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What caused early 20th Century warming?

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The skeptic argument...

"Of the rise in temperature during the 20th century, the bulk occurred from 1900 to 1940. It was followed by the aforementioned cooling trend from 1940 to around 1975. Yet the concentration of greenhouse gases was measurably higher in that later period than in the former. That drop in temperature came after what was described in the National Geographic as 'six decades of abnormal warmth'." (James Schlesinger)

What the science says...

Early 20th century warming was in large part due to rising solar activity and relatively quiet volcanic activity. However, both factors have played little to no part in the warming since 1975. Solar activity has been steady since the 50's. Volcanoes have been relatively frequent and if anything, have exerted a cooling effect.

A widespread misconception is that anthropogenic global warming has been dominant over the last century. In actuality, CO2 warming has only been dominant since the "modern global warming trend" began in the mid-70's.

To understand climate change, you need to factor in all the various forcings that influence global temperature. In the early 20th century, while CO2 levels were much smaller, solar activity was on the rise. Also, after a burst of volcanic activity in the late 18th century, there was a relative quiet volcanic period in the early 20th century. These were two dominant factors in the warming from 1900 to 1940.

However, both factors have played little to no part in the warming since 1975. Solar activity has been steady since the 50's. Volcanoes have been relatively frequent in the last few decades and if anything, have exerted a cooling effect that has somewhat masked the CO2 warming effect.

Papers studying climate change over the early 20th century

Estimation of natural and anthropogenic contributions to twentieth century temperature change (Tett 2002): "During 1907–57 we found that there was negligible net anthropogenic warming with the effect of greenhouse gases largely being balanced by other anthropogenic forcings. Therefore, in this period, the warming was largely naturally caused. The late century warming was largely explained by greenhouse gases offset by the effect of volcanic aerosol and the indirect effect of anthropogenic aerosols. Over the entire century natural forcings make no net contribution as they warm early in the century and cool from the 1960s on."
Solar Forcing of Global Climate Change Since The Mid-17th Century (Reid 1997) finds a link between solar variability and climate change, concluding that "solar forcing and anthropogenic greenhouse-gas forcing made roughly equal contributions to the rise in global temperature that took place between 1900 and 1955".

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Comments

Comments 1 to 3:

Mizimi at 23:30 PM on 18 September, 2008
One point to consider: oil and gas as fuels did not come into widespread use until the mid 1950's and began to supplant coal from 1960 onwards.
Coal was the major fuel before 1940 and emission controls virtually non-existant, so there would also have been a cooling effect from aerosols to (partially) balance GG emissions.
Quietman at 07:09 AM on 16 October, 2008
The fact of the matter is that there is no actual evidence for the 30 years of warming attributed to CO2 to be anything other than natural. Our contribution, of various sources, not just CO2 is negligable. Most of the arguments presented against CO2 have some merit and the answer is most likely in the combination of all.
chris at 08:28 AM on 24 October, 2008
Re: #2 (and #1)

..and yet there is a massive amount of evidence that the warming of the last 30 years is not "natural". The warming has followed the truely massive increases in CO2 emissions especially since the 1960's.

Atmospheric CO2 concentrations rose rather slowly throughout the early 20th century. They were approaching 300 ppm in 1900 and reached 320 ppm in 1962. Since then we've raced up to 386 ppm. That's the likely source of the large scale global warming of the last 30-odd years. So to suggest that "our contribution, of various sources, not just CO2 is negligible" (whatever that might mean!) just doesn't accord with the real world evidence.

I'd like to know what these "arguments presented against CO2" that "have some merit" actually are...can we have a list please?