Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.4.1.1 Global Water Cycle Intensity and P–E Over Land and Oceans ====

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As discussed in 8.3.1.1, the definition of global water cycle intensity varies from the simple metric of increases in global mean precipitation to broader joint considerations of water vapour and its transport, precipitation minus evaporation (P–E) rates and continental runoff (Figure 8.1). The AR5 determined that globally averaged precipitation is ''virtually certain'' to increase with temperature and that there is ''high confidence'' that the contrast of annual mean precipitation between dry and wet regions and seasons will increase over most of the globe as temperatures and moisture transports increase ( [[#Collins--2013|Collins et al., 2013]] ). The AR5 also highlighted that continued ocean warming for a few decades after GHG forcing stabilizes or begins to decrease will also lead to further increases in global mean precipitation and evaporation.

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'''Table 8.1 |''' '''Global and global land annual mean water cycle projections in the mid-term''' ( '''2041–2060''' ''') and long term''' ( '''2081–2100''' ''') relative to present day''' ( '''1995–2014''' '''), showing present day mean and 90% confidence range across CMIP6 models (historical experiment) and projected mean changes and the 90% confidence range across the same set of models and a range of Shared Socio-economic Pathway scenarios.''' Note that the exact value of changes can vary slightly based on the number of models assessed, but not sufficiently to affect the assessment. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

{| class="wikitable"
|-
|
| colspan="5"| '''Mid-term:''' '''2041–2060''' '''Minus Reference Period'''

| colspan="5"| '''Long Term:''' '''2081–2100''' '''Minus Reference Period'''

|-
|
| '''1995''' – '''2014''' '''reference period'''

| '''SSP1-1.9'''

| '''SSP1-2.6'''

| '''SSP2-4.5'''

| '''SSP3-7.0'''

| '''SSP5-8.5'''

| '''SSP1-1.9'''

| '''SSP1-2.6'''

| '''SSP2-4.5'''

| '''SSP3-7.0'''

| '''SSP5-8.5'''

|-
| colspan="12"| '''Global Annual'''

|-
| Precipitation (mm day <sup>–1</sup> )

| 2.96 [2.76 to 3.17]

| 0.06 [0.03 to 0.11]

| 0.07 [0.03 to 0.12]

| 0.07 [0.04 to 0.12]

| 0.06 [0.03 to 0.11]

| 0.08 [0.03 to 0.14]

| 0.06 [0.02 to 0.11]

| 0.09 [0.04 to 0.17]

| 0.12 [0.07 to 0.21]

| 0.15 [0.08 to 0.24]

| 0.2 [0.1 to 0.33]

|-
| Precipitable Water (kg m <sup>2</sup> )

| 24.79 [23.06 to 26.82]

| 1.42 [0.7 to 2.26]

| 1.84 [1.03 to 2.62]

| 2.29 [1.6 to 3.09]

| 2.7 [1.92 to 3.92]

| 3.15 [2.13 to 4.38]

| 1.11 [0.28 to 2.13]

| 2.11 [0.98 to 3.15]

| 3.76 [2.41 to 5.08]

| 6.2 [4.24 to 8.83]

| 7.92 [5.21 to 10.69]

|-
| colspan="12"| '''Global Land Annual'''

|-
| Precipitation (mm day <sup>–1</sup> )

| 2.27 [1.98 to 2.58]

| 0.07 [0.02 to 0.11]

| 0.07 [–0.0 to 0.13]

| 0.06 [0.01 to 0.13]

| 0.06 [0.02 to 0.12]

| 0.09 [0.01 to 0.16]

| 0.06 [0.01 to 0.1]

| 0.08 [0.02 to 0.16]

| 0.11 [0.02 to 0.19]

| 0.14 [0.03 to 0.22]

| 0.2 [0.07 to 0.32]

|-
| Precipitation – Evaporation (mm day <sup>–1</sup> )

| 0.87 [0.49 to 1.26]

| 0.02 [0.0 to 0.03]

| 0.02 [–0.01 to +0.05]

