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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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. Volcano

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

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".

Intermediate rebuttal written by John Cook

Update August 2015:

Updated on 2015-08-08 by MichaelK.

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What the science says...

Papers studying climate change over the early 20th century