According to Dr. Phil, the warming from 1915 to 1945 was due to the Sun and volcanoes. The subsequent cooling until 1975 was caused by burning coal – which produces aerosols. The subsequent warming until 1998 was caused by burning coal which produces CO2, and the subsequent cooling is again caused by burning Chinese coal which produces aerosols.
from: Phil Jones
subject: FW: Your graph regarding global temperature anomaly
There are a multitude of explanations of the course of temperature change during the 20th century. These are discussed in the various IPCC reports. The warming from 1915 to about 1940 is generally believed to be due to a slight increase in solar output and a reduction in volcanism. The recent warming from 1975 is due to the build-up of greenhouse gases, as they begin to dominate over sulphate aerosol releases. The slight cooling from 1940 to 1975 is thought to be due to industrial development (using lots of dirty coal) increasing aerosol emissions. WWII is not thought to have had any impact. On all the above there is natural variability which could have caused some of the changes even without any external forcing (greenhouse gases, aerosols, solar, volcanoes etc) as we have had warm and cold decades in the past when there was little changes in external influences. I hope this short reply helps. The best things to read are the IPCC reports.