetrating analytical powers of a lesser climate scientist.

From the text of BTW, the hypothetical business as usual (BAU) scenario for 2030 is 68 GtCO2e. The most optimistic scenario for emissions from the INDCs (and pessimistic for economic growth in the emerging economies) us that 2030 emissions will be 52 GtCO2e. The sophisticated climate projection models have whispered in code to the climate scientists that to be on target for the limit of 2.0°C, 2030 emissions show be not more than 36 GtCO2e. The mathematicians will be able to determine that 52 is exactly halfway between 36 and 68.

Now for the really difficult bit. I have just spent the last half hour in the shed manically cranking the handle of my patent beancounter extrapolator machine to get this result. By extrapolating this halfway result for the forecast period 2010-2030 through to 2100 my extrapolator tells me the INDCs are halfway to reaching the 2.0°C maximum warming target.

As Bob Ward will no doubt point out in his forthcoming rebuttal of Bjorn Lomborg’s paper, it is only true climate scientists who can reach such levels of analysis and understanding.

I accept no liability for any injuries caused, whether physical or psychological, by people foolishly trying to replicate this advanced result. Please leave this to the experts.

But there is a serious side to this policy advocacy. The Grantham Institute, along with others, is utterly misrepresenting the effectiveness of policy to virtually every government on the planet. Lomborg shows by rigorous means that policy is ineffective even if loads of ridiculous assumptions are made, whether on climate science forecasting, policy theory, technological solutions, government priorities, or the ability of  current governments to make policy commitments for governments for decades ahead. My prediction is that the reaction of the Grantham Institute, along with plenty of others, is a thuggish denunciation of Lomborg. What they will not consider is the rational response to wide differences of interpretation. That is to compare and contrast the arguments and the assumptions made, both explicit and implicit. 

Kevin Marshall