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World Water Development Report 2020: Water and Climate Change

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Executive Summary

Climate change will affect the availability, quality and quantity of water for basic human needs, threatening the effective enjoyment of the human rights to water and sanitation for potentially billions of people. The hydrological changes induced by climate change will add challenges to the sustainable management of water resources, which are already under severe pressure in many regions of the world.

Food security, human health, urban and rural settlements, energy production, industrial development, economic growth, and ecosystems are all water-dependent and thus vulnerable to the impacts of climate change. Climate change adaptation and mitigation through water management is therefore critical to sustainable development, and essential to achieving the 2030 Agenda for Sustainable Development, the Paris Agreement on Climate Change and the Sendai Framework for Disaster Risk Reduction.

Impacts on water resources

Global water use has increased by a factor of six over the past 100 years and continues to grow steadily at a rate of about 1% per year as a result of increasing population, economic development and shifting consumption patterns. Combined with a more erratic and uncertain supply, climate change will aggravate the situation of currently water-stressed regions, and generate water stress in regions where water resources are still abundant today. Physical water scarcity is often a seasonal phenomenon, rather than a chronic one, and climate change is likely to cause shifts in seasonal water availability throughout the year in several places.

Climate change manifests itself, amongst others, in the increasing frequency and magnitude of extreme events such as heatwaves, unprecedented rainfalls, thunderstorms and storm surge events.

Water quality will be adversely affected as a result of higher water temperatures, reduced dissolved oxygen and thus a reduced self-purifying capacity of freshwater bodies. There are further risks of water pollution and pathogenic contamination caused by flooding or by higher pollutant concentrations during drought.

Many ecosystems, particularly forests and wetlands, are also at risk. The degradation of ecosystems will not only lead to biodiversity loss, but also affect the provision of water-related ecosystem services, such as water purification, carbon capture and storage, and natural flood protection, as well as the provision of water for agriculture, fisheries and recreation.

Much of the impacts of climate change will be manifested in the tropical zones where most of the developing world can be found. Small island developing states are typically environmentally and socio-economically vulnerable to disasters and climate change, and many will experience increasing water stress. Across the planet, drylands are expected to expand significantly. Accelerated melting of glaciers is expected to have a negative effect on the water resources of mountain regions and their adjacent lowlands.

Despite the growing evidence that the changing climate will affect the availability and distribution of water resources, some uncertainties remain, especially at local and basin scales. While there is not much disagreement about the temperature increases, which have been simulated by different General Circulation Models (GCMs) under specific scenario conditions, more variability and ambiguity exist in projected precipitation trends. Often, trends in extremes (heavier precipitation, heat, prolonged droughts) show a clearer direction than trends in annual precipitation totals and seasonal patterns.