Lord Lawson, in a spirited attack on the Energy Bill passing through parliament, said in the House of Lords on 18^th June 2013

There is an emerging consensus among scientists that the climate sensitivity of carbon is probably less than they thought. That means, importantly, that any dangers from warming, if they occur, are postponed well into the next century. It means that there is no urgency to go ahead in this way, not only because the uncertainties are in the distant future but because we have no idea what technologies will develop over the next 100 years.

The analysis I have developed shows Lord Lawson understates how significant the climate sensitivity issue to the problem of catastrophic warming and mitigation policy.

In my analysis, the maximum warming would be 7^oC. There can be a case for warming topping out at some level, as

• There are diminishing returns to increases in greenhouse gas on temperature.
• There are diminishing returns at some point for unit rises in emissions on levels of GHGs. That is, higher levels of GHGs in the atmosphere will lead to higher levels of absorption.

I will assume that climate sensitivity is halved, but will assume that temperatures eventually reach 5^oC above pre-industrial levels.

The simple curve to work out the consequences is the policy curve. Any constraint of greenhouse gas levels will only have half the impact on temperature. The policy curve will shift to the right to PC1.

The climate costs curve is somewhat more difficult. The elements to consider in the curve are

E(C_GW)=f {M,1/t,L,A,│Pr-E()│,r,R,W}

I have highlighted the elements to consider.

Time t will be doubled. Warming rates will therefore be halved. Some of the harmful consequences of warming are from unprecedented rapid change. For many animals and plants, it is speculated sudden change is much more damaging than a slower change. More importantly, sudden changes in average temperature could jolt climate systems into different patterns. Savannahs could become deserts, or the monsoon could shift. Another aspect to consider is that rapid warming of the tundra could release massive amounts of CH4 into the atmosphere, further accelerating warming. Or rapid warming could lead to rapid disintegration and breakup of the polar ice caps, leading to rapid acceleration of sea level rise. The slower warming will make us much less likely to cross these climate tipping points.

Adaptations A
can be phased in more gradually. For instance, with sea level rise, the Thames Barrier will no longer be adequate. A replacement to last 50 years will need to be much less extensive. If warming causes crop yields to fall by increased drought there is more time to adjust.

With changes happening more slowly, (and less chaotically), the adaptation cost errors, │Pr-E()│, are likely to be less.

For any positive rate of discount r, then the current net present value will be lower for the much extended warming period. However, as Stern had a discount value of not much different to zero, allowing for a discount rate would totally cover the other issues.

For all of these reasons, the climate cost curve will move down to CC1 and total cost curve to TC1. The point where policy costs equals climate change costs moves from A to B. That is at a significantly higher temperature, and for a much lower level of policy cost.

I have steered away from the weighting W issues. But given that sensitivity is a core issue that the climate models have got consistently wrong, then any weighting given to other predictions should be viewed with greater scepticism.