Humans have built dams and impoundments for thousands of years for various purposes, including flood control, water supply, irrigation, recreation, navigation, and the generation of hydropower (WCD 2000). Yet the number and storage volumes of dams and reservoirs have in-

Human pressures on the environment are changing spatially and temporally, with profound implications for the planet's biodiversity and human economies. Here we use recently available data on infrastructure, land cover and human access into natural areas to construct a globally standardized measure of the cumulative human footprint on the terrestrial environment at 1 km2 resolution from 1993 to 2009. We note that while the human population has increased by 23% and the world economy has grown 153%, the human footprint has increased by just 9%. Still, 75% the planet's land surface is experiencing measurable human pressures. Moreover, pressures are perversely intense, widespread and rapidly intensifying in places with high biodiversity. Encouragingly, we discover decreases in environmental pressures in the wealthiest countries and those with strong control of corruption. Clearly the human footprint on Earth is changing, yet there are still opportunities for conservation gains.

Water scarcity severely impairs food security and economic prosperity in many countries today. Expected future population changes will, in many countries as well as globally, increase the pressure on available water resources. On the supply side, renewable water resources will be affected by projected changes in precipitation patterns, temperature, and other climate variables. Here we use a large ensemble of global hydrological models (GHMs) forced by five global climate models and the latest greenhouse-gas concentration scenarios (Representative Concentration Pathways) to synthesize the current knowledge about climate change impacts on water resources. We show that climate change is likely to exacerbate regional and global water scarcity considerably. In particular, the ensemble average projects that a global warming of 2°C above present (approximately 2.7°C above preindustrial) will confront an additional approximate 15% of the global population with a severe decrease in water resources and will increase the number of people living under absolute water scarcity (<500 m 3 per capita per year) by another 40% (according to some models, more than 100%) compared with the effect of population growth alone. For some indicators of moderate impacts, the steepest increase is seen between the present day and 2°C, whereas indicators of very severe impacts increase unabated beyond 2°C. At the same time, the study highlights large uncertainties associated with these estimates, with both global climate models and GHMs contributing to the spread. GHM uncertainty is particularly dominant in many regions affected by declining water resources, suggesting a high potential for improved water resource projections through hydrological model development.F reshwater is one of the most vital natural resources of the planet. The quantities that humans need for drinking and sanitation are relatively small, and the fact that these basic needs are not satisfied for many people today is primarily a matter of access to, and quality of, available water resources (1). Much larger quantities of water are required for many other purposes, most importantly irrigated agriculture, but also for industrial use, in particular for hydropower and the cooling of thermoelectric power plants (2, 3). These activities critically depend on a sufficient amount of freshwater that can be withdrawn from rivers, lakes, and groundwater aquifers. Whereas scarcity of freshwater resources already constrains development and societal well-being in many countries (4, 5), the expected growth of global population over the coming decades, together with growing economic prosperity, will increase water demand and thus aggravate these problems (6-8).Climate change poses an additional threat to water security because changes in precipitation and other climatic variables may lead to significant changes in water supply in many regions (6-11). The effect of climate change on water resources is, however, uncertain for a number of reasons. Climate model projections, although rather ...

[1] This paper demonstrates the potential of combining observed river discharge information with climate-driven water balance model (WBM) outputs to develop composite runoff fields. Such combined runoff fields simultaneously reflect the numerical accuracy of the discharge measurements and preserve the spatial and temporal distribution of simulated runoff. Selected gauging stations from the World Meteorological Organization Global Runoff Data Centre (GRDC) data archive were geographically coregistered to a gridded simulated topological network at 30 0 (longitude  latitude) spatial resolution (STN-30p). Interstation regions between gauging stations along the STN-30p network were identified, and annual interstation runoff was calculated. The annual interstation runoff was compared with outputs from WBM calculations, which were performed using long-term mean monthly climate forcings (air temperature and precipitation). The simulated runoff for each cell was multiplied by the ratio of observed to simulated runoff of the corresponding interstation region from the GRDC data set to create spatially distributed runoff fields at 30 0 resolution. The resulting composite runoff fields (UNH/GRDC Composite Runoff Fields V1.0) are released to the scientific community along with intermediate data sets, such as station attributes and long-term monthly regimes of the selected gauging stations, the simulated topological network (STN-30p), STN-30p derived attributes for the selected stations, and gridded fields of the interstation regions along STN-30p. These data sets represent high-resolution fields that are of value to a broad range of water-related research, including terrestrial modeling, climate-atmosphere interactions, and global water resource assessments.