3.1 Rain

Rain is particularly important for geoengineering because living things are very sensitive to the amount and intensity of rain they receive. A broader term for rain is precipitation, the collective word for all forms in which water from the air falls under gravity onto the Earth’s surface (rain, snow, sleet or hail).

Figure 6 One of nineteen illustrated climate haiku by Greg Johnson (Johnson, 2013), this one relating to rainfall.

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This water colour painting shows green hills and trees. It has the words 'History, Air' and the Haiku Carbon Increases. Air warms through century past. More heavy rains fall.

Figure 6 One of nineteen illustrated climate haiku by Greg Johnson (Johnson, 2013), this one relating to rainfall.

The haiku – a Japanese style of poem – in Figure 6 summarises three IPCC (2013) assessments of how climate change has affected rainfall:

• The atmospheric concentrations of the greenhouse gases carbon dioxide, methane, and nitrous oxide have all increased since 1750 due to human activity (‘Carbon increases’).
• The globally averaged surface temperature data show a warming of 0.85°C over the period 1880–2012 (‘Air warms through century past’). 
• Since 1950, there are likely more land regions where the number of heavy precipitation events has increased than where it has decreased (‘More heavy rains fall’).

The third statement refers to heavy precipitation events, which can cause devastating effects to human health and infrastructure. But changes to average rainfall – whether increased or decreased – are also important. Too little rainfall over a long period, for example, and supplies for drinking, agriculture and hydroelectric power can be at risk. Figure 7 shows the changes that have happened since the middle of the twentieth century.

Figure 7 Observed precipitation changes for the period 1951–2010. The trends are calculated with the same criteria for data as Figure 5 (IPCC, 2013).

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This global map has coloured cells colour coded for changes in rainfall, measured in mm yr-1 per decade. Blue tones show increases, up to 100 mm yr-1, brown tones show decreases down to -100 mm yr-1 and below. Data is almost exclusively over land, with the no data in Greenland, northern Canada and Asia, much of Africa and Antarctica. Increases can be seen in Europe and much of Asia, Norther America and parts of South America, southern Africa and most of Australia. Decreases can be seen in the Mediterranean, much of China, West Africa and southern and western Australia. Some of the data has crosses, including USA, Europe, Argentina, China and northern Australia.

Figure 7 Observed precipitation changes for the period 1951–2010. The trends are calculated with the same criteria for data as Figure 5 ...