Earth: the water planet
Around 70% of the Earth’s surface is covered in water. Over 97% of this is in the oceans, and only around 2.5% is in the form of freshwater (see table below). Of this freshwater, 69% is in permanent ice and snow cover (Antarctica, Arctic, mountain regions); 30% exists as groundwater, and is mainly inaccessible; and the remaining c. 1% is in rivers and lakes, accessible to us for drinking water. Small amounts of freshwater also exist in permafrost, the atmosphere, and living things. All of this water combined forms what we term the hydrosphere, which contains approximately 1.4 billion km3 water (Shiklomanov, 1998). This means that only 0.25% of all of the world’s freshwater is suitable for us to use. This represents around 90,000 km3 out of the hydrosphere’s 1.4 billion km3 (see figure below). It is evident, then, that this freshwater is a valuable and often scarce resource.
On average, there is around 2,000 m2 water available per person, per year (Shiklomanov, 1998). This is more than sufficient to cover our requirements, such as drinking and bathing. The main issues in water resources lie in the uneven distribution of this water over time and across the world. In general terms, water availability tracks global climatic zones, such that deserts receive much less water than temperate or tropical regions. This has clear implications for the ways in which communities have adapted to living in different environments, and managing their water resources in different ways. Over the last few decades however, there have been increasing pressures on global water stores, due to environmental change, population growth and pollution. The World Water Council (2010) have stated that:
Within the next fifty years, the world population will increase by another 40 to 50 %. This population growth – coupled with industrialization and urbanization – will result in an increasing demand for water and will have serious consequences on the environment.
Climate change and water availability
The hydrological cycle is very closely linked to changes in atmospheric temperature and radiation balance. As climate changes, due to human activity or otherwise, the hydrological cycle is also altered. This can cause changes in: weather patterns, snow cover, ice melt, atmospheric water content, and runoff (IPCC, Bates et al., 2008). Climate models from the Hadley Centre (HadCM2 and HadCM3 simulations) suggest that under increasing global temperatures, annual runoff will increase at high latitudes (such as polar areas) and in equatorial regions, and will decrease in mid-latitude or subtropical locations. These regions have some of the world’s highest population densities. The proportion of precipitation falling as snow, and the duration of snow cover are predicted to decrease in many parts of the world (Arnell, 1999). This may lead to significant changes in the timing of maximum streamflow and the delivery of water downstream. Some areas, for example, may experience considerably reduced water availability, and enter conditions of ‘water stress’. The IPCC (Bates et al., 2008) define a watershed/river basin as being water stressed when either water availability per person, per year is below 1,000 m3 (100 m2) based on long-term average runoff; or the ratio of water withdrawals to long-term average annual runoff is above 0.4. Such water stresses basins typically lie in: northern Africa, the Middle East, Australia, and the Mediterranean.
Changes in precipitation intensity and variability are projected to increase the frequency of extreme events such as drought and flooding. In some regions, changes in the frequency and magnitude of such events may also lead to an increase in water pollution, due to heightened levels of: dissolved nutrients and pesticides; pathogens; and dissolved organic carbon, for example. Under the scenarios that have been briefly described above, the ways that we currently manage water resources would no longer be suitable. Areas that experience more severe drought conditions for example, would need to adapt their water management strategies to account for decreased water availability. On the other hand, regions that may be affected by heightened flood risk would need to develop more robust flood defence strategies.
Scientists investigate changes in the hydrological cycle through the detailed monitoring of present-day hydrological systems and/or modelling the potential response of these systems to future climate change scenarios. Importantly, our understanding of the hydrological system is inherently complicated by the interconnectedness of the earth system and its cycles. Changes in the hydrological cycle for example lead to changes in oceanic and atmospheric systems. If we consider that rising global temperatures would lead to increased melting of ice masses, these vast stores of meltwater would be released into the oceans (either directly, or via runoff). Large influxes of freshwater can alter the salinity of the oceans, leading to changes in thermohaline circulation and associated evaporation-precipitation cycles. It is these feedback mechanisms that can lead to some of the uncertainties in our projections of future environmental change. What is known, however, is that increasing global temperatures are leading to significant changes in the hydrological cycle. Water is a vital resource for life on Earth. Our future management of water resources will depend on a more thorough understanding of the impacts of climate change on the hydrological system.
Facts and figures
The world’s water stores (taken from Shiklomanov, 1993):