In this week’s entry, the authors decided that, rather than try to estimate ECS itself, they would try to figure out the lower bound. Their primary focus was whether it was possible that extra CO2 in the air might lead to no warming at all or even cooling. Nope, said Lennart Bengtsson and Stephen Schwartz in their paper “Determination of a lower bound on Earth’s climate sensitivity”. At any rate the lowest plausible value of ECS seems to be positive at least to some degree. But along the way they also determined that the best estimate of ECS, based on real world observations, is about 2 degrees C, pretty much the same as all the other studies in this series. And a figure at which no significant policy to limit emissions does more good than harm.
Bengtsson and Schwartz began with the observation that, from what we know of the climate system, while the full adjustment to increased atmospheric CO2 takes centuries due to the slow overturning of ocean layers, most of the warming it causes (between 70% and 80% of it) happens over a decade or less. So using observations of temperature over the past few decades is not a bad way to proceed. And since the best and most comprehensive temperature coverage of the Earth only goes back to 1970, they limited their data collection to the 1970 to 2010 interval.
They also needed to estimate how much warming might have been concealed by the cooling effects of air pollution aerosols. Here they decided to assume that aerosol cooling hasn’t declined over the post-1970 interval, which deliberately biased their calculations towards the maximum possible sensitivity to warming from CO2. And when the data were all gathered and the computations done, they determined that the lower boundary figure, the number ECS was almost certainly above, was 1.16°C. But along the way they also got a best estimate for actual ECS of 2.0°C. And they noted that their estimate “coincides with the low end of the range for equilibrium climate sensitivity expressed as CO2 doubling temperature as given by the IPCC Assessment (Solomon et al., 2007).”
So yet again the IPCC range was plausible, but only the low end of it. This paper appeared in 2013, but the IPCC went right on peddling the idea that the best estimate is about 3C, and could be as high as 4.5 or 5C, based on how climate models behave.
Back in the real world, the data are stubbornly stuck at the low end of the range. According to their study, yes, CO2 warms the atmosphere. But not as much as most climate models say it will and not enough to worry about.