| 0.02 [–0.02 to +0.06]

| 0.03 [–0.0 to +0.06]

| 0.04 [0.0 to 0.1]

| 0.01 [–0.0 to +0.03]

| 0.03 [–0.01 to +0.08]

| 0.04 [–0.01 to 0.07]

| 0.07 [0.0 to 0.12]

| 0.1 [0.01 to 0.22]

|-
| Runoff (mm day <sup>–1</sup> )

| 0.79 [0.54 to 1.0]

| 0.02 [0.0 to 0.05]

| 0.04 [–0.0 to +0.1]

| 0.04 [–0.0 to +0.11]

| 0.04 [0.01 to 0.08]

| 0.06 [0.01 to 0.14]

| 0.02 [–0.0 to +0.03]

| 0.04 [–0.0 to +0.13]

| 0.06 [0.0 to 0.17]

| 0.1 [0.02 to 0.2]

| 0.15 [0.04 to 0.27]

|-
| Precipitable Water (kg m <sup>2</sup> )

| 18.86 [17.12 to 21.28]

| 1.23 [0.57 to 1.96]

| 1.58 [0.77 to 2.42]

| 1.96 [1.34 to 2.76]

| 2.33 [1.63 to 3.46]

| 2.72 [1.79 to 3.84]

| 0.95 [0.19 to 1.95]

| 1.78 [0.8 to 2.77]

| 3.18 [2.04 to 4.34]

| 5.33 [3.57 to 7.5]

| 6.81 [4.35 to 9.32]

|}

In this Report, [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] provides an updated assessment of global annual precipitation ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.1|Section 4.3.1]] ), finding that it is ''very likely'' that annual precipitation averaged over all land regions continuously increases as global surface temperatures increase in the 21st century ( ''high confidence'' ). CMIP6 projections for long-term changes in P–E (Figure 8.13) show that, for all scenarios, P–E increases over the tropics and high latitudes and decreases over the subtropics, resulting from a thermodynamically driven amplification of P–E patterns ( [[#8.2.2.1|Section 8.2.2.1]] ). Both the intensity of changes and the spread among the models is larger for the higher emissions scenarios. A less coherent latitudinal pattern and smaller magnitude of P–E changes over land reflect the complex influence of land–ocean warming contrast, atmospheric circulation change and vegetation feedbacks ( [[#8.2.2.1|Section 8.2.2.1]] ). However, stronger atmospheric moisture transport, increases in precipitation and evaporation over global land and ocean and larger continental runoff that is in part fed by melting of glaciers characterizes a more intense water cycle with global warming.

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[[File:283dc39a1b9cb529b8d3232957d5976f IPCC_AR6_WGI_Figure_8_13.png]]

