Executive Summary 
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}
4.1 Centrality of Water Security in Climate Change and Climate Resilient Development
Water security is defined as ‘the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability’ (Grey and Sadoff, 2007). Risks emanating from various aspects of water insecurity have emerged as a significant global challenge. The Global Risks Report by the World Economic Forum lists water crisis as one of the top five risks in all its reports since 2015 (WEF, 2015; WEF, 2016; WEF, 2017; WEF, 2018; WEF, 2019; WEF, 2020). Water also features prominently in the SDGs (Section 4.8) and plays a central role in various systems transitions needed for climate resilient development. Most SDGs cannot be met without access to adequate and safe water (Ait-Kadi, 2016; Mugagga, 2016). In addition, without adequate adaptation, future water-related impacts of climate change on various sectors of the economy are projected to lower the global GDP by mid-century, with higher projected losses expected in low- and middle-income countries (World Bank, 2017; GCA, 2019).
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