How global warming impacts water.
Water in its various forms is always on the move, in a complex process known as the water cycle. Global warming is already having a measurable effect on this cycle, altering the amount, distribution, timing, and quality of available water. Water users – from communities, to industries, to ecosystems – are in turn affected: their activities and functions depend, either directly or indirectly, on water.
Change is underway
With climate change, the water cycle is expected to undergo significant change. For example, a warmer climate causes more water to evaporate from both land and oceans; in turn, a warmer atmosphere can hold more water – roughly four percent more water for every 1ºF rise in temperature. Changes like this are expected to lead to specific, and in many cases negative, consequences. Some parts of the U.S. – in particular, the Northeast and Midwest – can expect increased precipitation and runoff, especially in winter and spring, leading to increased flooding. Other areas – notably the Southwest – can expect less precipitation, especially in the warm months, and longer, more severe droughts as storm tracks shift northward leaving arid areas increasingly dry.
Rain versus snow can make a critical difference
The form that precipitation takes is also subject to change in response to warming: climate projections for many regions of North America suggest less snow, overall, and more rain. In areas dependent on the gradual melting of snowpack to supply surface water through the warm months, this means lower flows and greater water stress in summer – a trend already in evidence in parts of the western U.S. While the effects of climate change on groundwater are not fully understood, rising water competition and stress at the surface are likely to drive greater use – and overuse – of this resource.
Overall, wet areas are expected to become wetter and dry areas drier, placing additional stress on the nation's over-taxed water systems as well as water-dependent sectors.
Water quality affects people and ecosystems
Declining water quality is another consequence of climate change. Water temperature, for example, will generally rise in streams, lakes, and reservoirs as air temperature rises. This tends to lead to lower levels of dissolved oxygen in water, hence more stress on the fish, insects, crustaceans and other aquatic animals that rely on oxygen. As more – and more intense – precipitation leads to increased runoff in certain regions, we can also expect more pollution to be washed into our waterways: sediments, nitrogen from agriculture, disease pathogens, pesticides, and herbicides. Naturally, the pollution load in streams and rivers will tend to be carried to larger bodies of water downstream – lakes, estuaries, and the coastal ocean – where one of the more dramatic consequences of heavy runoff can be blooms of harmful algae and bacteria.
The tide is rising
One of the starkest effects of climate change is the anticipated rise in sea level worldwide. This occurs for two main reasons – the expansion of the ocean as it warms, and the increased melt from ice sheets, ice caps and glaciers. Along with alarming threats to coastal communities, infrastructure, economies and ecosystems, this rise has implications for available freshwater, as rising sea levels drive saltwater into freshwater aquifers. To be useful for drinking or irrigating, more water from our aquifers, then, would need to be treated, usually by energy-intensive processes. Given the wide range of human activities that depend – directly or indirectly – on water, future climate-driven changes in water resources will affect many aspects of our lives.