Editing IPCC:AR6/WGII/Chapter-4 (section)

== Executive Summary ==

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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 confidence'' [[#footnote-001|1]] ). 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}

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'''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}'''

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Nearly half a billion people live in unfamiliarly wet areas, where the long-term average precipitation is as high as previously seen in only about one in six years ( ''medium confidence'' ). Approximately 163 million people live in unfamiliarly dry areas now ( ''medium confidence'' ). {4.2.1.1}

The intensity of heavy precipitation has increased in many regions since the 1950s ( ''high confidence'' ). Substantially more people (~709 million) live in regions where annual maximum one-day precipitation has increased than regions where it has decreased (~86 million) ( ''medium confidence'' ). At the same time, more people (~700 million) are also experiencing longer dry spells than shorter dry spells since the 1950s ( ''medium confidence'' ). {4.2.1.1}

During the last two decades, the global glacier mass loss rate exceeded 0.5 meters water equivalent per year ( ''high confidence'' ), impacting humans and ecosystems, including cultural uses of water among vulnerable high mountain and polar communities ( ''high confidence'' ). {4.2.2, 4.3.8}

There is a clear trend of increases in streamflow in the northern higher latitudes ( ''high confidence'' ), with climatic factors being more important than direct human influence in a larger share of major global basins ( ''medium confidence'' ). At the same time, groundwater in aquifers across the tropics has experienced enhanced episodic recharge from intense precipitation and flooding events ( ''medium confidence'' ), with implications for sectoral water use. {4.2.3, 4.2.6, 4.3.1, 4.3.4}

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'''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}'''

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Anthropogenic climate change has contributed to the increased likelihood and severity of the impact of droughts (especially agricultural and hydrological droughts) in many regions ( ''high confidence'' ). Between 1970 and 2019, 7% of all disaster events worldwide were drought-related. Yet, they contributed to 34% of disaster-related deaths, mostly in Africa. {4.2.5, 4.3.1, 4.3.2, Cross-Chapter Box DISASTER in Chapter 4}

Several recent heavy rainfall events, such as in western Europe, China, Japan, the USA, Peru, Brazil and Australia that led to substantial flooding, were made more likely by anthropogenic climate change ( ''high confidence'' ). There is ''high confidence'' that the warming in the last 40–60 years has led to ~10 d earlier spring floods per decade. Between 1970 and 2019, 31% of all economic losses were flood-related. {4.2.4, Cross-Chapter Box DISASTER in Chapter 4}

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'''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}'''

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Agriculture and energy production have been impacted by changes in the hydrological cycle ( ''high confidence'' ). Between 1983 and 2009, approximately three-quarters of the global harvested areas (~454 million hectares) experienced yield losses induced by meteorological drought, with the cumulative production losses corresponding to USD 166 billion. There is ''medium confidence'' that current global thermoelectric and hydropower production has been negatively affected due to droughts with ~4–5% reduction in plant utilisation rates during drought years compared to long-term average values since the 1980s. {4.3.1, 4.3.2}

Climate change and changes in land use and water pollution are key drivers of loss and degradation of freshwater ecosystems ( ''high confidence'' ), with impacts observed on culturally significant terrestrial and freshwater species and ecosystems in the Arctic and high-mountain areas ( ''high confidence'' ). In addition, precipitation and extreme weather events are linked to increased incidence and outbreaks of water-related diseases ( ''high confidence'' ). {4.3.3, 4.3.4, 4.3.5, 4.3.8}

Changes in water-related hazards disproportionately impact vulnerable populations such as the poor, women, children, Indigenous Peoples and the elderly in all locations, especially in the Global South, due to systemic inequities stemming from historical, socioeconomic and political marginalisation ( ''medium confidence'' ). {4.3.1, 4.3.3, 4.3.4, 4.3.8}

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'''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}'''

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Climate change impacts via water availability changes are projected to increase with every degree of global warming ( ''high confidence'' ), but there are high regional uncertainties. Between 3 and 4 billion people are projected to be exposed to physical water scarcity at 2°C and 4°C global warming levels (GWL), respectively ( ''low confidence'' ). {Box.4.1; 4.4.1, 4.4.3, 4.4.5, 4.6.1}

By 2100, one third of the 56 large-scale glacierised catchments are projected to experience a mean annual runoff decline by over 10%, with the most significant reductions in central Asia and the Andes ( ''low confidence'' ). Expected impacts may be felt by roughly 1.5 billion people who are projected to critically depend on runoff from the mountains by the mid-21st century (RCP6.0 scenario). {4.4.2, 4.4.3, 4.5.8}

By 2050, environmentally critical streamflow is projected to be affected in 42–79% of the world’s watersheds, causing negative impacts on freshwater ecosystems ( ''medium confidence'' ). Modified streamflow is also projected to affect inflows to urban storage reservoirs and increase the vulnerability of urban water services to hydro-meteorological extremes, particularly in less developed countries ( ''high confidence'' ). {4.4.6, 4.5.4, 4.4.5}

Future water-related impacts of climate change on various sectors of the economy are projected to lower global gross domestic product (GDP) (ranging from 0.49% of GDP by mid-century (SSP3) to less than 0.1% (RCP8.5, SSP5), with higher projected losses expected in low- and middle-income countries ( ''medium confidence'' ). {4.7.5}

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'''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}'''

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Drought risks are projected to increase over the 21st century in many regions ( ''very high confidence'' ), increasing economy-wide risks ( ''high confidence'' ). With RCP6.0 and SSP2, the global population exposed to extreme-to-exceptional total water storage drought is projected to increase from 3% to 8% over the 21st century ( ''medium confidence'' ). {4.4.5}

