Speaking of water, climate change affects the hydrologic cycle, leading to more frequent extreme weather events, such as droughts and floods. It is also causing the sea level to rise, which has a variety of impacts, including salination of surface waters and groundwater aquifers.
These changes often exacerbate existing water risks. For example, climate change will concentrate annual precipitation into a shorter time, putting stress on local infrastructure and storage capacity. Catastrophic weather events can also intensify existing water problems, such as hurricanes or a droughts. Climate change will be felt differently in different parts of the worlds, but it will have a greater impact in areas with inadequate infrastructure or lack of capacity to adapt.
According to the IPCC there are four main factors aggravating water scarcity (1):
- Population growth: in the last century, population of the world has tripled. It is expected to rise to 8.9 billion by 2050. Water use has been growing at more than twice the rate of population increase in the last century, and an increasing number of regions are chronically short of water.
- Increased urbanization will focus on the demand for water among a more concentrated population.
- High level of consumption: as the world becomes more developed, the amount of domestic water used by each person is expected to rise significantly.
- Climate change will shrink the resources of freshwater.
The problem of water scarcity depend on various reasons.
First, the distribution of precipitation in space and time is very uneven, leading to tremendous temporal variability in water resources worldwide (2). For example, the Atacama Desert in Chile, the driest place on earth, receives imperceptible annual quantities of rainfall each year. On the other hand, Mawsynram, Assam, India receives over 450 inches [1140 cm!!] annually. If all the freshwater on the planet were divided equally among the global population, there would be 5,000 to 6,000 m3 of water available for everyone, every year (3). Therefore it doesn’t exist a problem of worldwide water scarcity as such, but only an uneven distribution.
Second, the rate of evaporation varies depending on temperature and relative humidity, which impacts the amount of water available to replenish groundwater supplies. The combination of more intense rainfall of shorter duration [meaning less infiltration and more runoff] combined with increased evapotranspiration [the sum of plant transpiration and evaporation from earth’s land surface to atmosphere] and increased irrigation is expected to lead to groundwater depletion (4).
The Hydrological Cycle
The hydrological cycle starts with evaporation from the surface of ocean or land, continues with the formation of clouds in the atmosphere, and with the return of water to surface as precipitation. The cycle ends when the precipitation is either absorbed into the ground or runs off to the ocean, beginning the process over again.
Key changes to the hydrological cycle (associated with an increased concentration of greenhouse gases in the atmosphere and therefore resulting of climate change) include (5):
- Changes in the seasonal distribution and amount of precipitation.
- An increase in precipitation intensity under most situations.
- Changes in the balance between snow and rain.
- Increased evapotranspiration and a reduction in soil moisture.
- Changes in vegetation cover resulting from changes in temperature and precipitation.
- Consequent changes in management of land resources.
- Accelerated melting glacial ice.
- Increases in fire risk in many areas.
- Increased coastal inundation and wetland loss from sea level rise.
- Effects of CO2 on plant physiology, leading to reduced transpiration and increased water use efficiency.
If increased temperatures, as predicted by climate change research, will cause an intensification of the water cycle, there will be more extreme variations in weather events, as droughts will become prolonged and floods will increase in force (6).
Changes in Precipitation and Drought Patterns
Projections of changes in total annual precipitation indicate that increases are likely in the tropics and at polar regions, while decreases are likely in the sub-tropics (7). With the population of these sub-tropical regions likely increasing, water resources are expected to become more stressed in those regions, especially as climate change intensifies.
More precipitation means increasing a region’s susceptibility to different factors, including:
- Rate of soil erosion
- Mass movement of land
- Soil moisture availability
These factors affect key economic factors such as agricultural productivity, land values, and an area’s habitability. In addition, warming accelerates the rate of surface drying, leaving less water moving in near-surface layers of soil.
Projection on how changes in precipitation patterns affect runoff is not yet a precise science, however historical discharge records indicate that it is likely that for each 1°C rise in temperature, global runoff will increase by 4%. As a consequence, the global runoff is predicted to increase 7.8% by the end of the century (2). A region that experiences higher annual precipitation and more runoff increases the likelihood of flooding. Water availability is likely to be further exacerbated by poor management, overuse from increasing populations, and an increase in water demand from increased agricultural production (7).
