Saturday, August 27, 2016

Corrections to the Global Temperature Record and the Early Onset of Industrial Age Warming

A lot of fuss is often made about adjustments to estimates of global temperature. But here is the key figure from the paper by Karl et al. last year:

The changes to the temperature trend in the period of the "hiatus" are really small and hard to see in the context of the century scale temperature trend (Panel A). Most of the effect of corrections is in the 19th Century (Panel B). But even there, all the corrections make little difference to the overall trend.

The next graph shows 3 different estimates of global temperature since 1955:

The HADCRUT data has been criticized for not covering heating in the Arctic very well. The GISS series shows more warming due to that. But really the two series are not that different in the overall signal they provide. The Berkeley Earth series is somewhere in between these two. Berkeley Earth was funded by Koch and others to investigate whether the series from official agencies had distorted the record by their use of corrections. The result turned out almost the same as NASA's (GISS). So, there definitely doesn't seem to be any conspiracy to distort the data.

Graham Lloyd also mentions the paper published by Nerilie Abram et al. in Nature this week, that argues that "industrial era warming" began earlier than previously thought. Here is a key figure from their paper:

The brown graph is the reconstructed land surface temperature anomaly and the blue graph the sea surface anomaly. Their argument is based on the current warming trend starting in the first half of the 19th Century. But I don't see anything in the paper that actually associates this with anthropogenic forcing. So I tend to somewhat agree with the quote from Michael Mann "the Abram team misinterprets the cooling of the early 1800s from two giant volcanic eruptions as a cooler baseline instead of something unusual. That makes it look like human-forced warming started earlier than it did instead of climate naturally recovering from volcanoes putting cooling particles in the air". The paper compares the onset of warming in simulations to the onset of warming in the data and there is almost no correlation between the model results and the data (Panel A):

Yes, greenhouse gases were increasing already (CO2 chart is shown in the bottom right hand corner of the Figure) but it's likely that they only contributed a small part to the warming in that period and much of it is a bounceback from the volcanic eruptions, which had suppressed temperature.

The idea that people have been affecting the climate for a long time was first introduced by Ruddiman in a 2003 paper. I think that Ruddiman was likely right about this. My guess is that what we are seeing in the early 19th Century is mostly still the Ruddiman effect of increased human population, land-clearing, farming etc. Industrial CO2 emissions were very low: 54 million tonnes of carbon a year in 1850.

I'm working on a new climate change paper with Zsuzsanna Csereklyei and Stephan Bruns for the conference on climate econometrics in Aarhus at the end of October. We have got all the data together and we've reviewed the literature and so now comes the modeling phase. Watch this space :)

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