IPCC Special Report on the Ocean and Cryosphere in a Changing Climate

Relevance:
Mains: GSIII-

• Environmental conservation
• Environmental pollution & degradation

• The global ocean covers 71% of the Earth's surface and contains about 97% of the Earth’s water.
• The cryosphere refers to frozen components of the Earth.
• Around 10% of Earth’s land area is covered by glaciers or ice sheets.
• The ocean and cryosphere support unique habitats and are interconnected with other components of the climate system through the global exchange of water, energy, and carbon.

Observed changes in ocean

• The global ocean has warmed unabated since 1970 and has taken up more than 90% of the excess heat in the climate system.
• Marine heatwaves have very likely doubled in frequency since 1982.
• Density stratification has increased in the upper 200 m of the ocean since 1970.
• Observed surface ocean warming and high latitude addition of freshwater are making the surface ocean less dense relative to deeper parts of the ocean and inhibiting mixing between the surface and deeper waters.
• The ocean has taken up between 20–30% of total anthropogenic CO2 emissions since the 1980s causing further ocean acidification. 
• The open ocean has lost oxygen alongside a likely expansion of the volume of oxygen minimum zones.
• Oxygen loss is primarily due to increasing ocean stratification, changing ventilation, and biogeochemistry.
• Atlantic Meridional Overturning Circulation (AMOC) has weakened.
• Global mean sea level (GMSL) is rising due to increasing rates of ice loss from the Greenland and Antarctic ice sheets and ocean thermal expansion.
• Sea level rise is not globally uniform and varies regionally.
• Anthropogenic climate change has increased observed precipitation, winds, and extreme sea-level events associated with some tropical cyclones.

Observed changes in Cryosphere

• Global warming has led to a widespread shrinking of the cryosphere.
• Ice sheets and glaciers worldwide have lost mass.
• In nearly all high mountain areas, the depth, extent, and duration of snow cover have declined, especially at lower elevations.
• Permafrost temperatures have increased to record high levels.

• All people on Earth depend directly or indirectly on the ocean and cryosphere. 
• Human communities in close connection with coastal environments, small islands, polar areas, and high mountains are particularly exposed to ocean and cryosphere change, such as sea-level rise, extreme sea level, and shrinking cryosphere.
• Other communities further from the coast are also exposed to changes in the ocean, such as through extreme weather events. 

Impact of changes in the Ocean

• Many marine species across various groups have undergone shifts in geographical range and seasonal activities in response to ocean warming, sea ice change, and biogeochemical changes, such as oxygen loss, to their habitats.
• Cascading effects of multiple climate-related drivers on polar zooplankton have affected food web structure and biodiversity.
• There is a decline in the abundance of fish and shellfish stocks due to the direct and indirect effects of global warming.
• Increasing ocean acidification and oxygen loss is negatively impacting two of the four major upwelling systems: the California Current and Humboldt Current.
• Nearly 50% of coastal wetlands have been lost over the last 100 years, as a result of the combined effects of localized human pressures, sea-level rise, warming, and extreme climate events.
• In response to warming, distribution ranges of seagrass meadows and kelp forests are expanding at high latitudes and contracting at low latitudes since the late 1970s.
• Increased seawater intrusion in estuaries due to sea level rise has driven upstream redistribution of marine species and caused a  reduction of suitable habitats for estuarine communities.
• Warm-water coral reefs and rocky shores dominated by immobile, calcifying organisms such as corals, barnacles, and mussels, are currently impacted by extreme temperatures and ocean acidification.
• Marine heatwaves have already resulted in large-scale coral bleaching events at increasing frequency causing worldwide reef degradation since 1997.
• Harmful algal blooms, attributed partly to the effects of ocean warming, marine heatwaves, oxygen loss, eutrophication, and pollution have had negative impacts on food security, tourism, the local economy, and human health.

Impact of changes in the cryosphere

• Cryospheric and associated hydrological changes have impacted terrestrial and freshwater species and ecosystems in high mountain and polar regions through the appearance of land previously covered by ice, changes in snow cover, and thawing permafrost. 
• Some cold-adapted or snow-dependent species have declined in abundance, increasing their risk of extinction, notably on mountain summits.
• Permafrost thaw and glacier retreat have decreased the stability of high mountain slopes.
• Food and water security have been negatively impacted by changes in snow cover, lake and river ice, and permafrost in many Arctic regions.
• Glacier retreat and snow cover changes have contributed to localized declines in agricultural yields in some high mountain regions, including Hindu Kush Himalaya and the tropical Andes.
• In the Arctic, there is an increased risk of food- and waterborne diseases, malnutrition, injury, and mental health challenges, especially among Indigenous peoples.
• In some high mountain areas, water quality has been affected by contaminants, particularly mercury, released from melting glaciers and thawing permafrost.
• Summertime Arctic ship-based transportation (including tourism) increased over the past two decades concurrent with sea ice reductions (high confidence).
• This has implications for global trade and poses risks to Arctic marine ecosystems and coastal communities, such as invasive species and local pollution.
• Changes in snow and glaciers have changed the amount and seasonality of runoff and water resources in snow-dominated and glacier-fed river basins.
• Hydropower facilities have experienced changes in seasonality and both increases and decreases in water input from high mountain areas.
• High mountain aesthetic and cultural aspects have been negatively impacted by glacier and snow cover decline.