Precipitations in both western Africa and southern Asia have decreased by 7.5% between 1900 and 2005. Research has shown that area of land characterized as very dry has more than doubled since the 1970s, while the area of land characterized as very wet has slightly declined during the same time period (8).
Melting Glacial Ice
Water supplies are also affected by warmer winter temperatures that cause a decrease in the volume of snowpack with the result of diminishing water resources during summer months. This water is particularly important at midlatitudes and in mountain regions that depend upon glacial runoff to replenish rivers and groundwater supplies. Consequently, these areas will become increasingly susceptible to water shortages with time, because increased temperatures will initially result in a rapid rise in glacial meltwater during the summer months, followed by a decrease in melt as the size of glaciers shrinks. This reduction is projected to affect approximately one-sixth of the world’s population (7).
A reduction of glacial runoff has already been observed in the Andes, whereby the usual trend of glacial replenishment during winter months has been insufficient. This is due to increased temperatures, which have caused the glaciers to retreat. In these areas, approximately one-third of the drinking water is dependent upon these supplies, therefore the projections for the area are not reassuring if this same pattern continues (5).
Although the IPCC projects that the increase in temperatures of several degrees resulting of climate change will lead to an increase in average global precipitation over the 21st century (7), this amount does not necessarily relate to an increase in the amount of potable water available. A decline in water quality in fact can result from the increase in precipitations and runoff, because while the water will contain higher levels of nutrients, it will also carry more pathogens and pollutants. Pollutants and pathogens that were originally stored in groundwater reserves but that the increase in precipitation will flush out in the discharged water.
Similarly, when drought conditions are persistent and groundwater reserves are depleted, the residual water is often of inferior quality as a result of leakage of saline or contaminated water from the land surface, or adjacent water bodies that have highly concentrated quantities of contaminants. This occurs because decreased precipitation and runoff results in a concentration of pollution in the water.
Another significant cause of water degradation comes from the increase in water temperature. The increase can in fact lead to a proliferation of the microbial populations, which can have a negative impact on human health. Additionally, the rise in water temperature can affect the ecosystem due to the species’ sensitivity to temperature. The health of a body of water, such as a river, is dependent upon its ability to effectively self-purify through biodegradation, which is slowed when there is a diminished amount of dissolved oxygen. This occurs when water warms and its ability to hold oxygen decreases.
Effects on Coastal Populations
For coastal populations, water quality is likely to be affected by salinization. This will result from a rise in sea levels, which will increase salt concentrations in groundwater and estuaries. Sea-level rise will not only extend areas of salinity, but will also decrease freshwater availability in coastal areas. Saline intrusion is also a result of increased demand due in part to growing coastal populations that leave groundwater reserves increasingly vulnerable to contamination and diminishing water reserves (7).
- Bates, B., Z. W. Kundzewicz, et al. (2008). Climate Change and Water, IPCC.
- Oki, T. and S. Kanae (2006). “Global Hydrological Cycles and World Water Resources.” Science 313(5790): 1068-1072.
- Vorosmarty, C., P. Green, et al. (2000). “Global Water Resource: Vulnerability from Climate Change and Population Growth.” Science 289(5477): 284-288.
- Konikow, L. and E. Kendy (2005). “Groundwater Depletion: A GLobal Problem.” Hydrogeology 13(317-320).
- Goudie, A. (2006). “Global Warming and Fluvial Geomorphology.” Geomorphology 79(3-4): 384-394.
- Huntington, T. G. (2005). “Evidence for Intensification of the Global Water Cycle: Review and Synthesis.” Journal of Hydrology 319: 83-95.
- Confalonieri, U., M. B., et al. (2007). Human health. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK, IPCC.
- Dai, A., K. Trenberth, et al. (2004). ” A Global Dataset of Palmer Drought Severity Index for 1870-2002: Relationship with Soil Moisture and Effects of Surface Warming.” Journal of Hydrometeorology 5: 117-1130.