Changes in water resource availability, water quality and the destructive potential of storms and floods will play a central role in determining how climate change will affect human well-being and the functioning of the natural systems on which we depend. The critical role of water may appear obvious given its importance for agricultural productivity, human health and the functioning of ecosystems. It is perhaps less widely understood that water also plays a key role in the functioning of the climate system. In fact, global warming and changes in the water cycle are intricately linked.
While we have an imperfect understanding of the local-scale details of the changes to come, the scientific community now has considerable confidence in projections of some of the key features and broad-scale regional patterns of future changes in the world's water resources. The evidence strongly suggests that many areas of the world that are already grappling with intense competition and growing demands for scarce water supplies may face steadily worsening water supply conditions in the future. Everywhere, climate change will introduce new obstacles into the business of water resource planning and policy development because the climatic and hydrologic patterns of the past will no longer provide a reliable guide to the future.
Perhaps the most helpful way to begin grappling with future water resource changes is to start by taking stock of what we know, what we do not know and why. This analysis will first provide a rough outline of the current state of scientific understanding of the likely impacts of climate change on the world's water resources. It will then turn to the implications of these changes--and particularly the implications of unavoidable uncertainties--for water resource planning and policy negotiations.
SCIENTIFIC UNDERSTANDING OF CHANGES IN CLIMATE AND THE WATER CYCLE
The basic science of the greenhouse effect is well understood and has come to a consensus. Some of the major greenhouse gases--water vapor, carbon dioxide, methane and nitrous oxide--occur naturally in the atmosphere. They play a critical role in the earth's energy balance because they trap enough outgoing infrared radiation to make the surface of the earth warm enough to support life. Concerns about climate change arise from the fact that human activities are releasing large quantities of these substances--and other even more powerful manufactured greenhouse gases such as halocarbons--into the atmosphere. Because carbon dioxide and many of the halocarbons have very long atmospheric lifetimes, the increased concentrations are likely to result in an enhanced greenhouse effect in the future.
The climate system will react to such an increase in heat-trapping capacity by setting in motion processes that will adjust the earth's energy balance to a new equilibrium. These processes include the release of latent heat through increased evaporation, plant transpiration and precipitation--in other words, acceleration of the hydrologic cycle. (1) Hydrologic changes are, thus, an integral part of global climate change. In addition to accelerating evaporation, warming also increases the moisture-holding capacity of the atmosphere. Atmospheric water vapor, in turn, is a powerful greenhouse gas, so increases in the water content of the atmosphere will create a positive feedback that will tend to amplify the warming that humans have initiated by burning fossil fuels and engaging in other activities that release greenhouse gases. (2) It is estimated that the water vapor feedback may be large enough to roughly double the impact of an increase in carbon dioxide alone.
Cloud cover will also change. Clouds play a dual role--both amplifying warming by absorbing outgoing infrared radiation and producing a cooling effect by reflecting away incoming solar radiation. The net effect of cloud-cover changes will depend on the details of changes in cloud characteristics, altitude and location. It remains unclear whether cloud changes will have a positive or negative impact on global average temperatures. (3)
Other positive feedbacks include the warming effect of shrinking snow and ice cover as a darker earth's surface reflects less sunlight back to space, and the impacts of warming on natural sources and sinks of carbon dioxide and methane. Changes in the extent of wetlands and consequent methane generation and changes in the uptake and release of carbon from the oceans accompanied previous periods of warming and cooling. The expected effects of future warming include increased production of methane by tropical wetlands and a decline in the ability of the world's oceans to remove C[O.sub.2] from the atmosphere, because the solubility of C[O.sub.2] in seawater diminishes as the water warms. (4) In addition, we are generating other pollutants that play a role in the earth's energy budget. For example, tiny particles from combustion, especially sulphate aerosols, tend to produce cooling by reflecting incoming sunlight, while dust and soot deposits on snow surfaces have an opposite impact.
These feedbacks and attendant sources of uncertainty are incorporated in model simulations of future climate, and they result in a range of temperature change estimates for any given change in greenhouse gas concentrations. The physical uncertainties, however, are small compared to our inability to foresee the course of human activities and the resulting emissions of greenhouse gases.
Future greenhouse gas concentrations will depend on the pace and characteristics of future global economic development, changes in energy technology, land use change and population growth. Most importantly, greenhouse gas emissions will depend on the policies that we put in place to reduce the amount of climate change that will eventually occur. Pessimistic scenarios in which there is rapid population growth, slow technical progress and continued heavy reliance on fossil fuels are projected to result in much larger climate changes by the end of this century than are more optimistic scenarios in which slower population growth is coupled with a shift to clean, highly resource-efficient technologies and a transition toward a service and information-based economy. For the lowest emission scenario examined, the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report projects that global average temperatures would increase by 1.1[degrees]C to 2.9[degrees]C by the end of the century. (5) The projected increase for the high-end emission scenario is estimated to fall in the range of 2.4[degrees]C to 6.4[degrees]C. (6) Thus, different emission scenarios account for much of the uncertainty surrounding future projections of global temperature changes.
Despite international efforts to negotiate limits on the growth of greenhouse gas emissions, these emissions have increased rapidly in recent years. Measured in terms of C[O.sub.2]-equivalent, annual global emissions of the major greenhouse gases grew by 70 percent between 1970 and 2004, with an almost 10 percent jump between 2000 and 2004. (7) The current atmospheric concentration of C[O.sub.2] is estimated to be 381 parts per million (ppm), by far the highest level experienced over the past 650,000 years. (8) In the middle of the 18th century, prior to major industrialization, the concentration of C[O.sub.2] in the atmosphere stood at 280 ppm.
We are thus on course to experience substantial climate change before today's children have lived out their lives. In fact, there is compelling evidence that climate change is already occurring. The IPCC Fourth Assessment Report concludes that global average temperature increased by approximately 0.74[degrees]C in the hundred years up to 2006, and that eleven of the twelve hottest years in the instrumental record since 1850 occurred between 1995 and 2006. Furthermore, this warming is already having substantial impacts on many natural systems, including dramatic declines in summer sea ice across the Arctic, changes in species ranges, shrinking mountain glaciers, declining snow cover and changes in precipitation and runoff patterns. (9)
With regard to water resources, the local difference between precipitation and evaporation determines the amount of water available for runoff and groundwater recharge. Both will change. Precipitation changes will be critical, but evaporation-which is controlled by changes in other climate variables, such as temperature, humidity, radiation and wind speed--will also play a major role.
For any given emissions scenario, regional temperature change projections are reasonably consistent across climate models, with warming most pronounced in the Arctic and over land masses. Regional precipitation projections are less consistent, but global average precipitation will almost certainly increase with warmer temperatures. For a middle-of-the-road emissions scenario, climate models are projecting a 5 percent increase in global average annual precipitation over land masses by the end of this century. (10)
Warming will also tend to increase the intensity of rainfall and snowfall events because storms will be carrying heavier moisture loads. Cartoons sometimes portray global warming as leading to balmy tropical climates in currently cold locations. In reality, winter will still happen and, if it is cold enough to snow, the chances for a big snowfall will likely increase. When temperatures are above freezing, we can expect to see increases in the likelihood of deluges that may overwhelm storm sewers and cause localized flooding. In areas not on the receiving end of the storm track, dry spells are expected to lengthen and intensify as the warmer atmosphere accelerates the evaporation of any surface moisture. In other words, in different regions and seasons, global warming will increase the potential for both droughts and downpours.
Such global-scale hydrologic changes do not tell us much about...