'''Figure 8.13 |''' '''Zonal and annual-mean projected long-term changes in the atmospheric water budget.''' Zonal and annual mean projected changes (mm day <sup>–1</sup> ) in P (precipitation, left column), E (evaporation, middle column), and P–E (right column) over both land and ocean areas (coloured lines) and over land only (black lines) averaged across available CMIP6 models (number provided at the top left of each panel) in the SSP1-2.6 (top row), SSP2-4.5 (middle row) and SSP5-8.5 (bottom row) scenario, respectively. Shading denotes confidence intervals estimated from the CMIP6 ensemble under a normal distribution hypothesis. Colour shading denotes changes over both land and ocean. Grey shading represents internal variability derived from the pre-industrial control simulations. All changes are estimated for 2081–2100 relative to the 1995–2014 base period. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Global and global land mean water cycle changes from CMIP6 projections are shown in Table 8.1. Increases in global and continental precipitation, P–E and runoff in both the mid-term and long-term illustrate the future intensification of the water cycle, with the magnitude of change increasing with emissions scenarios. Consistent with AR5, CMIP6 simulations of global mean precipitation show a systematic multi-model mean increase of 1.6 to 2.9 % °C <sup>–1</sup> warming (apparent hydrological sensitivity; [[#8.2.1|Section 8.2.1]] ) by 2081 – 2100 relative to present day across the new SSP scenarios (using global surface air temperature change from Table 4.1). It is well understood that rising concentrations of CO <sub>2</sub> drive a long-term increase in global precipitation with warming, but with the increase partly offset by rapid atmospheric adjustments to the direct atmospheric heating from radiative forcing agents ( [[#8.2.1|Section 8.2.1]] ). The largest apparent hydrological sensitivity is found for SSP1-1.9, where the suppressing effects on precipitation from atmospheric heating by greenhouse gases (GHGs) rapidly reduce as their concentration falls. Additional warming due to reduced aerosol loadings under the SSP scenarios (Lund et al. , 2019) further increases global precipitation ( [[#Rotstayn--2013|Rotstayn et al., 2013]] ; [[#Wu--2013|Wu et al., 2013]] ; [[#Salzmann--2016|Salzmann, 2016]] ; [[#Richardson--2018|T.B. Richardson et al., 2018]] b; [[#Samset--2018b|Samset et al., 2018b]] ; [[#Westervelt--2018|Westervelt et al., 2018]] ), with particularly strong contributions from increased monsoon rainfall over East and South Asia ( [[#Levy--2013|Levy et al., 2013]] ; [[#Westervelt--2015|Westervelt et al., 2015]] ; [[#Dwyer--2017|Dwyer and O’Gorman, 2017]] ).

Over global land there is a small range in global mean multi-model mean precipitation increase across scenarios in the mid-term (2.6 – 4.0 %), which widens (to 2.6 – 8.8 %) in the long-term (Table 8.1). The long-term projections are consistent with the [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] assessment that global annual precipitation over land is projected to increase on average by 2.4 [ – 0.2 to +4.7] % ( ''likely'' range) in the SSP1-1.9 low-emissions scenario and by 8.3 [0.9 to 12.9] % in the SSP5-8.5 high emissions scenario by 2081–2100 relative to 1995–2014. Small differences in assessed model mean changes in Chapter 4, Table 4.2 result from a slightly different set of models considered for Table 8.1. Over land, P–E increases by around 2 – 3% in the mid-term (apart from SSP5-8.5 where increases are almost 5%) and around 1 – 12% in the long-term, determined by increased moisture transport from the ocean to land [[#8.4.1.2|Section 8.4.1.2]] ). Runoff increases are larger and less certain due to additional inputs from glacier melt and changes in groundwater storage ( [[#8.4.1.7|Section 8.4.1.7]] ). Overall, precipitation and runoff are ''very likely'' to increase over the global land in all scenarios in the mid- and long term. P–E is ''likely'' to increase over global land in the mid- and long term and ''very likely'' in SSP1-1.9, SSP3-7.0 and SSP5-8.5 pathways. The mid-term consistency in projections across scenarios is not apparent for precipitable water vapour, which increases over land by around 6 – 15% in the mid-term and 5 – 36% in the long-term across all scenarios. This implies that increases in extreme precipitation (closely related to atmospheric water vapour content; [[#8.2.3.2|Section 8.2.3.2]] ) are dependent on mitigation pathway, even in the mid-term ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.5|Section 11.4.5]] ). Water vapour residence time (computed as the ratio of precipitable water vapour to precipitation from values in Table 8.1) increases from eight days in the present to nine days in mid-term and up to about ten days in the long-term over land in SSP3-7.0, indicating a longer time to moisten the atmosphere between precipitation events. The CMIP6 projections are therefore consistent with an intensification but not acceleration of the global wa ter cycle.

In summary, it is ''virtually certain'' that global water cycle intensity, considered in terms of global and continental mean precipitation, evaporation and runoff, will increase with continued global warming. Global annual precipitation over land is projected to increase on average by 2.4 [–0.2 to +4.7] % ( ''likely'' range) in the SSP1-1.9 low-emissions scenario and by 8.3 [0.9 to 12.9] % in the SSP5-8.5 high emissions scenario by 2081–2100 relative to 1995–2014.

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