Secy. of State John Kerry last week exhorted all State Department officials to conclude a new international climate change agreement, integrate climate change with other priorities, and, in general, “elevate the environment in everything we do.” In the same week, climate researchers produced two more studies undercutting Kerry’s opinion that climate change is “perhaps the world’s most fearsome weapon of mass destruction.”
The studies address the core scientific issue of climate sensitivity — the question of how much warming results from a given increase in atmospheric carbon dioxide (CO2) concentrations.
There are two types of sensitivity estimates. Equilibrium climate sensitivity (ECS) is an estimate of the increase in ‘steady state’ surface temperature after the climate system has fully adjusted to a doubling of CO2 concentrations — a process assumed to take centuries or longer due to oceanic thermal inertia. Transient climate sensitivity (TCS) is the estimated increase in surface temperature during the 20-year period when CO2 doubling occurs, presumably during the final decades of this century.
ECS is the key variable in both climate model predictions of future global warming and model estimates of the “social cost of carbon” – the damage allegedly inflicted on society by an incremental ton of CO2 emissions.
The IPCC’s 2007 Fourth Assessment Report (AR4) estimated a “likely” ECS range of 2°C-4.5°C, with a “best estimate” of 3°C. Since 2011, however, the warming pause and the growing divergence of model predictions and observed global temperatures have been the impetus for several studies finding that IPCC sensitivity estimates are too hot.
Cato Institute scientists Patrick Michaels and Chip Knappenberger maintain a growing list of such studies, which totaled 18 as of February 2014.
The average sensitivity estimate of the 18 studies is just under 2°C. In other words, the AR4 “best estimate” of 3°C is 50% higher than the mean estimate of the new studies. That may be why the IPCC’s 2013-2014 Fifth Assessment Report (AR5) declines to offer a “best estimate.”
A new “best estimate” of 2°C would deflate the scary climate change impacts featured elsewhere in AR5, but recycling the same old 3°C “best estimate” would deflate the IPCC’s claim to be an honest broker. So instead the IPCC chose to lower the low end of the “likely” sensitivity range. Whereas the “likely” range in AR4 was 2°C-4.5°C, in AR5 it is 1.5°C-4.5°C.
That small concession, however, does not dispel the growing challenge to consensus climatology. As indicated in the chart above, the average sensitivity of the climate models used in AR5 is 3.2°C. That is 60% higher than the mean of recent estimates (<2°C). Let’s take a quick look at three studies that have come out this year. [click to continue…]