2.5 What is happening to global temperatures?

Over the last 100 years the global temperatures have risen by over 0.74°C. The most significant rise has taken place since the 1970s and the 10 warmest years on record have occurred since 1997. Global temperatures for 2000–2008 are almost 0.2°C warmer than the average for the decade 1990–1999.

The rate of increase has not been constant. The increase in temperature stabilised and even began to fall slightly from 1945 to 1970. This prompted concerns about the possibility of the Earth entering another ice age. This halt in the rising trend is explained by two main factors. First, and mainly, an increase in sulfate aerosols from post-War increase in economic activity and a dramatic increase in the amount of coal used (before any clean air acts were passed), and volcanic activity. Sulfate aerosols have a cooling effect, but are rained out within a decade or so. Thus, the main effect was short-term cooling, partially masking the longer term enhancing of the greenhouse effect. Second, changes in the way that sea surface temperatures were collected during the 1940s account for some of the observed temperature changes.

• See the IPCC FAQ ‘How are temperatures on Earth changing?’.

• See the briefing in the New Scientist on the drop in temperature between 1940 and 1970.

• The Fourth Assessment Report from the IPCC details historical and projected temperature change; see its ‘Synthesis report – summary for policymakers’.