Chapter 4: Water

This chapter assesses observed and projected climate-induced changes in the water cycle, their current impacts and future risks on human and natural systems and the benefits and effectiveness of water-related adaptation efforts now and in the future.

Currently, roughly half of worlds ~8 billion people are estimated to experience severe water scarcity for at least some part of the year due to climatic and non-climatic factors (medium confidence1 ). Since the 1970s, 44% of all disaster events have been flood-related. Not surprisingly, a large share of adaptation interventions (~60%) are forged in response to water-related hazards (high confidence). {4.1, Box 4.1, 4.2.1.1, 4.2.1.2, 4.2.2, 4.2.4, 4.2.5, 4.2.6, 4.3.8, 4.6, 4.7}

Intensification of the hydrological cycle due to human-induced climate change is affecting physical aspects of water security (high confidence), thereby exacerbating existing water-related vulnerabilities caused by other socioeconomic factors. {4.2, 4.2.1.1, 4.2.1.2, 4.2.1.3, 4.2.2, 4.2.4, 4.2.5, 4.2.6, 4.3}
Extreme weather events causing highly impactful floods and droughts have become more likely and (or) more severe due to anthropogenic climate change (high confidence). {4.2.4, 4.2.5, Cross-Chapter Box DISASTER in Chapter 4}
There is increasing evidence of observed changes in the hydrological cycle on people and ecosystems. A significant share of those impacts are negative and felt disproportionately by already vulnerable communities (high confidence). {4.3.1, 4.3.2, 4.3.3, 4.3.4, 4.3.5, 4.3.6, 4.3.8}
Water-related risks are projected to increase with every degree of global warming (high confidence), and more vulnerable and exposed regions and peoples are projected to face greater risks (medium confidence). {Box 4.1, 4.4.1, 4.4.1.1, 4.4.4, 4.5.4, 4.5.5, 4.5.6, Box 4.2}
Drought and flood risks and societal damages are projected to increase with every degree of global warming (medium confidence). {4.4.4, 4.4.5, 4.4.7, 4.5.1, 4.5.2}
Limiting global warming to 1.5°C would reduce water-related risks across regions and sectors (high confidence). {4.4.2, 4.4.5, 4.5.2, 4.5.3, 4.5.4, 4.5.6, 4.5.7, 4.6.1, 4.7.2}
Observed water adaptation responses have multiple benefits (high confidence), yet evidence of effectiveness of adaptation in reducing climate risks is not clear due to methodological challenges (medium confidence). {4.6, 4.7.1, 4.7.3}
Future projected adaptations are effective in reducing risks to a varying extent (medium confidence), but effectiveness falls sharply beyond 2°C, emphasizing the need for limiting warming to 1.5°C (high confidence). {4.6, 4.7.2, 4.7.3}
Water security is critical for meeting Sustainable Development Goals (SDGs) and systems transitions needed for climate resilient development, yet many mitigation measures have a high water footprint which can compromise SDGs and adaptation outcomes (high confidence). {4.1, Box 4.4, 4.6, 4.6.2, 4.6.3, 4.7, 4.7.1, 4.7.4, 4.7.5.7}
A common set of enabling principles underpinned by strong political support can help meet the triple goals of water security, sustainable and climate resilient development (high confidence). {4.8, 4.8.3, 4.8.4., 4.8.5, 4.8.6, 4.8.7}