Climate change in the past and its causes

Today's climate may have lasted about 10,000 years, with fluctuations, but the earth's climate has always changed. Evidence:

• Glacial landforms at high altitudes and latitudes show glaciers advancing and retreating.
• Sediments in glacial lakes reveal warm and cold periods.
• Tree rings show wet and dry periods.
• Historical records describe the vagaries of climate, crop failures, floods and migrations.

In India: archaeology shows the Rajasthan desert was wet and cool around 8,000 BC; 3,000-1,700 BC had higher rainfall; about 2,000-1,700 BC it was a centre of the Harappan civilisation; dry conditions have grown since.

In the geological past: the earth was warm about 500-300 million years ago (Cambrian, Ordovician, Silurian). In the Pleistocene, glacial and inter-glacial periods alternated; the last peak glacial was about 18,000 years ago, and the present inter-glacial began about 10,000 years ago.

Recent variability: the dust bowl, a severe drought on the south-western Great Plains of the USA in the 1930s; the devastating Sahel drought south of the Sahara (1967-1977); Europe's warm, dry 10th-11th centuries, when the Vikings settled Greenland; and the 'Little Ice Age' in Europe (1550 to about 1850). The 1990s saw extreme weather and some of the century's worst floods.

Causes:

• Astronomical: changes in the sun's output with sunspots (dark, cooler patches that rise and fall in cycles; some link more sunspots to cooler, wetter, stormier weather, though this is not statistically proven); and Milankovitch oscillations, cycles in the earth's orbit, its wobble and its axial tilt, which change the insolation received.
• Terrestrial: volcanism: eruptions throw aerosols high into the atmosphere, where they stay for some time and reduce the sunlight reaching the surface. After the eruptions of Pinatubo and El Chichón, average temperatures fell somewhat for a few years.
• Human (anthropogenic): the rising concentration of greenhouse gases, likely to cause global warming.

Example (new): cutting across an old tree trunk in a dry district may show a run of very thin rings: years when the tree grew little, which often match droughts in local records.