Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.2.1.1 Sea Surface Temperature ====

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The IPCC Fifth Assessment Report (AR5; [[#Hartmann--2013|Hartmann et al., 2013]] ) assessed that it is ''virtually certain'' that global sea surface temperature (SST) has increased since the beginning of the 20th century ( ''very high confidence'' ). The Special Report on Ocean and Cryosphere in a Changing Climate (SROCC) did not assess past SST change. Since AR5, improvements in the understanding of recent SST biases in the observational records, especially extending ship-based observations with buoy-based observations and improved treatment of sea ice, have had important consequences for key climate change indicators such as global mean surface temperature (GMST), global surface air temperature (GSAT), and SST (Cross-Chapter Box 2.3). The AR5 assessment is confirmed, and it is now ''very likely'' that global mean SST changed by 0.88 [0.68 to 1.01] °C from 1850–1900 to 2011–2020, and 0.60 [0.44 to 0.74] °C from 1980 to 2020 (Figure 9.3 and Table 2.4).

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'''Figure''' '''9.3 |''' '''Sea surface temperature (SST) and its changes with time. (a)''' Time series of global mean SST anomaly relative to 1950–1980 climatology. Shown are paleoclimate reconstructions and PMIP models, observational reanalyses (HadISST) and multi-model means from the Coupled Model Intercomparison Project (CMIP) historical simulations, CMIP projections, and HighResMIP experiment. '''(b)''' Map of observed SST (1995–2014 climatology HadISST). '''(c)''' Historical SST changes from observations. '''(d)''' CMIP 2005–2100 SST change rate. '''(e)''' Bias of CMIP. '''(f)''' CMIP change rate. '''(g)''' 2005–2050 change rate for SSP5-8.5 for the CMIP ensemble. '''(h)''' Bias of HighResMIP (bottom left) over 1995–2014. '''(i)''' HighResMIP change rate for 1950–2014. ( '''j)''' 2005–2050 change rate for SSP5-8.5 for the HighResMIP ensemble. 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 (see Cross-Chapter Box Atlas.1 for more information). Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

Regions vary in the rate of SST warming, with slight cooling in some regions (Figure 9.3). The SROCC ( [[#Collins--2019|Collins et al., 2019]] ) and [[IPCC:Wg1:Chapter:Chapter-7#7.4.4|Section 7.4.4]] assess SST changes over specific regions, which are consistent with the changes reported here. The tropical ocean has been warming faster than other regions since 1950, with the fastest warming in regions of the tropical Indian and western Pacific oceans (Figure 9.3), due to a combination of local atmosphere–ocean coupling, the Indonesian Throughflow ( [[#9.2.3.4|Section 9.2.3.4]] and Figure 9.11), and trends in the Walker circulation (Sections 2.3.1.4.1 and 3.3.3.1, and Figure 3.16). The western boundary currents of the subtropical gyres have warmed faster than the global mean over the past century. There remains ''low agreement'' in the changes of the location and the dynamical changes in western boundary current extensions (Sections 2.3.3.4.2 and 9.2.3.4, and Figure 9.3). In the Arctic, the mean SST increase over the last two decades is similar to, or only slightly higher than, the global average (J.-L [[#Chen--2019|]] [[#Chen--2019|Chen et al., 2019]] ). In contrast, the eastern Pacific Ocean, subpolar North Atlantic Ocean and Southern Ocean have warmed more slowly than the global average or cooled (Figure 9.3). Surface warming in the subpolar Southern Ocean has been slower than the global average since the 1950s, and this pattern is consistent with the upwelling around Antarctica renewing surface water with pre-industrial, deeper water masses ( [[#9.2.3.2|Section 9.2.3.2]] ; [[#Frölicher--2015|Frölicher et al., 2015]] ; J. [[#Marshall--2015|]] [[#Marshall--2015|Marshall et al., 2015]] ; [[#Armour--2016|Armour et al., 2016]] ). New evidence since SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) confirms slight cooling since the 1980s around the subpolar Southern Ocean, contrasting with marked warming directly northward of it ( [[#9.2.3.2|Section 9.2.3.2]] ; [[#Haumann--2020|Haumann et al., 2020]] ; [[#Rye--2020|Rye et al., 2020]] ; [[#Auger--2021|Auger et al., 2021]] ). In eastern boundary upwelling systems, SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) reported ''low agreement'' between SST trends in recent decades, due to varying spatio-temporal resolution and interannual to multi-decadal variability. Satellite evidence not included in SROCC shows that 92% of these regions warmed more slowly than neighbouring offshore locations between 1982 and 2015, so upwelling may buffer the near shore from warming ( [[#9.2.3.5|Section 9.2.3.5]] ; [[#Varela--2018|Varela et al., 2018]] ). Coupled ocean-atmospheric modes of variability strongly affect regional SST (Cross-Chapter Box 3.1 and Annex IV). In summary, a positive SST trend since 1950 is evident globally, but there is ''very high confidence'' that the Indian Ocean, western equatorial Pacific Ocean, and western boundary currents have warmed faster than the global average, while the Southern Ocean, the eastern equatorial Pacific, and the North Atlantic Ocean have warmed more slowly, or have slightly cooled.

