Environ. Sci. Technol. 2010, 44, 6019–6021
Reducing humanity’s water footprint BRADLEY G. RIDOUTT* Commonwealth Scientific and Industrial Research Organisation (CSIRO), Clayton, Australia STEPHAN PFISTER ETH Zurich, Institute of Environmental Engineering, Zurich, Switzerland
RHONDA SAUNDERS
Author’s Viewpoint
There are limits to the extent that humanity can continue to increase its appropriation of freshwater from the natural environment. Recent discussion has centered on the notion of a safe operating limit, being the level of global freshwater use beyond which widespread irreversible environmental change and harmful impacts on human well being are threatened (1). However, the issue is not only about planetary environmental boundaries, but also about equity, with consumption patterns in developed countries, underpinned by global supply chains, blamed for taking advantage of a disproportionate share of the Earth’s scarce freshwater resources. Water use models have produced estimates of global freshwater withdrawals of 3594 and 3824 km3 yr-1, representing the situation in 1995 and 2000, respectively (2, 3). Other authors estimate current withdrawals at around 4000 km3 yr-1, with around 2600 km3 yr-1 (65%) being consumptive use whereby water is removed from the local hydrological system by such processes as evaporation and crop transpi* E-mail:
[email protected]. 10.1021/es101907z
2010 American Chemical Society
Published on Web 06/28/2010
ration (1). With a proposed planetary boundary of 4000 km3 yr-1 of consumptive surface and groundwater use (with a zone of uncertainty of 4000-6000 km3 yr-1), it would appear that humanity’s freshwater use is currently within the safe operating limit (1). Indeed, these results suggest there may be scope for humanity’s total global freshwater withdrawals to increase and that freshwater scarcity is not such a critical issue compared to climate change, biodiversity loss, or perturbation of the global nitrogen and phosphorus cycles. However, in proposing limits for global freshwater use it is critical to also take into consideration the regional nature of freshwater scarcity. For each of the 11,050 watersheds represented in the WaterGap2 model (as described in 2), we multiplied withdrawals by the local Water Stress Index (WSI) (4). The global sum we refer to as humanity’s water footprint. Somewhat alarmingly, our analysis indicated that the majority of global freshwater withdrawals are currently from watersheds already experiencing extreme water stress (WSI > 0.9, Figure 1). This concurs with reports of an estimated 25% of rivers no longer flowing reliably to the sea, large-scale regional groundwater depletion, and the observed collapse of complete freshwater systems such as the Aral Sea. Therefore, irrespective of the proposed boundaries of global freshwater use, there is an immediate need to relieve excessive pressure on those already highly stressed watersheds where freshwater resources are now being depleted and damages to ecosystems and human health are already manifest. What is deceptive about freshwater use is that the vast majority, or around 90%, is associated with the life cycles of products and services. Globally, withdrawals for the domestic sector are comparatively minor. This means that citizens consume a lot more water than they might realize and that their water footprints extend far beyond their local area and even national boundary. Due to the interconnectedness of global business, the local consumption of products and services is intervening in the hydrological cycle throughout the world and to an extent rarely understood or appreciated. As such, there has been much recent interest in the concept of water footprinting, to make transparent the impacts of consumption and production on global freshwater scarcity (5), including a new ISO work program to develop a global water footprint standard (ISO/TC207/SC5/WG8). The interest in quantifying the environmental impacts related to products and services has also emerged in the area of life cycle assessment and life cycle management, led by the UNEPSETAC Life Cycle Initiative where a working group on water use is active. The importance of water footprinting has also been recognized by such organizations as the World Business Council for Sustainable Development and the UN Global Compact. From a policy standpoint, water footprinting is creating a capacity for change which in many ways is comparable to carbon footprinting. However, fundamental to any strategy to reduce the pressure humanity exerts on freshwater systems is the need for a stabilization target, without which it is difficult for governments, businesses, and individuals to conceptualize the extent of necessary action. In terms of climate change, the concentration of carbon dioxide and VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Distribution of global freshwater withdrawals by local water stress index (WSI, left) and distance-to-target withdrawals for each watershed (km3 yr-1, right) with a stabilization target of WSI e 0.5. Shaded in black are watersheds where withdrawals-to-availability
