There is a general consensus that global average surface air temperature increased during the 20th century and that future warming will happen. Global temperature increase, the key primary outcome of global warming, is expected to reach between 1.8 and 2.8° Celsius by the end of the 21st century relative to the 1980 – 1999 average even in rather moderate scenario. Climate Change and the Water Cycle are directly related. Water is an integral part of the climate system, and changes of the water cycle are expected to happen in nearer future. Climate change will increase hydrologic variability, resulting in extreme weather events such as droughts floods, and major storms. The observed warming over the last decades has led to changing precipitation patterns, to reduced snow cover and widespread melting of ice and to changes in soil moisture and runoff. Precipitation has increased in high northern latitudes and decreased in southern latitudes (IPCC). The probable changes in precipitation and evaporation will translate directly to shifts in the existing pattern of soil moisture deficits, groundwater recharge and runoff. Besides the observed effects of higher temperatures, a change of freshwater availability constitutes an even more important risk to the human society, food security and ecosystems. Although the changes of the water cycle in response to climate change remain difficult to predict, some major points should be addressed.
Background
The water cycle describes the continuous movement of water in its liquid, solid and vapour forms through the climate system as well as the storage in the reservoirs of ocean, cryosphere, land surface and atmosphere. The main physical processes of the water cycle are evaporation, condensation, precipitation, infiltration, runoff and subsurface flow.
The movement of water in the climate system is essential to life on land, as much of the water that falls on land as precipitation and supplies the soil moisture and river flow has been evaporated from the ocean and transported to land by the atmosphere. Water that falls as snow in winter can provide soil moisture in springtime and river flow in summer and is essential to both natural and human systems. The movement of fresh water between the atmosphere and the ocean can also influence oceanic salinity, which is an important driver of the density and circulation of the ocean. The latent heat contained in water vapour in the atmosphere is critical to driving the circulation of the atmosphere on scales ranging from individual thunderstorms to the global circulation of the atmosphere (IPCC).
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Impacts on the Water Cycle
The IPCC Report states a “medium confidence” that global change will influence precipitation patterns, but some significant trends can be summarized.
In the long term, global precipitation will increase due to the higher moisture holding capacity of a warmer atmosphere and higher evaporation rates from warmer water bodies. However, changes in response to the warming over the 21st century will not be uniform. Some regions will experience decreases and some will not experience any changes. There is high confidence that the contrast of annual mean precipitation between dry and wet regions will increase over most of the globe as temperatures increase. High latitudes and moist mid-latitude regions will experience greater amounts of precipitation, whereas many mid-latitude and subtropical arid-and semi-arid regions will experience less precipitation. Importantly, precipitation variability will increase across all regions. Furthermore there will be a shift to more intense extreme events. The intensity of precipitation events and flooding is projected to increase in areas that experience increases in mean precipitation, while there is also a greater risk of droughts in mid-continental areas. Intense and heavy episodic
rainfall events with high runoff amounts are interspersed with longer relatively dry periods
with increased evapotranspiration, particularly in the subtropics.
Specific changes to water resources and the hydrological cycle for example also include:
- Changes in mean surface flows due to natural climate variability at interannual and multidecadal time scales and climate change
- Changes in the seasonality (or timing) of flows, especially in snow melt basins
- Changes in flows from glaciers due to their retreat
- Decreasing snow and permafrost
- Rising sea levels caused by thermal expansion of seawater and melting of continental glaciers
- Changes in soil moisture
Impacts of climate change on annual and decadal weather cycles may also be significant. Examples include the southwest monsoon and the El Niño Southern Oscillation (ENSO) which affects weather in many portions of the globe, including sub-Saharan Africa. Due to the increase in moisture availability, ENSO-related precipitation variability will intensify and remain the dominant mode of interannual variability in tropical pacific.
There has been scientific discourse about this “intensification” of the water cycle.
