Climate change is already causing extreme events in many watersheds, impacting communities. The UNFCCC Climate Action and Support Trends 2019 report pointed to water as one of the most vulnerable sectors, poised to impact the greatest number of countries relative to the other sectors identified. Changes in extreme precipitation are likely to be stronger than changes in mean precipitation. Extreme events will continue to increase in intensity and frequency. Extreme precipitation will increase over all climate regions, but with greater intensity in humid and semihumid regions compared to semi-arid areas. This corresponds with a projected increase in the flood intensity for most areas.

Increase in seasonal variability of rainfall

Changes in precipitation impact the distribution and availability of water across geography and time. Specifically, this means that seasonally variable rainfall regimes are anticipated to become even more variable, whereas regimes with low seasonal variation will receive more rainfall in the monsoon. In terms of aridification, 72% of the global land area is likely to become drier (i.e., experience an increase in the aridity). Even when accounting for vegetation response to the increased CO₂ levels (higher CO₂ concentrations can help plants save water) the aridification is expected to have deleterious effects on ecosystems and the ability to sustain life. The aridification would especially affect the Middle East, North Africa, south Europe, and Australia. Urbanization is further altering rainfall, as part of shifts in regional climate patterns. For example, this is increasingly visible within large urban areas in China, where the magnitude and recurrence of extreme precipitation events is increasing. Increased droughts and floods will drive water scarcity due to physical shortage, or to the failure of institutions to ensure a regular supply, or because of a lack of adequate infrastructure.

Extreme rainfall events less studied

General Circulation Models (GCMs) are essential tools for decision-making related to climate. Yet, recent studies have suggested that current practices using them may understate the potential for significant changes in the hydrological cycle, including the risk of extreme events. In using these models, the focus on mean values and variance from that mean overshadows attention to extreme rainfall events. This is problematic since extreme climatic events are very important drivers of water crises, impacting water security in terms of degradation of quality and quantity. Global uncertainty in tropical and subtropical regions is high because of a combination of the difficulty in modeling convective rainstorms and the sparsity of weather observation networks for model validation and refinement. Extremes warrant greater attention in climate modeling and prediction research.

Inequity seen in water crisis impacts

Climate change can impact water quality, as well. For instance, shifts in monsoon timings can lead to dilution or concentration of nitrogen, phosphorus, and other pollutants. The impacts of water crises and climate risks are highly unequal, reflecting social inequality. Inadequate water, sanitation, and hygiene resources disproportionately affect women and girls, leading to negative health and social outcomes. Contamination also impairs the ability of sensitive ecosystems, such as coral reefs, to recover from extreme climate events.

Recent examinations of the 2018 Cape Town water crisis highlight how it is an important illustration of future water insecurity events spurred on in part and made worse by extreme climatic events. A multi-year drought provoked the severe water crisis in Cape Town in 2018, incurring complex political and social ramifications. While existing inequalities were reinforced and competition between water users increased, new opportunities for solidarity and collective action emerged. Water conservation efforts, particularly the city’s creative campaign to reduce demand among residents and businesses, reduced the severity of water scarcity.

Water crises can drive migration

There is increasing policy recognition that water-related extreme events are also contributing to the migration and displacement of millions of people. The United Nations World Water Development Report 2020: Water and Climate Change, documents these cases and suggests that – rather than trying to prevent climate-driven migration and to support achievement of the Sustainable Development Goals – the international policy community should begin considering migration as a potential adaptation strategy. Migration, urban development pathways, and climate change are disruptors that can catalyze shifts in values toward water use and management. An approach to climate change that recognizes the importance of water can combat both the causes and impacts of climate change. Combining climate change adaptation and mitigation strategies based on water can benefit water resources management, including disaster risk reduction, and improve the provision of water supply.