What caused early 20th Century warming?

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.

Climate Myth: 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)

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 19th 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

Intermediate rebuttal written by John Cook


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