Intensification of the Hydrological Cycle
Climate change adds a number of new aspects to the quantity and availability of water. The mentioned climate changed induced occurrences of extreme events like heavy precipitation, floods and droughts are discussed focusing on the redistribution of precipitation.
The theoretical basis of this intensification relies on the fact that specific humidity will increase approximately exponentially with temperature.
According to Durack et al. (2012), this intensification, the greater redistribution of precipitation patterns, is double to the projected response and will be substantial in a future 2° to 3° warmer world. Using the indirect method of ocean salinity patterns expressing the global water cycle, they state an intensification of 16 to 24 % per degree of surface warming. Ocean salinity patterns provide a sensitive and detectable measure of water cycle changes because increasing salinities are found in the evaporation-dominated midlatitudes and decreasing salinities in the rainfall-dominated regions.
Clementz and Sewall (2011) looked to the Eocene to investigate the relationships between climate and the water cycle. The global physiography was similar to todays, but the climate was different with atmospheric CO2 concentrations about five times higher. They used the phenomena that the stable isotopic composition of water masses is affected by imbalances in evaporation and precipitation and measured the O isotopic composition of carbonate in the tooth enamel of fossil sirenians. This study also offers evidence about the relationship of water cycles and climatic conditions.
Huntington summarizes numerous results of different publications about time-series analysis of hydroclimatic variables. Precipitation, runoff, tropospheric water vapor, droughts, soil moisture, actual evapotranspiration and growing season length show the trend to increase on regional or global scales. The observed trends are consistent with an intensification of the water cycle, but empirical evidence does not consistently support the idea. He also underlines the large effects human activities can have on the water cycle, for example through air pollution-induced suppression of rainfall or the effects on runoff and infiltration through land-use activities.
Impacts on agricultural water management - options to scope with water scarcity
An intensification of the hydrological cycle will lead to changes in the availability of water for agricultural purposes and therefore the risk of agricultural drought is likely in presently dry regions. The agricultural sector is the most sensitive to water scarcity, as it accounts for 70 per cent of global freshwater withdrawls, and through intensification and unsustainable practices agriculture is also a cause of water scarcity.
Given the importance of water for agriculture and food security, the predicted intensification of the water cycle and the probability of improved water scarcity in the future is a challenge that is there to be surmounted. The human society has to be well prepared for such changes and large numbers of people run the risk of living under water stress or seeing their livelihoods devastated by water related hazards. Comprehensive monitoring of water-related variables, in both quantity and quality aspects, supports decision making and is a prerequisite for adaptive management required under conditions of climate change.
Options to cope with water scarcity in agriculture can be divided between supply enhancement and demand management. Supply enhancement includes for example water conservation measures, access to conventional water resources, re-use of drainage water and wastewater, inter-basin transfers and desalination.
As the term demand management implies, actions controlling water demand are meant to be implemented. The three options to manage water demand are reduce water losses, increase water productivity and water re-allocation.
References and further Information
- Clementz, M.T. and Sewall, J.O. (2011): Latitudinal Gradients in Greenhouse Seawater 18O: Evidence from Eocene Sirenian Tooth Enamel. In: Science 332, 455
- Durack, P.J. et al. (2012): Ocean Salinities Reveal Strong Global Water Cycle Intensification During 1950 to 2000. In: Science 336
- FAO (2012): Coping with water scarcity. An action framework for agriculture and food security. Food and Agriculture Organisation of the United Nations, Rome.
- Held, I.M. and Soden, B.J. (2006): Robust Responses of the Hydrological Cycle to Global Warming. In: Journal of Climate 19, Issue217
- Huntington, T.G. (2006): Evidence for intensification of the global water cycle: Review and synthesis. In: Journal of Hydrology 319 83–95
- IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp
- Oki, T. and Kanae, S. (2006): Global Hydrological Cycles and World Water Resources. In: Science 313, 1068
- Sherwood, S. and Fu, Q.(2014): A Drier Future? In: Science 343, 737
