Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.5.2.2 Salinity Change Attribution ====

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AR5 concluded that it was ''very likely'' that anthropogenic forcings had made a discernible contribution to surface and subsurface ocean salinity changes since the 1960s ( [[#Bindoff--2013|Bindoff et al., 2013]] ; [[#Rhein--2013|Rhein et al., 2013]] ). It highlighted that the spatial patterns of salinity trends, and the mean fields of salinity and evaporation minus precipitation are all similar, with an enhancement to Atlantic Ocean salinity and freshening in the Pacific and Southern Oceans. Since AR5 all subsequent work on assessing observed and modelled salinity changes has confirmed these results.

Considerable changes to observed broad- or basin-scale ocean near-surface salinity fields have been reported (see [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ), and these have been linked to changes in the evaporation minus precipitation patterns at the ocean surface through model simulations, typically expressing a pattern of change where climatological mean fresh regions become fresher and corresponding salty regions becoming saltier ( [[#Durack--2012|Durack et al., 2012]] , [[#Durack--2013|2013]] ; [[#Zika--2015|Zika et al., 2015]] ; [[#Lago--2016|Lago et al., 2016]] ; [[#Skliris--2016|Skliris et al., 2016]] , [[#Skliris--2018|2018]] ; [[#Cheng--2020|Cheng et al., 2020]] ), also broadly present in the CMIP6 multi-model mean (Figure 3.27). At basin-scales, the depth-integrated effect of mean salinity changes as captured in halosteric sea level for the top 0 to 2000 m has also been assessed based on observational products, and these results mirror near-surface patterns in the CMIP5 and CMIP6 models, with most areas that are becoming fresher at the surface exhibiting increases in halosteric sea level, and areas becoming saltier exhibiting decreases ( [[#Durack--2014a|Durack et al., 2014a]] ; Figure 3.28). Further investigations using observations and models together have tied the long-term patterns of surface and subsurface salinity changes to coincident changes to the evaporation minus precipitation field over the ocean ( [[#Durack--2012|Durack et al., 2012]] , [[#Durack--2013|2013]] ; [[#Durack--2015|Durack, 2015]] ; [[#Levang--2015|Levang and Schmitt, 2015]] ; [[#Zika--2015|Zika et al., 2015]] , [[#Zika--2018|2018]] ; [[#Grist--2016|Grist et al., 2016]] ; [[#Lago--2016|Lago et al., 2016]] ; [[#Cheng--2020|Cheng et al., 2020]] ), however the rate of these changes through time continues to be an active area of active research ( [[#Skliris--2014|Skliris et al., 2014]] ; [[#Zika--2015|Zika et al., 2015]] , 2018; [[#Cheng--2020|Cheng et al., 2020]] ; [[#Sallée--2021|Sallée et al., 2021]] ).

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[[File:d595a2eb3fab5e2e6b766e28c0048d42 IPCC_AR6_WGI_Figure_3_27.png]]

Figure 3.27 | '''Maps of multi-decadal salinity trends for the near-surface''' '''ocean.''' Units are Practical Salinity Scale 1978 [PSS-78] per decade. '''(Top)''' The best estimate ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ) observed trend (1950 – 2019, [[#Durack--2010|Durack and Wijffels, 2010]] ). '''(Bottom)''' Simulated trend from the CMIP6 historical experiment multi-model mean (1950–2014). Black contours show the climatological mean salinity in increments of 0.5 PSS-78 (thick lines 1 PSS-78). Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

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[[File:802053c0a35a550a20c5b3cc04ca9b87 IPCC_AR6_WGI_Figure_3_28.png]]

Figure 3.28 | '''Long-term trends in halosteric and thermosteric sea level in CMIP6 models and observations.''' Units are mm yr <sup>–1</sup> . In The '''right-hand column''' , three observed maps of 0 to 2000 m halosteric sea level trends are shown: '''top (D&amp;W)''' from [[#Durack--2010|Durack and Wijffels (2010)]] , 1950–2019, updated; '''upper-middle (EN4)''' from [[#Good--2013|Good et al. (2013)]] , 1950–2019, updated; and '''lower-middle (Ishii)''' from [[#Ishii--2017|Ishii et al. (2017)]] , 1955–2019, updated. '''Bottom-right:''' the CMIP6 historical multi-model mean (1950–2014). Red and orange colours show a halosteric contraction (enhanced salinity) and blue and green a halosteric expansion (reduced salinity). In The '''left-hand column''' , basin-integrated halosteric '''(top)''' and thermosteric '''(bottom)''' trends for the Atlantic and Pacific, the two largest ocean basins, are shown, where Pacific anomalies are presented on the x-axis and Atlantic on the y-axis. Observational estimates are presented in black, CMIP6 historical (all forcings) simulations are shown in orange squares, with the multi-model mean shown as a dark orange diamond with a black bounding box. CMIP6 hist-nat (historical natural forcings only) simulations are shown in green squares with the multi-model mean as a dark green diamond with a black bounding box. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

Climate change detection and attribution studies have considered salinity, with the first of these assessed in AR5 ( [[#Bindoff--2013|Bindoff et al., 2013]] ). Since that time, the positive detection conclusions ( [[#Stott--2008|Stott et al., 2008]] ; [[#Pierce--2012|Pierce et al., 2012]] ; [[#Terray--2012|Terray et al., 2012]] ) have been supported by a number of more recent and independent assessments which have reproduced the multi-decadal basin-scale patterns of change in observations and models (Figures 3.27 and 3.28; [[#Durack--2014a|Durack et al., 2014a]] ; [[#Durack--2015|Durack, 2015]] ; [[#Levang--2015|Levang and Schmitt, 2015]] ; [[#Skliris--2016|Skliris et al., 2016]] ). Observed depth-integrated basin responses, contrasting the Pacific and Atlantic basins (freshening Pacific and enhanced salinity Atlantic) were also shown to be replicated in most historical (natural and anthropogenically forced) simulations, with this basin contrast absent in CMIP5 and CMIP6 natural-only simulations that exclude anthropogenic forcing ( [[#Durack--2014a|Durack et al., 2014a]] ; Figure 3.28).

While observational sparsity considerably limits quantification of regional changes, a recent study by [[#Friedman--2017|Friedman et al. (2017)]] assessed salinity changes in the Atlantic Ocean from 1896 to 2013 and confirmed the pattern of mid-to-low latitude enhanced salinity and high latitude North Atlantic freshening over this period exists even after accounting for the effects of the NAO and AMO.

Considering the bulk of evidence, it is ''extremely likely'' that human influence has contributed to observed near-surface and subsurface salinity changes across the globe since the mid-20th century. All available multi-decadal assessments have confirmed that the associated pattern of change corresponds to fresh regions becoming fresher and salty regions becoming saltier ( ''high confidence'' ). CMIP5 and CMIP6 models are only able to reproduce these patterns in simulations that include greenhouse gas increases ( ''medium confidence'' ). Changes to the coincident atmospheric water cycle and ocean-atmosphere fluxes (evaporation and precipitation) are the primary drivers of the basin-scale observed salinity changes ( ''high confidence'' ). This result is supported by all available observational assessments, along with a growing number of climate modelling studies targeted at assessing ocean and water cycle changes. The basin-scale changes are consistent across models and intensify on centennial scales from the historical period through to the projections of future climate ( ''high confidence'' ).

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