We have encountered Nicholas Lewis previously in this series, and will again subsequently, because he’s done such outstanding work in this area. Including in today’s entry, where he teamed up with now retired Georgia Tech climatologist Judith Curry to review how the updated data in the then-newly published IPCC 5th Assessment Report might affect empirical ECS estimates. In their paper, published in Climate Dynamics in 2015, they also examined whether the warming hiatus prior to 2015 was having much effect on ECS estimates. Their analysis yielded a new ECS best estimate of 1.64° C, very similar to Lewis’ 2013 estimate. But they also showed that it wasn’t a fluke due to the recent hiatus: pretty much any time span they looked at gave them the same answer.
The method Lewis and Curry used requires picking a base period and an end period, then estimating the changes in between in temperature, radiative forcings and ocean heat content. There isn’t much good data on the latter prior to 1950 so they had to use climate model-generated estimates. It was on the forcing data, such as cooling due to aerosols and warming due to solar output, that they wanted to take stock of the new and improved data series available in the new IPCC report.
Aerosols are indirectly key to ECS estimates because if they do a lot of cooling, then there must have been a lot of warming from greenhouse gases to offset it and yield the modest net warming we actually observe. By contrast, if aerosols don’t do much cooling then greenhouse warming can’t be as strong either given, again, the modest real-world warming. So Lewis and Curry added up all the numbers in the IPCC report and found that the overall aerosol cooling effect had been scaled back over the previous decade. Meaning ECS had to be as well.
Doing the usual number crunching that is Lewis’s particular speciality, comparing the 1995-2011 period to the 1859-1882 base period indicates an ECS best estimate of just 1.64° C. And to check that that wasn’t a fluke of the hiatus, they extended the final period to 1971-2011 and the ECS best estimate dropped further to 1.56° C.
When the IPCC’s 5th report came out all the hoopla and excitement was focused on the usual overwrought summaries and alarmist misrepresentations that politicians and journalists love to deliver. Fortunately a few dedicated scientists dug deeper into the numbers to see what the report really had to say. And as Lewis and Curry showed, its actual message was that in the real world #ECS was low and getting lower.