During the last 55 million years the Earth has been cooling; during the last million years there have been alternating glacials and interglacials. See little ice age.

External causes of climatic change include: changes in solar output (Haigh (2001) Science 294, 5549); changes in the number of sunspots, which seem to have an eleven-year cycle (Foukal et al. (2004) Science 306, 5693); changes in the ellipticity of the Earth’s orbit, which follow a 100 000-year cycle (Goldsmith (March 2007) Natural History); and changes in the Earth’s axis of rotation, which alters the season of perihelion, and which follow roughly a 100 000-year cycle (Zachos et al. (2001) Science 292, 5517). See milankovitch cycles.

Internal causes include changes in the distribution of land and sea, continental drift, and changes in the atmosphere–surface–ocean system. ‘Although vulcanicity is seen as a possible mechanism of climatic change, under present rates of eruptions it is difficult to envisage volcanic activity acting independently of other factors in causing significant and lasting global climate changes’ (Wyrwoll and McConchie (1986) Climate Change 8, 3).

Nyberg et al. (2007) Nature 447 report that the average frequency of major hurricanes decreased from the 1760s until the early 1990s, and that the phase of enhanced hurricane activity since 1995 appears to be a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperatures. Nicholls and Alexander (2007) PPG 31, 1 find a clear pattern of increasing warm extremes and decreasing cold extremes in global climates since the early 1990s. A. E. Dressler and E. A Parson (2006) summarize the key scientific literature on climate change. See Comrie (2007) Geog. Compass 1, 3 on climate change and human health.