Geoscience Reference
In-Depth Information
atmosphere when condensation takes place. This is the single largest component for
heating the atmosphere; it is even larger than the direct solar energy absorbed by the
atmosphere. This statement highlights the importance of the hydrological cycle for the
energy balance the atmosphere.
As water vapor is also an effective absorber of terrestrial radiation, it contributes sig-
nificantly to the regulation of the temperature of the lower atmosphere. The greenhouse
effect of water vapor is estimated to be *24 C. However, water vapor has a residence
time of 7-8 days in the atmosphere and responds effectively to temperature through the
Clausius-Clapeyron (CC) relation. In the present situation, it is the increase of the long-
lasting greenhouse gases, namely CO
2
,CH
4
,N
2
O and the CFCs, that are the drivers of
climate change while water vapor generally acts as a positive feedback factor (Lacis et al.
2013
). This is a fundamental factor in climate change. Model simulations suggest that
water vapor feedback can more than double the initial effect of the long-lasting greenhouse
gases.
With a population that has increased more the fourfold over the last 100 years and with
an infrastructure that has grown by more than a factor of ten, society at large has become
more exposed, in particular, to extremes in precipitation with associated flooding damages.
Society has also over time significantly increased the amount of water that is needed,
primarily for agriculture as well as for different kinds of industrial usage. This has con-
tributed to a severe lack of water in exposed regions, even affecting major water bodies
such as the Aral Sea and Lake Chad that have almost dried up completely during the last
decades because of excessive extraction of water.
Other potential problems are disruptions related to climate change. The most severe
prospects are systematic changes in weather zones such as a tendency for a poleward shift
of the extra-tropical storm tracks that is indicated in climate simulation studies; others are
the likelihood of more intense precipitation that will increase severe flooding. The pole-
ward shift of weather systems is expected to create regional water problems with increased
precipitation in some areas and decreased precipitation in other regions. Most severe here
are the increasing risks of persistent periods of droughts, preferentially in the subtropics of
both hemispheres (IPCC
2013
).
In summary, society will have to cope with a multitude of disruptive events related to
the water cycle due both to natural and anthropogenic effects such as (1) extreme events of
heavy and persistent precipitation as well as extended periods of drought, which are all
possible within the present climate, (2) anthropogenic actions unrelated to weather and
climate, such as large-scale environmental changes caused by changing practices in large-
scale agriculture and forestry, and (3) changes in the water cycle as a consequence of
climate change. Presently the first two are dominant but gradually, as the climate system is
getting warmer, the third factor is expected to be of increasing importance.
A scientific assessment of the Earth's hydrological cycle is a complex task which covers
a multitude of areas and applications. The scientific papers in the present volume address a
broad area of research related to the Earth's Hydrological Cycle. They represent the
outcome of the third workshop within the ISSI Earth's Science Programme. The workshop
took place from 6 to 10 February, 2012, in Bern, Switzerland, with the objective of
providing an in-depth overview of the Earth's hydrological cycle. The participants in the
workshop were experts in a wide range of disciplines; they included geophysicists,
meteorologists, hydrologists, oceanographers and climate modelers.
The increase in the world's population and the increasing need for food, energy and
natural resources have put increasing stress on the water requirements.
