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

=== 3.2.1 Introduction ===

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Climate change exposes ocean and coastal ecosystems to changing environmental conditions, including ocean warming, SLR, acidification, deoxygenation and other climatic impact-drivers (CIDs), which have distinct regional and temporal characteristics ( [[#Gruber--2011|Gruber, 2011]] ; [[#IPCC--2018|IPCC, 2018]] ). This section aims to build on the WGI AR6 assessment (Table 3.2) to provide an ecosystem-oriented framing of CIDs. Updating SROCC, projected trends assessed here are based on a new range of scenarios (Shared Socioeconomic Pathways, SSPs), as used in the Coupled Model Intercomparison Project Phase 6 (CMIP6; [[IPCC:Wg2:Chapter:Chapter-1#1.2.2|Section 1.2.2]] ).

'''Table 3.2 |''' Overview of the main global ocean climatic impact-drivers and their observed and projected trends from WGI AR6, with corresponding confidence levels and links to WGI chapters where these trends are assessed in detail

{| class="wikitable"
|-
! Climatic impact-drivers (hazards)

! Observed trends over the historical period

! WGI section

! Projected trends over the 21st century

! WGI section

|-
| ''Ocean temperature''

|
|-
| Ocean warming

| ‘At the ocean surface, temperature has on average increased by 0.88 [0.68–1.01] °C from 1850–1900 to 2011–2020.’

| 2.3.3.1, 9.2.1

( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| Ocean warming will continue over the 21st century ( ''virtually certain'' ), with the rate of global ocean warming starting to be scenario-dependent from about the mid-21st century ( ''medium confidence'' ).

| 9.2.1 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| Marine heatwaves (MHWs)

| MHWs became more frequent ( ''high confidence'' ), more intense and longer ( ''medium confidence'' ) over the 20th and early 21st centuries.

| Box 9.2 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

| MHWs will become ‘4 [2–9, ''likely'' range] times more frequent in 2081–2100 compared with 1995–2014 under SSP1-2.6, and 8 [3–15, ''likely'' range] times more frequent under SSP5-8.5.’

| Box 9.2 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| Climate velocities

| Not assessed in WGI

|
| Not assessed in WGI

|
|-
| ''Sea level''

|
|-
| Global mean sea level (GMSL)

| ‘Since 1901, GMSL has risen by 0.20 [0.15–0.25] m’, and the rate of rise is accelerating.

| 2.3.3, 9.6.1 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| There will be continued rise in GMSL throughout the 21st century under all assessed SSPs ( ''virtually certain'' ) ''.''

| 4.3.2.2, 9.6.3 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Lee--2021|Lee et al., 2021]] )

|-
| Extreme sea levels

| Relative sea level rise is driving a global increase in the frequency of extreme sea levels ( ''high confidence'' ).

| 9.6.4 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

| Rising mean relative sea level will continue to drive an increase in the frequency of extreme sea levels ( ''high confidence'' ).

| 9.6.4 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| ''Ocean circulation''

|
|-
| Ocean stratification

| ‘The upper ocean has become more stably stratified since at least 1970 […] ( ''virtually certain'' ) ''.'' ’

| 9.2.1.3 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

| ‘Upper-ocean stratification will continue to increase throughout the 21st century ( ''virtually certain'' ) ''.'' ’

| 9.2.1.3 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| Eastern boundary upwelling systems

| ‘Only the California current system has experienced some large-scale upwelling-favourable wind intensification since the 1980s ( ''medium confidence'' ) ''.'' ’

| 9.2.5 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

| ‘Eastern boundary upwelling systems will change, with a dipole spatial pattern within each system of reduction at low latitude and enhancement at high latitude ( ''high confidence'' ) ''.'' ’

| 9.2.5 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| Atlantic overturning circulation (AMOC)

| There is ''low confidence'' in reconstructed and modelled AMOC changes for the 20 th century.

| 2.3.3.4, 9.2.3 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| The AMOC will decline over the 21st century ( ''high confidence'' , but ''low confidence'' for quantitative projections).

| 4.3.2.3, 9.2.3 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Lee--2021|Lee et al., 2021]] )

|-
| ''Sea ice''

|
|-
| Arctic sea ice changes

| ‘Current Arctic sea ice coverage levels are the lowest since at least 1850 for both annual mean and late-summer values ( ''high confidence'' ).’

| 2.3.2.1, 9.3.1 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| ‘The Arctic will become practically ice-free in September by the end of the 21st century under SSP2-4.5, SSP3-7.0 and SSP5-8.5[…]( ''high confidence'' ).’

| 4.3.2.1, 9.3.1 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Lee--2021|Lee et al., 2021]] )

|-
| Antarctic sea ice changes

| There is no global significant trend in Antarctic sea ice area from 1979 to 2020 ( ''high confidence'' ).

| 2.3.2.1, 9.3.2 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| There is ''low confidence'' in model simulations of future Antarctic sea ice.

| 9.3.2 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| ''Ocean chemistry''

|
|-
| Changes in salinity

| The ‘large-scale, near-surface salinity contrasts have intensified since at least 1950 […] ( ''virtually certain'' ).’

| 2.3.3.2, 9.2.2.2 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| ‘Fresh ocean regions will continue to get fresher and salty ocean regions will continue to get saltier in the 21st century ( ''medium confidence'' ).’

| 9.2.2.2 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
| Ocean acidification

| Ocean surface pH has declined globally over the past four decades ( ''virtually certain'' ).

| 2.3.3.5, 5.3.2.2 ( [[#Canadell--2021|Canadell et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| Ocean surface pH will continue to decrease ‘through the 21st century, except for the lower-emission scenarios SSP1-1.9 and SSP1-2.6 […] ( ''high confidence'' ).’

| 4.3.2.5, 4.5.2.2, 5.3.4.1 ( [[#Lee--2021|Lee et al., 2021]] ; [[#Canadell--2021|Canadell et al., 2021]] )

|-
| Ocean deoxygenation

| Deoxygenation has occurred in most open ocean regions since the mid-20th century ( ''high confidence'' ).

| 2.3.3.6, 5.3.3.2 ( [[#Canadell--2021|Canadell et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )

| Subsurface oxygen content ‘is projected to transition to historically unprecedented condition with decline over the 21st century ( ''medium confidence'' ).’

| 5.3.3.2 ( [[#Canadell--2021|Canadell et al., 2021]] )

|-
| Changes in nutrient concentrations

| Not assessed in WGI

|
| Not assessed in WGI

|
|}

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