Sunday, May 19, 2019

How Much Had It Warmed by the 1940s?

Since some of you are discussing natural vs manmade warming in the comments, I whipped up this little chart that shows how much warming has occurred, when.

Click to enlarge
Blue dots are the decadal averages, and red dots are the cumulative sum of the decadal changes -- how a decade compares to the decade before it.

So about 30% of today's warming of almost 1°C happened by the 1940s, and, after a little cooling until the 1970s, about 80% of overall warming has happened since then.

What caused the warming before the '40s? I've actually asked a scientist or two about this over the years, and the answer I've gotten is
  1. some increase in solar irradiance from about 1910-1940
  2. greenhouse gases, especially CO2
  3. reversing of ice-albedo feedback initially caused by volcanic aerosols, which together caused the Little Ice Age
The increase in solar irradiance was maybe 1 W/m2, which is good for about 0.1°C of warming.

By 1945 atmospheric CO2 was about 310 ppm, compared to 1850's 285 ppm, which gives a forcing of about 0.45 W/m2. (Compared to today's value of about 2 W/m2. Remember, logarithms change fastest in the beginning.) If climate sensitivity is 3°C, that'd be a warming of about 0.35°C. But I should probably use the transient climate response here, which is the temperature change at the time of CO2 doubling, i.e. without the long-term feedbacks. If it's 1.5°C, that's a warming of about 0.15-0.2°C.

I don't know about the ice-albedo feedback, but anyway it's a feedback not a forcing. But it would have caused some warming.

So it's likely that 90% or more of warming-to-date is from man. This is at a time when, now, the climate should be naturally cooling, due to a slight decline in solar irradiance since the 1950s, and some slight Milankovitch cooling. (I don't have a numerical handle on the latter, but the cycles change so slowly (fastest is 40,000 years) it's gotta be tiny.) This is why some scientists I see say man is responsible for 110% of warming since 1850.

Does this sound about right?


Victor Venema said...

This is a good review on the topic:
Hegerl G.C., Broennimann S., Schurer A., Cowan T. (2018): The early 20th century warming . WIRES climate change, Vol 9, e522.

One should remember that the warming estimates in this period are much less reliable. The IPCC figure with the forcings is also helpful.


TL;DR: It is complicated, many causes contributed a little, greenhouse gases were a relatively big one. Climate "sceptics" pretending that in this period all warming was natural because not all warming was manmade are behaving like typical climate "sceptics".

P.S. We did not start from zero. So that the greenhouse warming responds logarithmically to the CO2 concentration does not make that much of a difference, but a bit.

David Appell said...

Good points, thanks Victor.

JoeT said...

I see that Victor is the third author on the brand new paper by Karsten Haustein et al, "A limited role for unforced internal variability in 20th century warming." Nice write-up at CarbonBrief:

Seems that the difference in the new paper from the Hegerl paper referenced above is that Hegerl says that natural variability has a significant contribution. But perhaps this is limited only to regional influences, not global. Perhaps Victor would care to comment?

Victor Venema said...

My paper with Karsten is about internal (unforced) variability of the climate system. Solar variations and volcanoes are natural, but are forced variations, not internal, they change the forcing at the top of the atmosphere. So the conflict is not as large as it may seem.