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).
- 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
- Radiosonde and satellite data
suggest that the mean atmospheric water vapour concentration has
increased. This in turn enables storms to generate more precipitation.
- 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.
- 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.
- An increased intensity of
frontal disturbances in the Northern Hemisphere has been observed over the
past few decades.
- 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.
- 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:
- Rising nighttime temperatures
exacerbate heat waves and reduce the beneficial effects of frost in
- 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.
- 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.
- Increased rainfall
intensities require more expensive flood control measures and imply
relatively less soil infiltration.
- More inclement storms, mainly
in winter, rises the risk of hazards along shorelines, especially as
coastal populations continue to increase.
- Fowler, A.M. and K.T.
Hennessy, 1995. Potential impacts of global warming on the frequency and
magnitude of heavy precipitation. Natural Hazards, 11,
- 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.
- 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.
- 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.
- 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.
- Hulme M.,1995. Estimating
Global Changes in Precipitation. Weather, 50, 36-45.
- Hulme M., and M. New, 1997.
Dependence of large-scale precipitation climatologies on temporal and
spatial sampling. J. Climate, 10, 1099-1113.
- 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.