The global water cycle is intimately tied to our changing climate. As the planet warms, the water cycle accelerates, with multiple changes in precipitation patterns putting pressure on freshwater ecosystems (Oki and Kanae 2006). The quantity of salt water is now increasing relative to freshwater due to global warming, land-use changes, melting ice and snow reserves, pumping of groundwater, drying of the continents, and rising sea levels (Bates et al. eds. 2008).
Lakes and wetlands are important in regulating water cycles, for example by creating more moderate local climates (Kodama, Eaton and Wendler 1983; Laird et al. 2001; Saaroni and Ziv 2003; McInnes 2016; Dai et al. 2018). They warm up during the day and lose heat more slowly at night than the land surface, reducing temperature extremes in their basins. Through evaporation, they provide water vapour and precipitation during winter, and they cool and stabilize the local climate in summer. Urban wetlands have been shown to provide a local cooling effect of at least 1-3°C (Filho et al. 2017).
Climate change alters water cycles over lakes, wetlands and other standing (lentic) water systems, reducing the quantity of fresh water and waterbody surface area. A warmer climate increases evaporation over the waterbody and adjacent land, but a warmer atmosphere also takes more time to become saturated with water to subsequently produce rainfall. Thus, moisture evaporated from a waterbody may blow away before it can fall as rainfall in its own basin. The basin then becomes drier, with less run-off into the waterbody and associated rivers and wetlands, increasing the need for agricultural irrigation water. These factors collectively accelerate the shrinking of a waterbody, as illustrated in the case of Lake Chad (below), which has lost 90 per cent of its surface area, with an enormous loss of its associated biodiversity, especially fish, and loss of livelihoods for the millions of people dependent upon the lake. Human water use is estimated to account for 50 per cent of the shrinkage and climate change for the remainder (Coe and Foley 2001; Gao et al. 2011). The resulting change in microclimate establishes a cycle that further contributes to the drying and desertification of the continent and intensifies the impacts of global climate change.
Many areas now receive less precipitation than in the past, while others receive more, with most regions experiencing increasingly unpredictable and variable temperature and precipitation patterns. Polar regions and high mountain regions are warming much faster than other parts of the world, with unforeseeable consequences (see Section 4.3.2). A 12 per cent increase in record-breaking high rainfall events occurred globally during 1981-2010 (Lehmann, Coumou and Frieler 2015). By contrast, there is evidence of increasing drought severity in Europe (Vicente-Serrano et al. 2014), with historical records indicating increased aridity over many areas since the 1950s (Dai 2011).
Global climate change interacts with weather and local-scale climate effects, as well as unsustainable water uses and diversions, leading to dramatic impacts such as shrinking freshwater bodies (e.g. Lake Chad, see Box 9.1; the Aral Sea; the disappearing wetlands of Islamic Republic of Iran [e.g. Lake Urmia] and the Iraqi Marshes; and even the Caspian Sea (Rodell et al. 2018)).
Too much rainfall brings pollution, soil erosion, avalanches and mud slides which, together with floods, tornadoes and cyclones, are responsible for much physical damage to infrastructure, loss of life and injury. Too little rainfall causes drought, extreme wildfires, sandstorms, soil degradation and increased competition over water sources, often leading to the accelerated shrinkage and loss of these goods. Collectively, these realities and risks have grave socio-political, economic, environmental and ecological implications, making better management and governance of freshwater resources an imperative.