The projected increase in precipitation intensity ( ''high confidence'' ) will increase rain-generated local flooding ( ''medium confidence'' ). Direct flood damages are projected to increase by four to five times at 4°C compared to 1.5°C ( ''medium confidence'' ). {Box 4.1, 4.4.1, 4.4.1.1, 4.4.4, 4.5.4, 4.5.5}

At 4°C global warming, by the end of the century, approximately 10% of the global land area is projected to face simultaneously increasing high extreme streamflow and decreasing low extreme streamflow, affecting roughly over 2.1 billion people ( ''medium confidence'' ). {4.4.3} The increase in extreme events is projected to compromise the efficacy of WaSH services and slow progress towards reductions in WaSH-related disease burdens ( ''medium confidence'' ). {4.5.3}

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'''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}'''

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Projected increases in hydrological extremes pose increasing risks, with a potential doubling of flood risk between 1.5°C and 3°C of warming and an estimated 120–400% increase in population at risk of river flooding at 2°C and 4°C, respectively. Projected losses include a 1.2- to 1.8-fold increase in GDP loss due to flooding between 1.5°C and 2°C warming ( ''medium confidence'' ). {4.4.3, 4.4.4, 4.4.5, 4.5.6, 4.6.1, 4.7.2}

Over large areas of northern South America, the Mediterranean, western China and high latitudes in North America and Eurasia, extreme agricultural droughts are projected to be at least twice as likely at 1.5°C global warming, 150 to 200% more likely at 2°C warming, and over 200% at 4°C ( ''medium confidence'' ). Due to the combined effects of water and temperature changes, risks to agricultural yields could be three times higher at 3°C compared to 2°C ( ''medium confidence'' ). {4.5.1, 4.6.1}

In Mediterranean parts of Europe, hydropower potential reductions of up to 40% are projected under 3°C warming, while declines below 10% and 5% are projected under 2°C and 1.5°C warming levels, respectively.

Climate-induced hydrological changes are projected to increase migration in the last half of the century, with an almost seven-fold increase in asylum seekers to the European Union (EU) for RP4.5 compared to RCP2.6. The number of internally displaced people in sub-Saharan Africa, South Asia and Latin America increased almost five times for RCP8.5 compared to RCP2.6 ( ''low confidence).'' {4.5.7}

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'''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}'''

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A large share of adaptation interventions (~60%) are shaped in response to water-related hazards ( ''high confidence'' ) and involve water interventions (irrigation, rainwater harvesting, soil moisture conservation). Adaptation responses in developing countries tend to be autonomous, incremental and focused on managing water-related risks in agriculture. In contrast, responses are more policy-oriented and urban-focused in developed countries ( ''high confidence'' ). {4.6.2, box 4.3, 4.6.5, 4.7.1, 4.7.2}

Irrigation helps stabilise and increase crop yields and is often a preferred strategy for farmers and policymakers for risk reduction, but irrigation is also associated with a range of adverse outcomes, including groundwater over-extraction ( ''medium confidence'' ). In addition, large-scale irrigation also affects local to regional climates, both in terms of temperature and precipitation change ( ''high confidence'' ). {4.2.6, 4.6.2, Box. 4.2} .

Water adaptation measures tend to have positive economic outcomes in developing countries and positive environmental outcomes in developed countries ( ''high confidence'' ) ''.'' Roughly one third and one fourth of case studies on water adaptation also documents maladaptation and co-benefits, respectively ( ''high confidence'' ). A significant knowledge gap remains in knowing if observed adaptation benefits also translate to climate risk reduction, if so, by how much and under what conditions ( ''medium confidence'' ). {4.7.1, 4.7.2, 4.7.4}

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'''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}'''

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Adaptations that are beneficial now (e.g., crop- and water-related ones) are also projected to effectively reduce specific future risks to a moderate to a large extent ( ''medium confidence'' ). However, residual impacts remain for some options and regions at all levels of warming, and the overall effectiveness decreases at higher warming levels ( ''high confidence'' ), further emphasizing the need for limiting warming to 1.5°C. {Box 4.2, 4.7.1, 4.7.2, 4.7.3, 4.7.4}

At warming levels beyond 1.5°C, the potential to reach biophysical limits to adaptation due to limited water resources are reported for small islands ( ''medium confidence'' ) and regions dependent on glaciers and snowmelt ( ''medium confidence'' ). {4.7.4}

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'''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}'''

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Water features prominently in nationally determined contributions (NDCs) and national adaptation plans (NAPs) of most countries. SDGs cannot be met without adequate and safe water ( ''high confidence'' ), and water is fundamental to all systems transition ( ''high confidence'' ). {4.1, 4.7, 4.7.1, 4.8, 4.8.7}

Water garners a significant share of public and private adaptation funds ( ''high confidence'' ). However, barriers remain for low-income countries to access funds ( ''medium confidence'' ), and there is insufficient evidence on benefits for marginalised groups ( ''medium confidence'' ). {4.8.2}

Many mitigation measures, such as carbon capture and storage, bio-energy and afforestation and reforestation, can have a high-water footprint ( ''high confidence'' ). The water intensity of mitigation must be managed in socially and politically acceptable ways to increase synergies with SDGs, improve water security and reduce trade-offs with adaptation ( ''medium confidence'' ). {4.7.6}

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'''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}'''

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Many countries and social groups most threatened by climate change have contributed the least to the problem and do not have the adequate resources to adapt (high confidence). Water adaptation policies enabled through ethical co-production between holders of Indigenous knowledge, local knowledge and technical knowledge (medium confidence), through cooperation and coordinated actions among multiple actors, including women and all marginalised groups, at various levels of governance (medium confidence) is needed for effective transitions towards climate resilient development. {4.8, 4.8.3, 4.8.4, 4.8.5, 4.8.6}

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