Is the hydrologic cycle intensifying?

B. Geerts and E. Linacre


Globally there is a balance between the evaporation from the oceans, and precipitation into the oceans plus run-off from the continents. One intuitive consequence of global warming is that higher sea surface temperature would increase the vapour pressure difference between the sea surface and the ambient atmosphere. This would enhance the evaporation rate (Dalton's Law, Note 4.E), and hence increase the other components of the hydrologic cycle. The results of using three GCM’s, together with empirical observations in Australia, Europe, India, China and the US, confirm the hypothesis that global warming enhances the global hydrologic cycle (1). For instance, a warming by 4 K is expected to increase global precipitation by about 10%. Models suggest that the increase is more likely to come as heavier rainfall, rather than as more frequent falls or longer rainfall duration.

The surface temperatures have increased by nearly 1K during the 20th century, so one might wonder whether there are signs that the hydrological cycle has already measurably intensified. The following five arguments suggest that this is the case (2).

  1. There has been a reduction in the day/night temperature range over land. Nighttime temperatures have increased at almost twice the rate of daytime temperatures since 1950 (roughly 0.9 K versus 0.5 K) suggesting increased cloudiness and/or increased evaporative cooling during the daytime (not unlike how body heat evaporates rubbing alcohol from one's skin, leaving one's body somewhat cooled in the process).
  2. Radiosonde and satellite data suggest that the mean atmospheric water vapour concentration has increased. This in turn enables storms to generate more precipitation.
  3. Precipitation amounts have changed in different ways in various regions during the last 80 years, but they have generally increased in the middle and high latitudes, often in excess of 10%, for instance in the southern hemisphere (Fig 1). In the northern hemisphere tropics, especially Africa, a significant decrease has occurred since 1950. Over the Pacific ITCZ, rainfall amounts have increased during the last few decades (3), and they have risen by about 10% since 1910 in the coterminous USA.
  4. The observed increase in precipitation has been due in large part to a disproportionate increase in heavy and extreme precipitation rates, as projected by climate models.
  5. An increased intensity of frontal disturbances in the Northern Hemisphere has been observed over the past few decades.
  6. On a longer time scale, polar ice cores show a dramatic decrease of airborne dust particles since the last glacial maximum, apparently because of a post-glacial enhanced hydrological cycle, washing out the aerosols before they can settle on the Greenland or Antarctic ice caps (4). A more recent decrease in global mean aerosol content would indicate an intensifying hydrological cycle.
  7. More rain tends to fall if daily mean temperature is above normal, as shown by observations since 1910 in Australia (5), and by GCM modeling elsewhere.

Fig 1: Precipitation trend from 1900 to 1992. This record is based on rain gauge data only. Top three images are for the northern hemisphere (90° -60° N, 60° -30° N, and 30° N-equator), and bottom picture is for the southern hemisphere (from (6), (7)).

An enhancing hydrological cycle, in turn, enhances global warming. There are several positive feedback mechanisms. One is the water vapour feedback (item 2 above), since water vapour is a greenhouse gas (8). Also, increased convective (deep) cloudiness heats the planet, because it reduces the outgoing longwave radiation more than the net incoming shortwave radiation.

While a larger global mean annual rainfall and a smaller number of frost days may have beneficial effects, especially for agriculture, the various factors listed above also have the following five undesirable consequences:

  1. Rising nighttime temperatures exacerbate heat waves and reduce the beneficial effects of frost in killing pests.
  2. Water vapour is a leading greenhouse gas, and an increase in water vapour concentration is a key component of the leading positive feedback mechanism in global warming.
  3. In middle and high latitudes, soils are generally close to saturation, and therefore seemingly small increases in rainfall can cause large increases in runoff, resulting in more frequent floods.
  4. Increased rainfall intensities require more expensive flood control measures and imply relatively less soil infiltration.
  5. More inclement storms, mainly in winter, rises the risk of hazards along shorelines, especially as coastal populations continue to increase.



  1. Fowler, A.M. and K.T. Hennessy, 1995. Potential impacts of global warming on the frequency and magnitude of heavy precipitation. Natural Hazards, 11, 283-303.
  2. Karl, T.R. 1997: Changes and variations in temperature and precipitation extremes: evidence for an enhanced hydrologic cycle? Preprint Volume, 10th Conf. on Appl. Climat., AMS, Reno, 20-23 Oct 1997, p1. Thomas R. Karl is a Senior Scientist at the National Climate Data Center in Asheville, NC. 
  3. Morrissey, M.L. & N. E. Graham, 1996. Recent trends in rain gauge measurements from the tropical Pacific: evidence for an enhanced hydrologic cycle. Bull. Amer. Meteor. Soc., 77, 1207-19.
  4. Yung, Y.L., T. Lee, C.-H. Wang, Y-T. Shieh, 1996. Dust: A Diagnostic of the Hydrologic cycle During the Last Glacial Maximum. Science, 271, 962-963.
  5. Power, S., F. Tseitkin, S. Torok, B. Lavery, R. Dahni and B. McAvaney 1998. Australian temperature, Australian rainfall and the Southern Oscillation, 1910 - 1992. Aust. Meteor. Mag., 47, 85-101.
  6. Hulme M.,1995. Estimating Global Changes in Precipitation. Weather, 50, 36-45.
  7. Hulme M., and M. New, 1997. Dependence of large-scale precipitation climatologies on temporal and spatial sampling. J. Climate, 10, 1099-1113.
  8. Bates, J.J., X. Wu, and D.L. Jackson, 1996. Interannual variability of upper-tropospheric water vapor band brightness temperature. J. Climate, 9, 427-438.