In AR5 ( [[#Flato--2013|Flato et al., 2013]] ), a marginal improvement was noted in Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model SST biases compared to Phase 3 (CMIP3) models in AR4, with a reduction in the magnitude of biases. The AR5 noted that, in several regions, large SST biases are symptomatic of errors in the representation of important processes, such as dynamics in the equatorial Pacific and North Atlantic, and Southern Ocean. Common regional biases in SST or historical SST trends are not exclusively linked to the representation of the ocean ( ''high confidence'' ), but can have multiple causes, including: errors in the representation of long-term historical trends in equatorial winds ( [[#9.2.1.2|Section 9.2.1.2]] ); misrepresentation of the forced equatorial ocean response ( [[#Karnauskas--2012|Karnauskas et al., 2012]] ; [[#Kohyama--2017|Kohyama et al., 2017]] ; [[#Coats--2018|Coats and Karnauskas, 2018]] ); thermocline depth errors ( [[#Linz--2014|Linz et al., 2014]] ); errors in atmospheric model cloud-related shortwave radiation ( [[#Hyder--2018|Hyder et al., 2018]] ); biases in ocean circulation variability ( [[#Wang--2014|]] [[#Wang--2014|C. Wang et al., 2014]] ); and deficiencies in upper ocean (Q. [[#Li--2019|]] [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ) and atmospheric ( [[#Bates--2012|Bates et al., 2012]] ) boundary layer parametrizations. In CMIP6, the mid-latitude biases in the Northern Hemisphere are improved in the multi-model mean, and the inter-model standard deviation of the zonal mean SST error is significantly decreased in the northern Hemisphere south of 50°N compared to CMIP5, though biases in equatorial regions remain essentially unchanged ( [[IPCC:Wg1:Chapter:Chapter-3#3.5.1.1|Section 3.5.1.1]] and Figures 3.23, 3.24 and 9.3). Some long-standing ocean model biases have been reduced through increases in model resolution in CMIP6 ( [[#Bock--2020|Bock et al., 2020]] ) and improved parametrizations ( [[#Fox-Kemper--2011|Fox-Kemper et al., 2011]] ; Q. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Qiao--2016|Qiao et al., 2016]] ; [[#Reichl--2018|Reichl and Hallberg, 2018]] ). The High Resolution Model Intercomparison Project (HighResMIP) ensemble (Figure 9.3) has smaller cold biases in the North Atlantic and the tropical Pacific, and smaller warm biases in the upwelling regions off the western coasts of Africa, North and South America ( [[#Roberts--2018|Roberts et al., 2018]] , 2019; [[#Caldwell--2019|Caldwell et al., 2019]] ; [[#Docquier--2019|Docquier et al., 2019]] ). In summary, CMIP6 models show persistent regional biases in representing the climatological SST state ( ''very high confidence'' ), but higher resolution reduces some biases, particularly in the North Atlantic and eastern boundary upwelling systems (Figure 9.3; ''high confidence'' ).

