The new Intergovernmental Panel on Climate Change (IPCC) Special Report on The Ocean and Cryosphere in a Changing Climate (SROCC), released on 25 September, is the first IPCC Report to focus on the role of the ocean in the global climate and the effects of climate change on the ocean. Ocean acidification and ocean deoxygenation are two biogeochemical phenomena which are extensively covered throughout the report. One of the headline statements of the report reads:

"Over the 21^st century, the ocean is projected to transition to unprecedented conditions with increased temperatures (virtually certain), greater upper ocean stratification (very likely), further acidification (virtually certain), oxygen decline (medium confidence), and altered net primary production (low confidence). Marine heatwaves (very high confidence) and extreme El Niño and La Niña events (medium confidence) are projected to become more frequent. The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken (very likely). The rates and magnitudes of these changes will be smaller under scenarios with low greenhouse gas emissions (very likely). {3.2; 5.2; 6.4; 6.5; 6.7; Box 5.1; Figures SPM.1, SPM.3}"

The full Report, Summary for Policymakers, SROCC factsheet and other materials are available for download from https://www.ipcc.ch/srocc/home/.

Adapted from SROCC, Figure 1.

A few relevant excerpts from the Summary for Policymakers are cited below:

OBSERVED CHANGES AND IMPACTS

Observed Physical Changes

A2.5 The ocean has taken up between 20–30% (very likely) of total anthropogenic CO2 emissions since the 1980s causing further ocean acidification. Open ocean surface pH has declined by a very likely range of 0.017–0.027 pH units per decade since the late 1980s, with the decline in surface ocean pH very likely to have already emerged from background natural variability for more than 95% of the ocean surface area. {3.2.1; 5.2.2; Box 5.1; Figures SPM.1, SPM.2}

Observed Impacts on Ecosystems

A5.3 Eastern Boundary Upwelling Systems (EBUS) are amongst the most productive ocean ecosystems. Increasing ocean acidification and oxygen loss are negatively impacting two of the four major upwelling systems: the California Current and Humboldt Current (high confidence). Ocean acidification and decrease in oxygen level in the California Current upwelling system have altered ecosystem structure, with direct negative impacts on biomass production and species composition (medium confidence). {Box 5.3, Figure SPM.2}

A6.4 Warm-water coral reefs and rocky shores dominated by immobile, calcifying (e.g., shell and skeleton producing) organisms such as corals, barnacles and mussels, are currently impacted by extreme temperatures and ocean acidification (high confidence). Marine heatwaves have already resulted in large-scale coral bleaching events at increasing frequency (very high confidence) causing worldwide reef degradation since 1997, and recovery is slow (more than 15 years) if it occurs (high confidence). Prolonged periods of high environmental temperature and dehydration of the organisms pose high risk to rocky shore ecosystems (high confidence). {SR1.5; 5.3.4, 5.3.5, 6.4.2.1, Figure SPM.2}

PROJECTED CHANGES AND RISKS

Projected Physical Changes

B2.3 Continued carbon uptake by the ocean by 2100 is virtually certain to exacerbate ocean acidification. Open ocean surface pH is projected to decrease by around 0.3 pH units by 2081–2100, relative to 2006– 2015, under RCP8.5 (virtually certain). For RCP8.5, there are elevated risks for keystone aragonite shell-forming species due to crossing an aragonite stability threshold year-round in the Polar and sub-Polar Oceans by 2081–2100 (very likely). For RCP2.6, these conditions will be avoided this century (very likely), but some eastern boundary upwelling systems are projected to remain vulnerable (high confidence). {3.2.3, 5.2.2, Box 5.1, Box 5.3, Figure SPM.1}

B2.4 Climate conditions, unprecedented since the preindustrial period, are developing in the ocean, elevating risks for open ocean ecosystems. Surface acidification and warming have already emerged in the historical period (very likely). Oxygen loss between 100 and 600 m depth is projected to emerge over 59–80% of the ocean area by 2031– 2050 under RCP8.5 (very likely). The projected time of emergence for five primary drivers of marine ecosystem change (surface warming and acidification, oxygen loss, nitrate content and net primary production change) are all prior to 2100 for over 60% of the ocean area under RCP8.5 and over 30% under RCP2.6 (very likely). {Annex I: Glossary, Box 5.1, Box 5.1 Figure 1}

Projected Risks for Ecosystems

B5.3 Warming, ocean acidification, reduced seasonal sea ice extent and continued loss of multi-year sea ice are projected to impact polar marine ecosystems through direct and indirect effects on habitats, populations and their viability (medium confidence). The geographical range of Arctic marine species, including marine mammals, birds and fish is projected to contract, while the range of some sub-Arctic fish communities is projected to expand, further increasing pressure on high-Arctic species (medium confidence). In the Southern Ocean, the habitat of Antarctic krill, a key prey species for penguins, seals and whales, is projected to contract southwards under both RCP2.6 and RCP8.5 (medium confidence). {3.2.2, 3.2.3, 5.2.3}

B5.4 Ocean warming, oxygen loss, acidification and a decrease in flux of organic carbon from the surface to the deep ocean are projected to harm habitat-forming cold-water corals, which support high biodiversity, partly through decreased calcification, increased dissolution of skeletons, and bioerosion (medium confidence). Vulnerability and risks are highest where and when temperature and oxygen conditions both reach values outside species’ tolerance ranges (medium confidence). {Box 5.2, Figure SPM.3}

B6.1 All coastal ecosystems assessed are projected to face increasing risk level, from moderate to high risk under RCP2.6 to high to very high risk under RCP8.5 by 2100. Intertidal rocky shore ecosystems are projected to be at very high risk by 2100 under RCP8.5 (medium confidence) due to exposure to warming, especially during marine heatwaves, as well as to acidification, sea level rise, loss of calcifying species and biodiversity (high confidence). Ocean acidification challenges these ecosystems and further limits their habitat suitability (medium confidence) by inhibiting recovery through reduced calcification and enhanced bioerosion. The decline of kelp forests is projected to continue in temperate regions due to warming, particularly under the projected intensification of marine heatwaves, with high risk of local extinctions under RCP8.5 (medium confidence). {5.3, 5.3.5, 5.3.6, 5.3.7, 6.4.2, Figure SPM.3}