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

==== 4.5.1.3 Near-surface Relative Humidity ====

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The AR5 contrasted future changes in near-surface relative humidity (RH) over land and ocean, concluding with ''medium confidence'' that reductions in near-surface RH over many land areas are ''likely'' . The decrease in near-surface RH over most land areas is associated with the larger warming rates over land than over the ocean and is termed the last-saturation-temperature constraint, as explained in AR5.

Since AR5, significant effort has been devoted to understanding the mechanisms for the decrease in near-surface land RH under global warming, and the relevance of RH changes for the land–sea warming contrast and the water cycle. For the near-surface RH decrease over land, both the moisture transport from the ocean and land–atmosphere feedback processes contribute. For changes in specific humidity over land, the moisture transport from the ocean is dominant while the role of evapotranspiration is secondary ( [[#Byrne--2016|Byrne and O’Gorman, 2016]] ; [[#Chadwick--2016|Chadwick et al., 2016]] ). Nevertheless, the changes in near-surface land RH are also strongly influenced by evapotranspiration, which is suppressed by the drying of soils and plant responses to increasing CO <sub>2</sub> related to stomatal closure under climate change ( [[#Byrne--2015|Byrne and O’Gorman, 2015]] ; [[#Berg--2016|Berg et al., 2016]] ; [[#Chadwick--2016|Chadwick et al., 2016]] ; [[#Swann--2016|Swann et al., 2016]] ; [[#Lemordant--2018|Lemordant et al., 2018]] ). The combination of oceanic and continental influences can explain the spatially diverse trends in the near-surface RH over land in the observations for the recent decades, with a generally dominant negative trend at the global scale ( [[#Vicente-Serrano--2018|Vicente-Serrano et al., 2018]] ). There is a strong feedback between the near-surface land RH decrease and land–ocean warming contrast under future warming projections ( [[#4.5.1.1|Section 4.5.1.1]] ).

Changes in land RH can modulate the response of the water cycle to global warming ( [[#Chadwick--2013|Chadwick et al., 2013]] ; [[#Byrne--2015|Byrne and O’Gorman, 2015]] ). Most CMIP5 models project higher precipitation associated with higher near-surface RH and temperature under climate change ( [[#Lambert--2017|Lambert et al., 2017]] ). Over land, the spatial gradients of fractional changes in near-surface RH contribute to a drying tendency in precipitation minus evapotranspiration with warming, which partly explains why the ‘wet gets wetter, dry gets drier’ paradigm does not hold over land ( [[#Byrne--2015|Byrne and O’Gorman, 2015]] ). Terrestrial aridity is projected to increase over land, as manifested by a decrease in the ratio of precipitation to potential evapotranspiration, in which the decrease in near-surface land RH has a contribution of about 35% in CMIP5 models under doubled CO <sub>2</sub> forcing ( [[#Fu--2014|Fu and Feng, 2014]] ). The aridity can be further amplified by the feedbacks of projected drier soils on land surface temperature, RH, and precipitation ( [[#Berg--2016|Berg et al., 2016]] ).

The CMIP6 multi-model ensemble projects general decreases in near-surface relative humidity over a large fraction of land areas, but moderate increases over the ocean (Figure 4.23). The projected changes depend on emissions scenario and season. Changes in near-surface RH under SSP1-2.6 are insignificant compared to natural variability. Under SSP3-7.0, during boreal summer, significant decreases relative to natural variability are projected in continental Europe and the Middle East, North America, South America and South Africa.

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[[File:3cb603d098b10e17f8f8965d1788fdd4 IPCC_AR6_WGI_Figure_4_23.png]]

'''Figure''' '''4.23 |''' '''Long-term changes in seasonal mean relative humidity.''' Displayed are projected spatial patterns of multi-model mean change (%) in seasonal '''(top)''' December–January–February (DJF) and '''(bottom)''' June–July–August (JJA) mean near-surface relative humidity in 2081–2100 relative to 1995–2014, for (left) SSP1-2.6 and (right) SSP3-7.0. The number of models used is indicated in the top right of the maps. No overlay indicates regions where the change is robust and ''likely'' emerges from internal variability, that is, where at least 66% of the models show a change greater than the internal variability threshold ( [[#4.2.6|Section 4.2.6]] ) and at least 80% of the models agree on the sign of change. Diagonal lines indicate regions with no change or no robust significant change, where fewer than 66% of the models show change greater than the internal-variability threshold. Crossed lines indicate areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

In summary, there is ''medium confidence'' that continued warming will lead to decreased near-surface relative humidity over a large fraction of land areas, but moderate increases over the ocean. There is ''high confidence'' that near-surface relative humidity will decrease over parts of the tropical and subtropical latitudes over land.

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