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What caused early 20th Century warming?

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The skeptic argument...

It warmed before 1940 when CO2 was low

"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...

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Early 20th century warming is due to several causes, including rising CO2.

The climate at any one time is affected by several factors which can act independently or together. The main factors include solar variability, volcanic activity, atmospheric composition, the amount of sunlight reflected back into space, ocean currents and changes in the Earth's orbit.

Before 1940, the increase in temperature is believed to have been caused mainly by two factors:

Increasing solar activity; and
Low volcanic activity (as eruptions can have a cooling effect by blocking out the sun).

Other factors, including greenhouse gases, also contributed to the warming and regional factors played a significant role in increasing temperatures in some regions, most notably changes in ocean currents which led to warmer-than-average sea temperatures in the North Atlantic. Does this mean that solar activity is also primarily responsible for late 20th century warming? In short, no. Solar activity since the 1950s has been relatively stable and therefore cannot explain recent trends. Similarly, increased volcanic activity may actually have had a cooling effect in recent decades. On the other hand, greenhouse gas concentrations, which were relatively low pre-1940, have increased considerably and are now dominating the climate system. This highlights the need to look at all factors when determining which factors are likely to be affecting climate at any one time.

In short, there's no reason to assume that because the sun was responsible for early 20th century, it is responsible for all warming. The evidence strongly suggests that current warming is mainly the result of increasing greenhouse gas levels.

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Comments

Comments 1 to 4:

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?
barry at 00:02 AM on 28 March, 2010
"Also, after a burst of volcanic activity in the late 18th century, there was a relative quiet volcanic period in the early 20th century."

Did you mean 19th century for the former period?