Over 100 years ago, the Swedish chemist Svante Arrhenius first pointed out that the release of carbon dioxide into the atmosphere by the burning of fossil fuels was warming the Earth. At the time, neither he nor anybody else was particularly concerned. Greenhouse gases, such as CO2, water vapour and methane, absorb infrared radiation emitted from the Earth's surface and radiate the heat back towards the planet. The natural concentrations of these gases increase the temperature of the Earth by about 35 °C, and are thus essential for a habitable planet.
In Arrhenius's day, the additional greenhouse warming due to human activities was negligible. However, after continuous measurements of the atmospheric CO2 concentration began in 1957, computations soon showed that the rise in CO2 would lead to a warming of our planet by several degrees if it continued unabated. This temperature rise is comparable with the increase since the last ice age, but was predicted to occur on a 100 year, rather than a 10 000 year, timescale. The implications for regional climate, sea levels, ecology and human living conditions were far from clear.
In the December issue of Physics World magazine, Klaus Hasselmann from the Max Planck Institute of Meteorology, Hamburg, Germany writes about climate models and our current understanding of climate change.