Projected changes in Ocean

• Over the 21st century, the ocean is projected to transition to unprecedented conditions with increased temperatures, greater upper ocean stratification, further acidification, oxygen decline, and altered net primary production.
• Marine heatwaves and extreme El Niño and La Niña events are projected to become more frequent.
• Projections indicate that extreme Indian Ocean Dipole(IOD) events also increase in frequency.
• The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken.
• Any substantial weakening of the AMOC is projected to cause a  decrease in marine productivity in the North Atlantic, more storms in Northern Europe, less Sahelian summer rainfall, and South Asian summer rainfall.
• Sea level continues to rise at an increasing rate. 
• Projected changes in waves and tides vary locally in whether they amplify or ameliorate these hazards.
• The proportion of Category 4 and 5 tropical cyclones, and the associated average precipitation rates are projected to increase for a 2ºC global temperature rise above any baseline period.
• The rates and magnitudes of these changes will be smaller under scenarios with low greenhouse gas emissions.

Projected changes in Cryosphere

• Global-scale glacier mass loss, permafrost thaw, and decline in snow cover and Arctic sea ice extent are projected to continue in the near term (2031–2050) due to surface air temperature increases, with unavoidable consequences for river runoff and local hazards.
• The Greenland Ice Sheet is currently contributing more to sea-level rise than the Antarctic Ice Sheet, but Antarctica could become a larger contributor by the end of the 21st century as a consequence of rapid retreat.
• In many high mountain areas, glacier retreat and permafrost thaw are projected to further decrease the stability of slopes, and the number and area of glacier lakes will continue to increase. 
• Floods due to glacier lake outbursts or rain-on-snow, landslides, and snow avalanches, are projected to occur also in new locations or different seasons.
• Increased plant growth is projected to replenish soil carbon in part, but will not match carbon releases over the long term.

Projected risks from changes in Ocean

• Ocean warming, oxygen loss, acidification, and a decrease in the flux of organic carbon from the surface to the deep ocean are projected to harm habitat-forming cold-water corals.
• Almost all warm-water coral reefs are projected to suffer significant losses of the area and local extinctions, even if global warming is limited to 1.5ºC.
• Vulnerability and risks are highest where and when temperature and oxygen conditions both reach values outside species’ tolerance ranges.
• For sensitive ecosystems such as seagrass meadows and kelp forests, high risks are projected if global warming exceeds 2ºC above pre-industrial temperature.
• Global warming compromises seafood safety through human exposure to elevated bioaccumulation of persistent organic pollutants and mercury in marine plants and animals.
• Long-term loss and degradation of marine ecosystems compromises the ocean’s role in cultural, recreational, and intrinsic values important for human identity and well-being

Projected risks from changes in the cryosphere

• Future land cryosphere changes will continue to alter terrestrial and freshwater ecosystems in high mountain and polar regions with major shifts in species distributions resulting in changes in ecosystem structure and functioning and eventual loss of globally unique biodiversity.
• Wildfire is projected to increase significantly for the rest of this century across most tundra and boreal regions, and also in some mountain regions.
• Permafrost thaw-induced subsidence of the land surface is projected to impact overlying urban and rural communication and transportation infrastructure in the Arctic and high mountain areas.
• High mountain tourism, recreation, and cultural assets are projected to be negatively affected by future cryospheric changes.

• People with the highest exposure and vulnerability are often those with the lowest capacity to respond.
• The temporal scales of climate change impacts in the ocean and cryosphere and their societal consequences operate on time horizons that are longer than those of governance arrangements.
• Governance arrangements are, in many contexts, too fragmented across administrative boundaries and sectors to provide integrated responses.
• Financial, technological, institutional and other barriers exist for implementing responses to current and projected negative impacts.

• Terrestrial and marine habitat restoration, and ecosystem management tools such as assisted species relocation and coral gardening, can be locally effective in enhancing ecosystem-based adaptation.
• Such actions are most successful when they are community-supported, are science-based whilst also using Indigenous knowledge.
• Restoration of vegetated coastal ecosystems, such as mangroves, tidal marshes, and seagrass meadows (coastal ‘blue carbon ecosystems), could provide climate change mitigation through increased carbon uptake.
• Ocean renewable energy can support climate change mitigation and can comprise energy extraction from offshore winds, tides, waves, thermal and salinity gradient, and algal biofuels.
• The emerging demand for alternative energy sources is expected to generate economic opportunities for the ocean renewable energy sector.
• Integrated water management approaches across multiple scales can be effective at addressing impacts and leveraging opportunities from cryosphere changes in high mountain areas.
• Intensifying cooperation and coordination among governing authorities across scales, jurisdictions, sectors, policy domains and planning horizons can enable effective responses.