Observations show that changes are occurring in the amount, intensity, frequency and type
of precipitation. These aspects of precipitation generally exhibit large natural variability,
and El Niño and changes in atmospheric circulation patterns such as the North Atlantic
Oscillation have a substantial influence. Pronounced long-term trends from 1900 to 2005 have
been observed in precipitation amount in some places: significantly wetter in eastern North
and South America, northern Europe and northern and central Asia, but drier in the Sahel,
southern Africa, the Mediterranean and southern Asia. More precipitation now falls as rain
rather than snow in northern regions. Widespread increases in heavy precipitation events have
been observed, even in places where total amounts have decreased. These changes are associated
with increased water vapour in the atmosphere arising from the warming of the world’s oceans,
especially at lower latitudes. There are also increases in some regions in the occurrences of
both droughts and floods.
Precipitation is the general term for rainfall, snowfall and other forms of frozen or liquid
water falling from clouds. Precipitation is intermittent, and the character of the precipitation
when it occurs depends greatly on temperature and the weather situation. The latter determines
the supply of moisture through winds and surface evaporation, and how it is gathered together in
storms as clouds. Precipitation forms as water vapour condenses, usually in rising air that expands
and hence cools. The upward motion comes from air rising over mountains, warm air riding over cooler
air (warm front), colder air pushing under warmer air (cold front), convection from local heating of
the surface, and other weather and cloud systems. Hence, changes in any of these aspects alter
precipitation. As precipitation maps tend to be spotty, overall trends in precipitation are indicated
by the Palmer Drought Severity Index (see Figure 1), which is a measure of soil moisture using
precipitation and crude estimates of changes in evaporation.
A consequence of increased heating from the human-induced enhanced greenhouse effect is increased
evaporation, provided that adequate surface moisture is available (as it always is over the oceans
and other wet surfaces). Hence, surface moisture effectively acts as an ‘air conditioner’, as heat
used for evaporation acts to moisten the air rather than warm it. An observed consequence of this is
that summers often tend to be either warm and dry or cool and wet. In the areas of eastern North and
South America where it has become wetter (Figure 1), temperatures have therefore increased less than
elsewhere (see FAQ 3.3, Figure 1 for changes in warm days). Over northern
continents in winter, however, more precipitation is associated with higher temperatures, as the water
holding capacity of the atmosphere increases in the warmer conditions. However, in these regions, where
precipitation has generally increased somewhat, increases in temperatures
(FAQ 3.1) have increased drying, making the precipitation changes less
evident in Figure 1.
As climate changes, several direct influences alter precipitation amount, intensity, frequency and
type. Warming accelerates land surface drying and increases the potential incidence and severity of
droughts, which has been observed in many places worldwide (Figure 1). However, a well-established
physical law (the Clausius-Clapeyron relation) determines that the water-holding capacity of the
atmosphere increases by about 7% for every 1°C rise in temperature. Observations of trends in
relative humidity are uncertain but suggest that it has remained about the same overall, from the
surface throughout the troposphere, and hence increased temperatures will have
resulted in increased water vapour. Over the 20th century, based on changes in sea surface temperatures,
it is estimated that atmospheric water vapour increased by about 5% in the atmosphere over the oceans.
Because precipitation comes mainly from weather systems that feed on the water vapour stored in the
atmosphere, this has generally increased precipitation intensity and the risk of heavy rain and snow
events. Basic theory, climate model simulations and empirical evidence all confirm that warmer climates,
owing to increased water vapour, lead to more intense precipitation events even when the total annual
precipitation is reduced slightly, and with prospects for even stronger events when the overall
precipitation amounts increase.
The warmer climate therefore increases risks of both drought − where it is not raining − and floods
− where it is − but at different times and/or places. For instance, the summer of 2002 in Europe
brought widespread floods but was followed a year later in 2003 by record-breaking heat waves and
drought. The distribution and timing of floods and droughts is most profoundly affected by the cycle
of El Niño events, particularly in the tropics and over much of the mid-latitudes of
In areas where aerosol pollution masks the ground from direct sunlight, decreases in evaporation
reduce the overall moisture supply to the atmosphere. Hence, even as the potential for heavier
precipitation results from increased water vapour amounts, the duration and frequency of events
may be curtailed, as it takes longer to recharge the atmosphere with water vapour.
Local and regional changes in the character of precipitation also depend a great deal on atmospheric
circulation patterns determined by El Niño, the North Atlantic Oscillation (NAO; a measure
of westerly wind strength over the North Atlantic in winter) and other patterns of variability. Some
of these observed circulation changes are associated with climate change. An associated shift in the
storm track makes some regions wetter and some − often nearby − drier, making for complex patterns of
change. For instance, in the European sector a more positive NAO in the 1990s led to wetter conditions
in northern Europe and drier conditions over the Mediterranean and northern African regions (Figure 1).
The prolonged drought in the Sahel (see Figure 1), which was pronounced from the late 1960s to the late
1980s, continues although it is not quite as intense as it was; it has been linked, through changes in
atmospheric circulation, to changes in tropical sea surface temperature patterns in the Pacific, Indian
and Atlantic Basins. Drought has become widespread throughout much of Africa and more common in the
tropics and subtropics.
As temperatures rise, the likelihood of precipitation falling as rain rather than snow increases,
especially in autumn and spring at the beginning and end of the snow season, and in areas where
temperatures are near freezing. Such changes are observed in many places, especially over land in
middle and high latitudes of the Northern Hemisphere, leading to increased rains but reduced snowpacks,
and consequently diminished water resources in summer, when they are most needed. Nevertheless, the
often spotty and intermittent nature of precipitation means observed patterns of change are complex.
The long-term record emphasizes that patterns of precipitation vary somewhat from year to year, and
even prolonged multi-year droughts are usually punctuated by a year of heavy rains; for instance as
El Niño influences are felt. An example may be the wet winter of 2004-2005 in the southwestern
USA following a six-year drought and below-normal snowpack.