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

===== 4.5.1.1.1 Land–ocean warming contrast =====

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It is ''virtually certain'' that future average warming will be higher over land than over the ocean. [[IPCC:Wg1:Chapter:Chapter-2#2.2.1|Section 2.2.1]] of SRCCL (G. [[#Jia--2019|]] [[#Jia--2019|Jia et al., 2019]] ) assessed that it is certain that land temperatures have increased more than global mean temperatures since the pre-industrial period. This so-called land–ocean warming contrast is a striking feature of observed trends ( [[#Lambert--2007|Lambert and Chiang, 2007]] ; [[#Byrne--2018|Byrne and O’Gorman, 2018]] ) and projected changes in surface-air temperature ( [[#Sutton--2007|Sutton et al., 2007]] ; [[#Joshi--2008|Joshi and Gregory, 2008]] ; [[#Dong--2009|Dong et al., 2009]] ; [[#Lambert--2011|Lambert et al., 2011]] ; [[#Drost--2012|Drost et al., 2012]] ; [[#Bayr--2013|Bayr and Dommenget, 2013]] ; [[#Byrne--2013b|Byrne and O’Gorman, 2013b]] ; [[#Izumi--2013|Izumi et al., 2013]] ; [[#Joshi--2013|Joshi et al., 2013]] ). Between 1979 and 2016, average temperature over land increased by 42% more than over the ocean ( [[#Byrne--2018|Byrne and O’Gorman, 2018]] ). A similar warming contrast is found in CMIP5 projections though with large differences across models and latitudes ( [[#Sutton--2007|Sutton et al., 2007]] ; [[#Drost--2012|Drost et al., 2012]] ; [[#Byrne--2013b|Byrne and O’Gorman, 2013b]] ; [[#Joshi--2013|Joshi et al., 2013]] ), which is also consistent with paleoclimate evidence ( [[#Izumi--2013|Izumi et al., 2013]] ; [[#Schmidt--2014|Schmidt et al., 2014]] ). The ratio of land-to-ocean warming is greater than one for almost all regions ( ''high confidence'' ) and is larger for dry subtropical continents (about 1.5) than for moist regions in the tropics and mid-latitudes (about 1.2; [[#Byrne--2013a|Byrne and O’Gorman, 2013a]] ). Projected warming over land and ocean only is shown in Table 4.2 for different scenarios, and the global average ratio of land-to-ocean warming in CMIP6 is 1.5 with a ''likely'' range of 1.4 to 1.7, which is consistent with estimates based on CMIP5.

Since AR5, a robust physical understanding of the warming contrast been developed. A simple theory based on atmospheric dynamics and moisture transport shows that surface-air temperature and relative humidity over land are strongly coupled, and demonstrates that the warming contrast occurs because air over land is drier than over the ocean ( [[#Joshi--2008|Joshi et al., 2008]] ; [[#Byrne--2013a|Byrne and O’Gorman, 2013a]] , b, 2018). The warming contrast causes land relative humidity to decrease ( [[#Byrne--2016|Byrne and O’Gorman, 2016]] , 2018; [[#Chadwick--2016|Chadwick et al., 2016]] ) and this feeds back on and strengthens the warming contrast. Differences in land-relative humidity responses across models are the primary cause of uncertainty in the land–ocean warming contrast ( [[#Byrne--2013b|Byrne and O’Gorman, 2013b]] ). These land-relative humidity changes are ultimately controlled by moisture transport between the land and ocean boundary layers ( [[#Byrne--2016|Byrne and O’Gorman, 2016]] ; [[#Chadwick--2016|Chadwick et al., 2016]] ) and are also sensitive to characteristics of land surfaces that are challenging to model, including stomatal conductance and soil moisture ( [[#Berg--2016|Berg et al., 2016]] ; [[#Zarakas--2020|Zarakas et al., 2020]] ).

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