Geoscience Reference
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Foreword: International Space Science Institute (ISSI)
Workshop on the Earth's Hydrological Cycle
Lennart Bengtsson
Received: 6 November 2013 / Accepted: 7 November 2013 / Published online: 13 December 2013
Springer Science+Business Media Dordrecht 2013
Water is a central component in the Earth's system. It is indispensable for life on Earth in
its present form and influences virtually every aspect of our planet's life support system.
On relatively short time scales, atmospheric water vapor interacts with the atmospheric
circulation and is crucial in forming the Earth's climate zones that determine where
habitable areas can exist. On the longest time scales of hundreds of millions of years, water
contributes to the lubrication of the movements of the tectonic plates, creating a pattern of
change that has shaped and is continuing to shape the Earth.
In the atmosphere, water vapor plays a key role in the Earth's energy balance and
regulates the Earth's climate in a significant way. Water vapor is the most powerful of the
greenhouse gases and serves to enhance the tropospheric temperature because water vapor
is physically and dynamically controlled by atmospheric temperature and atmospheric
circulation. The total amount of available water on the Earth amounts to some
1.5 9 10 9 km 3 . The dominant part of this, 1.4 9 10 9 km 3 , resides in the oceans. About
29 9 10 6 km 3 are locked up in the land ice on Greenland and Antarctica, and some
15 9 10 6 km 3 are estimated to exist as groundwater. If all the ice over the land and all the
glaciers were to melt, as has happened several times in the Earth's history, the sea level
would rise by some 80 m. In comparison, the total amount of water vapor in the atmo-
sphere is small; it amounts to *25 kg/m 2 , or the equivalent of 25 mm water for each
column of air. Yet atmospheric water vapor is crucial for the Earth's energy balance.
The annual mean global values of evaporation and precipitation are *1,000 mm of
water/m 2 . However, these values vary enormously in space and time from areas that are
almost completely dry to areas where the annual precipitation is more than an order of
magnitude larger than the global mean value. An evaporation of 1,000 mm of water/year
corresponds to 80 W/m 2
in energy loss for the surface and a corresponding gain for the
 
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