Editing IPCC:AR6/WGI/TS (section)

== Box TS.6 | Water Cycle ==

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'''Human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century (''high confidence''), and it is projected to cause substantial further changes at both global and regional scales (''high confidence'').'''

'''Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s (''medium confidence''). Atmospheric water vapour has increased throughout the troposphere since at least the 1980s (''likely'') . Annual global land precipitation will increase over the 21st century as global surface temperature increases (''high confidence''). Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P–E) patterns over the ocean (''high confidence'').'''

'''The severity of very wet and very dry events increase in a warming climate (''high confidence''), but changes in atmospheric circulation patterns affect where and how often these extremes occur. Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emissions scenarios (''high confidence'').'''

'''Over the 21st century, the total land area subject to drought will increase and droughts will become more frequent and severe (''high confidence''). Near-term projected changes in precipitation are uncertain mainly because of internal variability, model uncertainty and uncertainty in forcings from natural and anthropogenic aerosols (''medium confidence'').'''

'''Over the 21st century and beyond, abrupt human-caused changes to the water cycle cannot be excluded (''medium confidence''). Links to chapters 2.3, 3.3, 4.3, 4.4, 4.5, 4.6, 8.2, 8.3, 8.4, 8.5, 8.6, 11.4, 11.6, 11.9'''

There is ''high confidence'' that the global water cycle has intensified since at least 1980 expressed by, for example, increased atmospheric moisture fluxes and amplified precipitation minus evaporation patterns. Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s (''medium confidence''), and a ''likely'' human contribution to patterns of change, particularly for increases in high-latitude precipitation over the Northern Hemisphere. Increases in global mean precipitation are determined by a robust response to global surface temperature (''very likely'' 2–3% per °C) that is partly offset by fast atmospheric adjustments to atmospheric heating by greenhouse gases (GHGs) and aerosols (Section TS.3.2.2). The overall effect of anthropogenic aerosols is to reduce global precipitation through surface radiative cooling effects (''high confidence''). Over much of the 20th century, opposing effects of GHGs and aerosols on precipitation have been observed for some regional monsoons (''high confidence'') (Box TS.13). Global annual precipitation over land is projected to increase on average by 2.4% (–0.2% to +4.7% ''likely'' range) under SSP1-1.9, 4.6% (1.5% to 8.3% ''likely'' range) under SSP2-4.5, and 8.3% (0.9% to 12.9% ''likely'' range) under SSP5-8.5 by 2081–2100 relative to 1995–2014 (Box TS.6, Figure 1). Inter-model differences and internal variability contribute to a substantial range in projections of large-scale and regional water cycle changes (''high confidence''). The occurrence of volcanic eruptions can alter the water cycle for several years (''high confidence''). Projected patterns of precipitation change exhibit substantial regional differences and seasonal contrast as global surface temperature increases over the 21st century (Box TS.6, Figure 1). Links to chapters 2.3.1, 3.3.2, 3.3.3, 3.5.2, 4.3.1, 4.4.1, 4.5.1, 4.6.1, Cross-Chapter Box 4.1, 8.2.1, 8.2.2, 8.2.3, Box 8.1, 8.3.2.4, 8.4.1, 8.5.2, 10.4.2

Global total column water vapour content has ''very likely'' increased since the 1980s, and it is ''likely'' that human influence has contributed to tropical upper tropospheric moistening. Near-surface specific humidity has increased over the ocean (''likely'') and land (''very likely'') since at least the 1970s, with a detectable human influence (''medium confidence''). Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P–E) patterns over the ocean (''high confidence''). It is ''virtually certain'' that evaporation will increase over the ocean and ''very likely'' that evapotranspiration will increase over land, with regional variations under future surface warming (Box TS.6, Figure 1). There is ''high confidence'' that projected increases in precipitation amount and intensity will be associated with increased runoff in northern high latitudes (Box TS.6, Figure 1). In response to cryosphere changes (Section TS.2.5), there have been changes in streamflow seasonality, including an earlier occurrence of peak streamflow in high-latitude and mountain catchments (''high confidence''). Projected runoff (Box TS.6, Figure 1c) is typically decreased by contributions from small glaciers because of glacier mass loss, while runoff from larger glaciers will generally increase with increasing global warming levels until their mass becomes depleted (''high confidence''). Links to chapters 2.3.1, 3.3.2, 3.3.3, 3.5.2, 8.2.3, 8.4.1, 11.5

Warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events (''high confidence''). Greater warming over land than over the ocean alters atmospheric circulation patterns and reduces continental near-surface relative humidity, which contributes to regional drying (''high confidence''). A ''very likely'' decrease in relative humidity has occurred over much of the global land area since 2000. Projected increases in evapotranspiration due to growing atmospheric water demand will decrease soil moisture over the Mediterranean region, south-western North America, South Africa, South-Western South America and south-western Australia (''high confidence'') (Box TS.6, Figure 1). Some tropical regions are also projected to experience enhanced aridity, including the Amazon basin and Central America (''high confidence''). The total land area subject to increasing drought frequency and severity will expand (''high confidence''), and in the Mediterranean, South-Western South America, and Western North America, future aridification will far exceed the magnitude of change seen in the last millennium (''high confidence''). Links to chapters 4.5.1, 8.2.2, 8.2.3, 8.4.1, Box 8.2, 11.6, 11.9

Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle (''high confidence''). Large-scale deforestation ''likely'' decreases evapotranspiration and precipitation and increases runoff over the deforested regions relative to the regional effects of climate change (''medium confidence''). Urbanization increases local precipitation (''medium confidence'') and runoff intensity (''high confidence'') (Box TS.14). Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions (''medium confidence''). There is ''high confidence'' that groundwater depletion has occurred since at least the start of the 21st century, as a consequence of groundwater withdrawals for irrigation in agricultural areas in drylands. Links to chapters 8.2.3, 8.3.1, 11.1.6, 11.4, 11.6, FAQ 8.1

Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emissions scenarios (''high confidence''). A warmer climate increases moisture transport into weather systems, which intensifies wet seasons and events (''high confidence''). The magnitudes of projected precipitation increases and related extreme events depend on model resolution and the representation of convective processes (''high confidence''). Increases in near-surface atmospheric moisture capacity of about 7% per 1ºC of warming lead to a similar response in the intensification of heavy precipitation from sub-daily up to seasonal time scales, increasing the severity of flood hazards (''high confidence''). The average and maximum rain-rates associated with tropical and extratropical cyclones, atmospheric rivers and severe convective storms will therefore also increase with future warming (''high confidence''). For some regions, there is ''medium confidence'' that peak tropical cyclone rain-rates will increase by more than 7% per 1°C of warming due to increased low-level moisture convergence caused by increases in wind intensity. In the tropics year-round and in the summer season elsewhere, interannual variability of precipitation and runoff over land is projected to increase at a faster rate than changes in seasonal mean precipitation (Figure TS.12e,f) (''medium confidence''). Sub-seasonal precipitation variability is also projected to increase, with fewer rainy days but increased daily mean precipitation intensity over many land regions (''high confidence''). Links to chapters 4.5.3, 8.2.3, 8.4.1, 8.4.2, 8.5.1, 8.5.2, 11.4, 11.5, 11.7, 11.9

[[File:dc131ab5f093e225937615a4c3afd167 IPCC_AR6_WGI_TS_Box_6_Figure_1.png]]

'''Box TS.6, Figure 1 |'''

'''Projected water cycle changes.''' ''The intent of this figure is to give a geographical overview of changes in multiple components of the global water cycle using an intermediate emissions scenario. Important key message: without drastic reductions in greenhouse gas emissions, human-induced global warming will be associated with widespread changes in all components of the water cycle.'' Long-term (2081–2100) projected annual mean changes (%) relative to present-day (1995–2014) in the SSP2-4.5 emissions scenario for '''(a)''' precipitation, '''(b)''' surface evapotranspiration, '''(c)''' total runoff and '''(d)''' surface soil moisture. Numbers in top right of each panel indicate indicate the number of Coupled Model Intercomparison Project Phase 6 (CMIP6) models used for estimating the ensemble mean. For other scenarios, please refer to relevant figures in Chapter 8. Uncertainty is represented using the simple approach: No overlay indicates regions with high model agreement, where ≥80% of models agree on sign of change; diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Links to chapters 8.4.1; Figures 8.14, 8.17, 8.18, and 8.19

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'''Infographic TS.1 |''' '''Climate Futures'''

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'''Infographic TS.1 |''' '''Climate Futures.''' ''The intent of this figure is to show possible climate futures: The climate change that people will experience this century and beyond depends on our greenhouse gas emissions, how much global warming this will cause and the response of the climate system to this warming.''

'''(top left)''' Annual emissions of CO <sub>2</sub> for the five core Shared Socio-economic Pathway (SSP) scenarios (very low: SSP1-1.9, low: SSP1-2.6, intermediate: SSP2-4.5, high: SSP3-7.0, very high: SSP5-8.5). '''(bottom left)''' Projected warming for each of these emissions scenarios. '''(top right)''' Response of some selected climate variables to four levels of global warming (°C). Changes in the ‘Today’ column are based on a global warming level of 1°C. '''(bottom right)''' The long-term effect of each global warming level on sea level. See Section TS.1.3.1 for more detail on the SSP climate change scenarios. This infographic builds from several figures in the Technical Summary: Figure TS.4 (for top left panel), Figure TS.6 (bottom left), Figure TS.12 (top right) and Box TS.4, Figure 1b (bottom right).
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