It may help to clarify some of the facts related to the lively exchange between Bob Ward and Lord Ridley about the Transient Climate Response (TCR). The TCR is defined by the IPCC as “the change in the global mean surface temperature, averaged over a 20-year period, centred at the time of atmospheric carbon dioxide doubling, in a climate model simulation in which CO2 increases at 1% yr – 1.”
Lord Ridley is right that the IPCC in its recently published AR5 Working Group I report gave an estimated likely (66–100% probability) range for TCR of 1 – 2.5°C. The IPCC also increased its confidence that the TCR is not greater than 3.0°C though it is not immediately clear what confidence the IPCC places on the 1°C lower limit, rather than the range it provides.
As Bob Ward notes, however, the TCR is unlikely to reflect how global mean surface temperatures might increase during this century, as claimed by Lord Ridley. This is for two reasons. First, our carbon dioxide emissions are rising by around three percent not one percent per year. Second, the TCR only refers to changes due to carbon dioxide; climate change will be greater than this due to increases in other greenhouse gases such as methane and nitrous oxide, though it is also offset to some degree by atmospheric aerosols.
Our current emissions are broadly in line with the most emissions intensive of the IPCC’s scenarios for AR5 – the so-called RCP8.5 scenario. This is likely to lead to an average increase in global mean surface temperature for the final two decades of this century of 3.2-5.4°C relative to the second half of the nineteenth century. This is a huge change and will be even larger in high latitudes and over continents. The scale of change is broadly comparable to that which separates our present climate from that of the Ice Age.
I don’t think we have any clear idea of what the full economic, social and political impact of such huge changes would be. I am all in favour of policies that lift people out of poverty, insecurity and hunger now. But that does not mean we should ignore the huge risks we are taking with the climate to which humans have become adapted. Presenting this as a sharp dichotomy is wrong.
The “transient climate response” is a model metric – rather than a projection of climate change. It was primarily developed to facilitate comparisons between different climate models and their rate of climate change. In part, it was created because the previous model metric of “climate sensitivity” (the equilibrium global mean temperature response to an instantaneous doubling of carbon dioxide) gives only part of the story in a world where carbon dioxide levels are constantly changing. The “transient climate response” also incorporates some knowledge about the rate of ocean heat uptake. It is something we modellers find useful for our inter-comparison studies, but little more than that.
Chris