Human evidence: Matching patterns of surface warming
Climate models predict that global warming caused by increased levels of greenhouse gases such as carbon dioxide should follow certain geographic patterns, with higher rates of warming in some regions than others. These include faster rates of warming over land, at higher latitudes, at night-time and during the winter. Measurements over the past 50 years show patterns of surface warming consistent with those predicted for greenhouse-gas-induced warming.
Climate models predict that an increase in atmospheric greenhouse gases results in faster surface warming at higher latitudes than in areas closer to the Equator. This ‘polar amplification’ occurs partly because of a reduction in the extent of ice and snow. The melting causes darker land and water surfaces that were beneath the ice and snow to be revealed. Darker surfaces absorb more solar radiation, i.e. there is a decrease in albedo. The observed trend of greater warming at the poles is broadly consistent with the predictions resulting from greenhouse-gas-induced warming.
Climate models predict that an increase in atmospheric greenhouse gases results in faster warming of night-time temperatures than daytime temperatures. The mechanisms for this are not well understood but are thought to be related to changes in cloud cover and soil moisture. Measurements show that this daytime – night-time temperature range has decreased by about 0.4 °C over the past 50 years, although climate models give a smaller magnitude for this decrease. Scientists think that this is partly because of difficulties in simulating clouds and that changes in cloud cover and soil moisture may have contributed to the reduction in this temperature range. The models indicate that warming caused by an increase in sunlight would result in daytime temperatures rising at about the same rate as night-time temperatures, contrary to the observed trends.
Climate models predict that an increase in atmospheric greenhouse gases will result in greater increases of winter temperatures than summer temperatures in the northern hemisphere. Scientists think this effect may be caused by snow and sea ice retreat and the albedo effect resulting from increased absorption and re-emission of heat energy by larger quantities of greenhouse gases in the atmosphere. Measurements show that the seasonal cycle is reducing over land areas outside the tropics.