The CMIP6 models represent the observed trends in SST patterns with greater fidelity than CMIP5, with the ocean area that is inconsistent with the observed trends decreasing by about three quarters from CMIP5 to CMIP6 ( [[#Olonscheck--2020|Olonscheck et al., 2020]] ). In some regions, the direction of SST changes in observations are consistent with CMIP6 only when including internal variability ( [[#Olonscheck--2020|Olonscheck et al., 2020]] ). This is notably the case in the equatorial Pacific, North Atlantic, and Southern Ocean, which are regions where SST is of known importance in controlling heat uptake ( [[#9.2.2.1|Section 9.2.2.1]] ) and the global radiative feedback parameter ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.3|Section 7.4.4.3]] ). Overall, despite some persistent regional biases, CMIP6 coupled climate models reproduce the observed SST trends or high internal variability over the past century over a range of different multi-decadal periods (Figure 9.3; [[#Olonscheck--2020|Olonscheck et al., 2020]] ; [[#Watanabe--2021|Watanabe et al., 2021]] ), highlighting their skill to inform future large-scale SST changes at regional scale. Warming is projected at varying rates in all regions by 2050, except the North Atlantic Subpolar Region, the equatorial Pacific, and the Southern Ocean where models disagree ( ''high confidence'' ).

It is ''virtually certain'' that SST will continue to increase in the 21st century, at a rate depending on future emissions scenarios. The future global mean SST increase projected by CMIP6 models for the period 1995–2014 to 2081–2100 is 0.86 [5–95% range: 0.43–1.47] °C under SSP1-2.6, 1.51 [1.02 to 2.19] °C under SSP2-4.5, 2.19 [1.56 to 3.30] °C under SSP3-7.0, and 2.89 [2.01 to 4.07] °C under SSP5-8.5 (Figure 9.3). While under SSP1-2.6, the CMIP6 ensemble consistently projects that it is ''very likely'' at least 83% of the world ocean surface will have warmed by 2100, and under SSP5-8.5, at least 98% of the world ocean surface will have warmed. The spatial pattern of future change is consistent with observed SST change over the 20th century, though with notable regional differences (Figure 9.3). Long-term change in SST patterns is important for regional impacts but also affects radiative feedbacks, and therefore long-term change in climate sensitivity ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.3|Section 7.4.4.3]] ). In the Southern Ocean, CMIP6 models project that SSTs will eventually consistently increase in the 21st century, at a rate dependent on future scenarios (Figure 9.3 and [[#9.2.3.2|Section 9.2.3.2]] ; [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). Yet, there is only ''low confidence'' that this Southern Ocean warming will emerge by the end of the century ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.1|Section 7.4.4.1]] ), due to the inconsistent historical and near-term simulations and observations over the 20th century (Figure 9.3). Furthermore, the equilibrium SST pattern from proxy records or simulated by climate models under CO <sub>2</sub> forcing stand in contrast with the cooling trends in the Southern Ocean observed over the past decades ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.1.2|Section 7.4.4.1.2]] ). Similarly, the SST change pattern observed in the tropical Pacific Ocean will transition on centennial time scales to a mean pattern resembling the El Niño pattern ( ''medium confidence'' ) (Annex IV). However, it is difficult to delineate a climate change trend ressembling an El Niño pattern and El Niño variability ( [[#Wittenberg--2009|Wittenberg, 2009]] ; [[#Collins--2010|Collins et al., 2010]] ) without large ensembles ( [[#Kay--2015|Kay et al., 2015]] ). Several Pliocene SST reconstructions indicate enhanced warming in the centre of the eastern Pacific equatorial cold tongue upwelling region, consistent with reconstruction of enhanced subsurface warming and enhanced warming in coastal upwelling regions ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.2.2|Section 7.4.4.2.2]] ). The North Atlantic subpolar gyre is projected to continue to warm more slowly than surrounding regions ( [[#Suo--2017|Suo et al., 2017]] ), as the Gulf Stream concurrently warms rapidly (Figure 9.3; [[#Cheng--2013|Cheng et al., 2013]] ) and the Atlantic Meridional Overturning Circulation further declines under greenhouse gas forcing, although models disagree about the rate of change (Figure 9.3 and [[#9.2.3.1|Section 9.2.3.1]] ). In summary, CMIP6 models show a future pattern of SST change comparable to historical trends with intensity depending on future emissions scenario, and some of the observed cooling trends over the 20th century will eventually transition to a warming SST on centennial time scales, in particular in the Southern Ocean ( ''high confidence'' ) and in the equatorial Pacific ( ''medium confidence'' ), while the North Atlantic subpolar gyre will continue to warm more slowly than the global average ( ''high confidence'' ).

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