Global - Climate

Global Drylands: Temperature and Precipitation Patterns

The drylands of the world show great diversity in their temperature characteristics and precipitation patterns. This map captures this diversity in a limited number of classes with the following characteristics:

NON-TROPICAL DRYLANDS

  •  Cold (< 10° C) to mild winters (10-20° C);
  •  Mild (10-20° C) to very warm (> 30° C) summers.;
  •  Moisture regime ranging from arid to semi-arid to sub-humid;
  •  Variable precipitation patterns;
  •  Cold is a general but variable constraint, depending on severity of the winters, requiring adaptation of plants and crops;
  •  Agricultural systems are dominated by non-tropical crops with low water requirements (wheat, barley)

With winter-rainfall patterns
Water needs of crops are lower as compared to tropical areas, therefore higher water use efficiency. C3-crops have a productivity advantage.

With summer rainfall patterns
Water needs of crops are higher than in winter rainfall patterns, but usually lower than in tropical areas. C3-crops adapted to higher temperatures, or C4-crops adapted to lower temperatures, have a productivity advantage.

With transitional rainfall patterns:
Regimes with two rainy seasons, in summer and in winter, or with irregular unpredictable patterns.

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Global Drylands: Moisture Regimes

The general moisture regime of the drylands of the world can be expressed by a simple measure, the aridity index (ratio annual precipitation to annual potential evapotranspiration). The following classes are shown on this map:

 Hyper-arid: aridity index <0.03
 Characterized by very low and irregular rain, which may fall in any season
 Arid: aridity index 0.03 – <0.2
 Characterized by annual rainfall of 80-200 mm (in areas with winter rainfall) up to 200-350 mm (in areas with summer rainfall). Inter-annual rainfall variability 50-100 percent
 Semi-arid: aridity index 0.2 – <0.5
 In winter rainfall areas: mean annual rainfall from 200-250 mm to 450-500 mm.
 In summer rainfall areas: mean annual rainfall from 300-400 mm to 700-800 mm
 Inter-annual rainfall variability 25-50 percent
 Sub-humid: aridity index 0.5 – <0.65. Inter-annual rainfall variability is less than 25 percent

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Global Drylands and Climate Change: Change of mean annual temperature 1980/1999 to 2080/2099

This map visualizes what climate change actually means in terms of temperature changes in different parts of the globe over a 100-year period (from 1980/89 to 2080/99) under the greenhouse gas emission scenario A1b (see further). With respect to the global drylands, the following table summarizes these changes by relating different dryland regions to temperature change classes.

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Global Drylands and Climate Change: Relative change of mean annual precipitation 1980/1999 to 2080/2099

This map visualizes what climate change actually means in terms of precipitation changes in different parts of the globe over a 100-year period (from 1980/89 to 2080/99) under the greenhouse gas emission scenario A1b (see further). With respect to the global drylands, the following table summarizes these changes by relating different dryland regions to precipitation change classes.

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Climatic resource constraints and global drylands

Drylands are usually associated with ‘hot’ areas but can in fact be affected by both severe moisture shortage and low temperature.
A severe moisture constraint is defined as where the aridity index is below 0.35.
A severe coldness constraint is defined where the annual sum of growing degree days (above 0°C) is less than 2000.
This simple map shows areas where such constraints occur either single or combined.

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