Water is integral to life on Earth. It is essential to the survival of people, organisms, and economies. But issues surrounding water resources and management are not rooted in the question of whether there is enough water on our planet. Rather, they are driven by the state of the water available to us. Is the water salty or fresh? Is it frozen or liquid? Is it clean or contaminated? Is it here or elsewhere? Is it available when it is needed, or does it arrive when it is harmful? The effectiveness of strategies for dealing with water availability, quality, and variability is a defining determinant of the persistence of species, the functions of ecosystems, the vibrancy of societies, and the strength of economies. How can we describe the world’s water?
Water on Earth can be divided into five main pools totalling 1.38 billion cubic kms. Water vapor in the atmosphere is the smallest pool, making up less than 0.001 percent of the total. Lakes, rivers, and streams hold about 0.013 percent of Earth’s water, of which nearly half is in the form of salty lakes. Groundwater holds about 1.7 percent of the total, but, again, more than half of all groundwater is salty. Ice caps and permanent snow–including the massive continental ice sheets on Antarctica and Greenland, alpine glaciers, and seasonal snow–constitute another 1.7 per - cent of the total. The fifth and largest pool comprises the oceans of salty water, making up 96.5 percent of Earth’s total. Put another way, only about 2.5 percent of the world’s water is fresh; the remainder is salty. About half a million cubic kms of water, or 0.036 percent of the total, evaporates and falls as precipitation each year. Of this, about 21 percent falls on land– more than one half of which evaporates directly back into the atmosphere–while the remainder falls back to the oceans.
Relative to the enormity of the total, human impact on Earth’s water may initially appear quite small. For example, the total water in ice on land has been decreasing by about 300 cubic kms per year as a result of warming temperatures and changing precipitation patterns, and groundwater that serves the world’s arid and semiarid areas has been decreasing by about 150 cubic kms per year as a result of human extraction. The total water footprint of human activities (all of the water used for crops, manufacturing, and domestic purposes) is on the order of 7500 cubic kms per year.
But, as we will see, the effects of our water use are massive. Much of the challenge of understanding and managing water arises from the fact that it is central to so many activities. As a consequence, decisions about water often tell us more about our priorities than they do about the total amount of available water. Many of the trade-offs in allocating water involve three big water users: food, energy, and environment. A world with an increasing human population, burgeoning energy demands, evolving food preferences, and a rapidly changing global climate means that everything about the water equation is dynamic. The result is a complicated web of interconnections with potentially unexpected risks, but also with many points for intelligent intervention.
Changing the distribution of water among pools, storing huge quantities of water, or moving water long distances is feasible at a scale that is modest relative to global totals but that is crucial locally. The constraints are physical (as with the large inputs of energy required for desalination), geographical (many of the logical locations for reservoirs have already been used), financial (building and sustaining the infrastructure required for managing water is expensive), political (nobody wants to relinquish rights to scarce water without compensation), and ethical (what uses deserve to be prioritized, and how do they relate to the needs of the environment?).
The concepts of trade-offs and cycles are fundamental to understanding the linkages among water, climate, food, and the broader environment. For many kinds of water uses, allocation to one use intrinsically means less water for other uses. Consumptive use for agriculture, industry, or cities almost always involves trade-offs, as do mandates for instream flows to protect ecosystems or fisheries. But even consumptive use leaves the total amount of global water unchanged; the real issue is that consumption shifts water to a different part of the hydrological cycle: for example, from liquid to vapour, clean to contaminated, or fresh to salty. Choices about managing water trade-offs involve more than hydrology and economics. They involve values, ethics, and priorities evolved and embedded in societies over thousands of years. The juxtaposition of hydrology, economics, and values is at the crux of the water-climate-food-energy-nature nexus.
Water and climate are inextricably linked. Climate defines the amount, variability, and type of precipitation; the rate of evaporation; and the conversion of water to its various phases (snow, ice, liquid, vapor). Climate also influences how water moves through land and water bodies, and how it changes throughout the journey. Climate change alters all of these processes.
For some processes, the impacts of recent and future climate changes are clear. For others, the complexity of the climate system and the uncertainty of future human actions make both detection of past changes and prediction of future patterns fiendishly difficult. At the simple end of the scale, a warming climate leads to a glob al increase in evaporation, which in turn leads to an increase in global precipitation. On the other end of the scale, it is far more difficult to understand how changes in temperatures, cloud cover, the incidence and intensity of extreme meteorological events, and other aspects of a changing climate will impact our ability to provide a stable, plentiful, and safe supply of water within the context of a growing global population. We will use two examples to illustrate the complex and multifaceted interactions between climate and water availability: water quantity and the role of extremes; and water quality and the links to eutrophication.
The spatial patterns and intensity of precipitation are far from uniform, and climate change only increases this variability. The general pattern is that wet regions tend to experience increased precipitation, while dry areas tend to get drier, thereby leading to increased risks of both wet (flooding) and dry (drought) extremes.
Many parts of the world have already experienced an increase in the fraction of all precipitation that falls in the heaviest weather events (which are more likely to lead to flooding). A warmer atmosphere can hold more moisture, increasing the likelihood of conditions that release huge amounts of precipitation over a short period of time. As a consequence, flood risks can rise even while increased evaporation is challenging water supplies. In the Eastern and Midwestern United States, which have experienced an increase of more than 30 percent in heavy downpours over the last fifty years, the motivation for recognizing and preparing for an increased risk of heavy rainfall is clear. One recent paper concluded that 18 percent of moderate precipitation extremes over land (in addition to 75 percent of moderate heat extremes) are a result of global warming that has already occurred.
Conversely, the trend toward drying is amplified by increased evaporation caused by warming, reflecting not only more rapid moisture loss from reservoirs but also increased water demands for crops and natural ecosystems. With warming, many areas will face increased risks of severe water shortages, even if average precipitation does not change. Beyond precipitation, climate change is also altering global patterns of the physical state of water. In most regions, climate change is leading to decreases in snow and ice. In areas with winter temperatures not too far below freezing, even modest warm - ing can lead to a dramatic decrease in the fraction of precipitation that falls as snow. For example, although California’s record breaking low snowpack in spring 2015 is partly a reflection of low precipitation, it is also a consequence of warmer storms that bring rain instead of snow, reducing the ability of mountainous regions to store water for dryer seasons. The melting of alpine glaciers further threatens water supplies, especially in parts of Asia and South America, where thaw leads initially to an increase in river flow and eventually to a loss of year-to-year buffering.
This is the first part of a two-part series