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Global Change
Instruction Program
The Water Cycle
The water cycle is a balanced system, with
water stored in many places at any one time
(Table 3). The cycle involves the transfer of water
in its various forms of liquid, vapor, ice, and
snow through the land, air, and water environments. Both matter and energy are involved in
the transfer. The transfer begins when heat from
the sun warms the ocean and land surfaces and
causes water to evaporate. The water vapor enters
the atmosphere and generally moves with the
The water cycle (Figure 21) is important in the
context of biogeochemical cycling in the C, N, P, S,
and O system, as well as being important in its
own right. Water circulating through the ecosphere
is part of a continuous hydrologic cycle that makes
life on earth possible. The water cycle is the driver
cycle for transport of many elements at the earth’s
surface. In the atmosphere, water vapor is the most
important greenhouse gas, and its behavior during
a global warming is of concern.
Figure 21. The global biogeochemical cycle of water (the hydrologic cycle), showing the major processes of water movement.
Numbers in parentheses show the water budget in thousands of cubic kilometers per year (after Maurits la Rivière, 1990.)
Atmospheric water vapor transport
(40)
So
lar
rad
iat
Precipitation of
Snow, ice, rain
(111)
(71)
ion
Cloud
Evaporation
r
cie
Su
a
Gl
rfa
ce
ru
Transpiration
no
Evaporation
(425)
ff
Surface water
Lake
River
Precipitation
(385)
Soils
Return flow (40)
Percolation
Land
Ocean
Groundwater flow
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Understanding Global Change: Earth Science and Human Impacts
circulation of the air. On a global
Table 3. Distribution of water in the ecosphere
scale, warm air rises in the atmosphere and cooler air descends. The
Percent of total
Reservoir
Volume (106 km3)
________________________________________________________________
water vapor rises with the warm
Ocean
1370
97.25
air. The farther from the warm planCryosphere
etary surface the air travels, the
(ice caps and glaciers)
29
2.05
cooler it becomes. Cooling causes
Groundwater
9.5
0.68
water vapor to condense on small
Lakes
0.125
0.01
particles (cloud condensation
Soils
0.065
0.005
nuclei) in the atmosphere and to
Atmosphere
0.013
0.001
precipitate as rain, snow, or ice and
Rivers
0.0017
0.0001
fall back to the earth’s surface.
When the precipitation reaches the
Biosphere
0.0006
0.00004
___________________________________________________________
land surface, it is evaporated directTotal
1408.7
100
ly back into the atmosphere, runs
off or is absorbed into the ground,
After Berner and Berner, 1996.
or is frozen in snow or ice. Also,
plants require water and absorb it,
retaining some of the water in their
tissue. The rest is returned to the atmosphere
the planet will probably lead to more water
through transpiration. Precipitation on land is
vapor in the atmosphere. The increased water
equal to evapotranspiration plus runoff to the
vapor has the potential to absorb more infrared
ocean. That is, over the land, there is more preradiation reradiated from the planetary surface
cipitation than evapotranspiration.
and thus lead to further warming. This is another
For the ocean, the situation is reversed. Much
example of a positive feedback.
of the water evaporated from the ocean returns
Water droplets form in the troposphere by
there directly; however, a small amount (about
condensation of water on cloud condensation
8% of that evaporated) is carried by atmospheric
nuclei. These particles may absorb or reflect enerwinds over the continents, where it precipitates.
gy. The amount of water vapor in part deterOnce on the ground, the water finds its way to
mines the types and distribution of clouds that
streams, lakes, or rivers in runoff or by percolaform in the atmosphere. In terms of predicting
tion into and through groundwater. In due time,
the effects of increasing concentrations of greenthe water will return to the ocean, mainly in
house gases in the atmosphere on temperature
stream and river flows and less importantly in
and other climatic variables, general circulation
groundwater. This return flow balances the net
models (GCMs) and other types of models have
loss from the ocean surface by evaporation.
been used. The GCMs are very complex computSnow and ice may remain on the land for a
er representations of the atmosphere or atmolong time before the water in these forms of presphere-ocean system that are used in the modelcipitation evaporates to the atmosphere or
ing of global climate change. In these models, the
returns via rivers or as direct glacial meltwater to
effects of clouds on the radiant energy budget of
the oceans. The snow and ice that feed glaciers
the planet are a major source of uncertainty in
may remain locked up in the cryosphere for
attempting to predict future climate change.
thousands of years, but finally the ice will melt,
Clouds regulate the radiative heating of the
and the water will travel to another part of the
planet. They reflect a significant part of the
hydrologic cycle.
incoming solar radiation. Clouds also absorb
Water vapor in the atmosphere is the princilongwave, infrared radiation emitted by the earth.
pal greenhouse gas. Because the amount of water
At the cold tops of clouds, energy is emitted to
vapor in the atmosphere is dependent on the
space. In 1984 the Earth Radiation Budget
temperature of the planet, any initial warming of
Experiment (ERBE) was launched. This
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Global Biogeochemical Cycles and the Physical Climate System
GCMs only perform calculations at widely
separated points over the globe and relatively
infrequently. Cloud formation involves very
dynamic processes at short time and spatial
scales.
Clouds may act as a positive or negative feedback in a future earth warmed by the enhanced
greenhouse effect. This ambiguity accounts in
part for the range of estimates of the average
temperature increase predicted by the GCMs for
a doubling of the atmospheric carbon dioxide
concentration.
A final comment on the water cycle is that it
is being significantly affected by water usage and
contamination of water stocks. Currently, humans
use an amount of water equivalent to about 25%
of total terrestrial evapotranspiration and 55%, or
6,800 cubic kilometers per year, of the runoff of
water from the continents that is accessible. Only
about 20% of the world’s drainage basins have
pristine water quality. There is little doubt that
the world’s water resources will be significantly
strained in the 21st century.
experiment involves a system of three satellites
that provide data on incoming and outgoing radiation. One result of this experiment so far is the
demonstration that clouds have a net cooling
effect on the earth. On a global scale, clouds
reduce the amount of radiative heating of the
planet by –13 watts per square meter. This is a
large number when compared to the +2.5 watts
per square meter attributed to the increase in
atmospheric greenhouse gases during the last
century. It is also large compared to the radiative
heating that could arise from a doubling of
atmospheric carbon dioxide concentrations in the
next century—about +4 watts per square meter.
Thus, a small change in the types and distribution
of clouds may have a large effect on the radiation
budget compared to the effect of changing greenhouse gas composition owing to human activities.
In a world already warmed by greenhouse
gases released from human activities, it is difficult
to predict what will happen to the types and distribution of clouds. Too little is known about the
complex processes of cloud formation and, in particular, the response of clouds to a warming earth.
Also, these complex processes are difficult to simulate in the general circulation models. One difficulty lies with the problem of reproducing cloud
physics in the models. A second difficulty is that
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