A commentary published in Nature this week has opened up a discussion about the value of using the goal of keeping global warming to below 2°C.
David Victor and Charles Kennel are concerned that the below 2°C target for global warming is not useful, partly because they consider it is no longer achievable and partly because global mean surface temperature does not present a full picture of climate change. The problem comes, of course, in identifying an alternative approach to establishing what is required from attempts to mitigate global warming.
The 2 degree target is in a sense nominal, in that it there is no precise threshold at which everything goes from bearable to unbearable, but it does have the advantage of being easy to understand, for both policy makers and the wider public . The proposed alternative indicators, including ocean heat content and high latitude temperature, have scientific validity but the implications of changes in these parameters may not be obvious to people living away from these areas. Furthermore, monitoring any measure on a real time basis will not avoid the intrinsic variability seen in the global temperature record. Ocean heat content shows an apparently unremitting upward trend at present but a climate change denialist would have been happy to point out a “hiatus” in that trend during the 1960s.
Victor and Kennel also state that “the 2°C target has allowed politicians to pretend that they are organizing for action when, in fact, most have done little”. This criticism would apply to any target – providing it is still at such sufficient distance to remain broadly plausible – and their proposal for “a global goal for average [greenhouse gas] concentrations in 2030 or 2050” would provide equal opportunity for prevarication.
According to a review of recent emissions reduction modelling studies conducted for the AVOID 2 programme co-authored by Ajay Gambhir of the Grantham Institute, it is still possible to meet the 2 degrees C target, provided that a broad portfolio of technologies is available and that there are no significant delays in global coordinated mitigation action. A continuation of relatively weak policies to 2030, or the absence of specific technologies such as carbon capture and storage, could however greatly increase mitigation costs and in some models render the target unachievable.
Nevertheless, Victor and Kennel are right to point out the problems with the current over-simplistic approach and I hope that their initiation of a search for “indicators of planetary health” will spur someone to invent a useful new measure for monitoring and assessing climate change.