Precipitation, Floods and Drought

Precipitation has increased in many regions of the world and decreased in others, with little or no net change in the total amount of precipitation. Drought has increased in most places, consistent with expectations for a warming climate. Generally, wet areas have become wetter, and dry areas have become drier in the past 40 years. Precipitation has increased in the eastern parts of North and South America, Northern Europe, and northern and central Asia. Precipitation decreases have been observed in the subtropics and the tropics outside the monsoon trough, namely the Sahel, the Mediterranean, Southern Africa, and Southern Asia.1

Some areas have experienced widening swings between the two precipitation extremes.2 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. In the summer of 2007, widespread flooding in central England (the wettest since records began in 1766) was accompanied by drought and record-breaking heat waves in southeast Europe.3

Precipitation Trend in millimeters per day, 1950-2008

Trend map for observed annual precipitation.

Dai, 2010

Significantly, dramatic and widespread increases in heavy precipitation have also been observed, even in areas where the total precipitation has decreased. And the fingerprint of human influence has been found in these increases.4 This is of particular concern as heavy precipitation contributes to destructive and costly events such as floods.5

Natural variability cannot explain the observed changes in precipitation intensity or geographic distribution of precipitation. Rather, the observed changes follow from basic physical principles and are consistent with a combination of natural factors and human influence.6

Heavy Precipitation

Change in % of  Global Precipitation Falling on Wettest Days

Changes in the proportion of precipitation falling on very wet days (wettest 5% of days). The vertical scale shows percent change from 22.5%, the average of the base period (1961-1990). The smooth red curve accounts for variation within a decade.

Trenberth et al. 2007

The water holding capacity of the atmosphere increases in a warmer world. A 4% increase in atmospheric moisture has been observed and that is consistent with a warming climate.7 Natural variability cannot account for the changes, and the fingerprint of human influence has been identified in these changes.8

The increased moisture in the atmosphere is driving the shift to heavier but less frequent rains — “when it rains, it pours.” While an atmosphere that holds more moisture has greater potential to produce heavier precipitation, precipitation events also become less frequent and shorter, as it takes longer to recharge the atmosphere with moisture.9 By analogy, a larger bucket holds and dumps more water, but takes longer to refill.

Increases in Amounts of Very Heavy Precipitation, 1958-2007

The map shows the percentage increases in very heavy precipitation (defined as the heaviest 1 percent of all events) from 1958 to 2007 for each region. There are clear trends toward more very heavy precipitation for the nation as a whole, and particularly in the Northeast and Midwest.

Karl et al. 2009

Storms are producing more intense precipitation, even in areas where total precipitation is decreasing.10 Storms reach out to gather water vapor over regions that are 10-25 times as large as the precipitation area, multiplying the effect of increased atmospheric moisture. As water vapor condenses to form clouds and rain, it releases latent heat energy that adds buoyancy to the air and fuels the storm. This increases the gathering of moisture into storm clouds and further intensifies precipitation.11

In the wettest areas of the tropical oceans, the most extreme rainfall events have increased by 60% per degree of warming.12

While the geographic pattern and intensity of precipitation has changed dramatically, total global precipitation has changed little.13

The higher latitudes have become wetter in recent years, due mainly to the warmer air holding more moisture and in part to alterations in atmospheric circulation driven by climate change. At the same time, the subtropics and parts of the tropics have become drier as winds carry the moisture away to the monsoon rain areas or to mid-latitude storms.14

In the United States, total average precipitation has increased by about 7% in the past century, while the amount of precipitation falling in the heaviest 1% of rain events has increased 20%.15 Regions such as the Northeast and Midwest have seen considerably larger increases in the heaviest rains.

Next Page (Floods) →

(Full List of References)


  1. Trenberth et al. 2007, Trenberth 2011
  2. Trenberth et al. 2007
  3. Trenberth 2011
  4. Min et al. 2011
  5. Solomon et al. 2007
  6. Stott et al. 2010, Trenberth 2011, Zhang et al. 2007
  7. Trenberth et al 2007
  8. Stott et al. 2010
  9. Trenberth 2011
  10. Trenberth 2011
  11. Trenberth 2011
  12. Allan et al 2010
  13. Trenberth 2011
  14. Trenberth 2011
  15. Karl et al. 2009