How will the global water cycle evolve in response to global warming?

No element of the climate system has as much impact on society as the water cycle, yet we remain ignorant of the largest part of the water cycle, that over the oceans. The oceans are the main reservoir of free water on the planet, the source of nearly 90% of global evaporation and the site of ~80% of global precipitation. A mere 1% of Atlantic ocean precipitation matches the discharge of the Mississippi River. Water evaporates more readily from a warmer ocean, so an intensification of the water cycle is expected with anthropogenic warming. The signature of the water cycle within the oceans is in the distribution of salinity, which must be monitored in the future if we are to understand how the water cycle is changing. In addition, salinity influences ocean mixing and circulation and thus the ability of the ocean to absorb, store and transport heat and CO2. Can we initiate an observing system for upper ocean salinity that will help us to understand and predict the future evolution of the global water cycle? Can we develop a better understanding of the smaller terrestrial water cycle, where plants and drainage basins are responding to rising warmth and CO2? Can we understand the interactions between ocean, atmosphere and the high latitude ice sheets that are leading to increased melting and discharge to the ocean? The global water cycle is truly a central unifying problem for climate change, and of fundamental importance to society.

9 comments

1. elorentzen says:

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   I would point to the evaporation phase of the earth’s water cycle as a focus of study. If we developed means to manage this process, we can send snow to replenish the icecaps and preserve the glaciers; we can send rain to arid areas.
   Theoretically, if sea water is pumped over large areas of desert lands, evaporation can take place at a large scale, over time, and precipitation would follow. How much rain falls and when rain falls may soon be established, but it would be interesting to know where rain would fall or be made to fall.
   A by-product could be some lowering of the earth’s temperature.
2. jyang says:

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   In the next 100 years or so, the societal impact from water cycle change would likely be greater than that from a gradual rise of sea level.
3. chrishartnady says:

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   I wish to draw attention to the existing IGCP Project 565 – “Developing the Global Geodetic Observing System into a Monitoring System for the Global Water Cycle” (see http://geodesy.unr.edu/igcp565/about_igcp/ ). Its rationale is simply stated on its website: “At time scales from weeks to decades, hydrological loading of the Earth’s surface dominates non-secular variation in each of the three fundamental areas of geodesy. Thus, geodesy inherently provides valuable constraints on the global water cycle at multiple spatial and temporal scales. In particular, the space geodetic sensors capture the signals of variation in the entire fluid envelope of the solid Earth, including the terrestrial water storage. Space geodetic observations of surface mass variability are inherently strong at regional to global scales. Therefore, GGOS offers a monitoring system that can complement traditional in-situ measurements of terrestrial water storage through geodetic remote sensing.”
4. Richard@CSFD says:

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   One of the key issues is how to use the enhanced knowledge we will gain about the water cycle to increase the benefits for Mankind. How to maximise the amount of water that is used by ecosystems and agriculture to produce biomass (biodiverse, including FOOD), and to minimise water losses by excessive runoff and direct evaporation from soils. In other words how to increase the proportion of ‘green water’, which is also a way to increase resilience and even adaptation to climate change.
5. Matschullat says:

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   The question seems too restricted to me. I suggest to open it to the full feedback process and include the terrestrial realm. Not only does the water cycle react on global warming, but the cycle is being influenced by global change at large. We have plenty of evidence for the regional impact of land-use change on the hydrologial cycle and thus on the regional climate signal, too.
6. clim101 says:

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   The intesnification of the water cycle will not only affect mean precipitation but also how it occurs. A combination of heavier, less frequent rainfall events in parts of the tropics together with longer periods of drying will, confusingly, lead to a greater susceptibility to both drought and flood conditions.
7. xubinzeng says:

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   It is crucial to emphasize in this question the availability of fresh water regionally and globally and its variation with climate change and human population growth.
8. valmasdel says:

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   I would suggest to broaden the scope of this question to the long term monitoring of the water cycle. New techniques now offer the ability to monitor not only water vapour but its stable isotope composition both by remote sensing and in situ. This offers completely new perspectives for monitoring the atmospheric water cycle continuously with constrains on water origin. This would be a very innovative monitoring system which, together with the explicit modelling of water stable isotopes, would bring our understanding of the water cycle one step forward.
9. Belouve says:

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   You may read a good paper from PhD Roy Spencer here: http://www.nov55.com/rws.html “Global Warming and the Nature Thermostat”. Use NOAA online calculator to draw graphs showing temperatures,relative and specific humidity at different altitudes, precipitation rates, everywhere you want in the world, and also globally, for the period from 1948 until now. A marvelous tool: http://www.cdc.noaa.gov/cgi-bin/data/timeseries/timeseries1.pl

   You’ll be perhaps astonished, doing it,to observe that atmospheric relative and specific humidity have steadily decreased from 1975 to now, at different atmospheric levels, while temperatures where getting higher, which infer that global greenhouse effect would possibly be not the cause of the temperature increase.