In September 2019 the Intergovernmental Panel on Climate Change (IPCC) published the third of its special reports entitled “The Ocean & Cryosphere In A Changing Climate”. This is a review of the Summary for Policymakers (SPM) which outlines the key findings of the report.
It has three sections:
- Section A: Observed Changes and Impacts.
- Section B: Projected Changes and Risks.
- Section C: Implementing Responses to Ocean and Cryosphere Change.
The Ocean covers over 70% of the world’s area and the cryosphere is the area of the planet covered by ice: currently around 10% of the land surface area is covered by ice or glaciers. Human communities with a close connection with coastal environments, small islands (including Small Island Developing States, SIDS), 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.
Section A covers the observed changes, which are confirmed through confidence level statements from very high confidence through to low confidence statements. Over the last decades the cryosphere with melting ice sheets and glaciers, reduced snow cover, and reduced Arctic sea ice and thickness.
Ice sheets and glaciers worldwide have lost mass (very high confidence). Between 2006 and 2015, the Greenland Ice Sheet lost ice mass at an average rate of 278 ± 11 Gtyr–1 (equivalent to 0.77 ± 0.03 mm yr–1 of global sea level rise), mostly due to surface melting. During the period 2006–2015, the Antarctic Ice Sheet lost mass at an average rate of 155 ± 19 Gtyr–1 (0.43 ± 0.05 mm yr–1), mostly due to rapid thinning and retreat of major outlet glaciers draining the West Antarctic Ice Sheet. Glaciers worldwide outside Greenland and Antarctica lost mass at an average rate of 220 ± 30 Gt yr–1(equivalent to 0.61±0.08 mm yr–1sea level rise) in 2006–2015. Arctic June snow cover extent on land declined by 13.4 ± 5.4% per decade from 1967 to 2018, a total loss of approximately 2.5 million km2, predominantly due to surface air temperature increase. Depth, extent and duration of snow has declined over recent decades. Permafrost temperatures have increased to record high levels (1980s-present) (very high confidence) including the recent increase by 0.29°C ±0.12°C from 2007 to 2016 averaged across polar and high-mountain regions globally.
Between 1979 and 2018, Arctic sea ice extent has very likely decreased for all months of the year. September sea ice reductions are very likely 12.8 ±2.3% per decade. These sea ice changes in September are likely unprecedented for at least 1000 years. Arctic sea ice has thinned, concurrent with a transition to younger ice: between 1979 and 2018, the areal proportion of multi-year ice at least five years old has declined by approximately 90%. There has been an amplified warming in the Arctic where higher air temperatures have contributed to further warming.
The global ocean has had a period of unabated warming since 1970 and has taken up 90% of the excess heat in the atmosphere. Since 1993 the rate of ocean warming has more than doubled. There have been more marine heatwaves since 1982 to the point that they have more than likely doubled. The warming has varied across the world with the Southern Ocean accounting for 35–43% of the total heat gain, in the upper 2000 m global ocean between 1970 and 2017. There is also density stratification which has increased in the top 200 metres of oceans since 1970 meaning that the upper part of the ocean is now less dense than it once was. The cause is an influx of freshwater from melting ice making the ocean surface less dense than the deeper ocean. Associated with the stratification is a loss of oxygen from the open oceans. Oceans have taken up around 20-30% of anthropogenic CO2 meaning that they are now more acidic since the 1980s. Overall the global mean sea level (GMSL) is rising at an accelerating rate as there has been an increase in the rate of Greenland and Antarctic ice melt. Antarctica ice mas loss tripled from 2007–2016. The Greenland ice mass loss doubled over the same time period. The rate of GMSL rise for 2006–2015 of 3.6 mm yr–1 (3.1 – 4.1 mm yr–1, very likely range), is unprecedented over the last century and is around 2.5 times the rate for 1901–1990 of 1.4 mm yr–1 (0.8–2.0 mm yr–1, very likely range). There is accelerated ice flow in Antartica in two areas of the continent could lead to several metres of sea level rise over the next few centuries. There may be an irreversible change in ice instability. The sea level rises are not uniform around the globe and will be within ±30% of the global mean sea-level rise due to warming of water and impacts of land ice melt.
The report uses assessments of projected future changes which are based largely on CMIP514 climate model projections using Representative Concentration Pathways (RCPs). RCPs represent scenarios that include time series of emissions and concentrations of the full suite of greenhouse gases (GHGs) and aerosols and chemically active gases. In addition land use / land cover is included. RCPs provide only one set of many possible scenarios that would lead to different levels of global warming. Two scenarios RCP2.6 and RCP8.5 represent a lower emission scenario (RPC2.6) and a higher emission scenario (RPC8.5). RCP2.6 is based upon low greenhouse gas emissions and high mitigation future based upon phase 5 of the Coupled Model Intercomparison Project (CMIP5) to give 66.6% confidence of having less than a two Celsius temperature rise by 2100. RPC8.5 is a higher emission scenario where there is little intervention to reduce greenhouse gases: it represents a continued and sustained growth of greenhouse gases. It represents the worst case scenario with highest greenhouse gas emissions.
There have been changes to ecosystems as seasons have been changing and the temperature regions adjusting. This has positive and negative impacts on ecosystems. There are more ecosystem disturbances with an increased frequency. Tundra areas have shown an increased greening but there is also a browning that negatively affects the ecosystem and its services.
Ocean Eastern Boundary Upwelling Systems (EBUS) are some of the most productive ocean ecosystems. Increased ocean acidification and oxygen loss are negatively impacting two of the four major upwelling systems: the California and Humboldt Currents. Ecosystem structures have been affected impacting the biomass production and species composition. There are negative impacts on overall fish catch potentials which are linked to over fishing. Biogeochemical composition changes and effects of warming may also contributed to reduced fish stocks. There has been a reduction, by 50%, of coastal wetlands over the last 100 years. Vegetated coastal areas are an important carbon store. Coral reefs and rocky shores dominated by immobile, calcifying (e.g. shell and skeleton producing) organisms (e.g. corals, barnacles and mussels), are currently being impacted by extreme temperatures and ocean acidification. Marine heatwaves have already affected coral areas through coral bleaching since 1997. Recovery is slow, longer than 15 years, for this ecosystem.
Since the mid-20th century, the shrinking cryosphere in the Arctic and high-mountain areas has led to predominantly negative impacts on food security, water resources, water quality, livelihoods, health and well-being, infrastructure, transportation, tourism and recreation. There are also cultural impacts on indigenous peoples. There has been a rise in natural disasters linked to changes in the changes in cryosphere. High mountain environments are changing affecting the tourism potential although it may be offset by artificial snow production for example. Harmful algal blooms, which have increased in frequency, have shown an increase in their range since the 1980s. There will be further impacts on reclaimed land and hard coastal defences as the sea levels rise. More natural solutions include the creation of wetland habitats to enable better protection.
Projected Changes & Risks
Section B considers the projected changes and risks. During 2031-2050 there will continue to be declines in permafrost, snow cover and glacier mass loss. The Greenland and Antarctic ice sheets will continue to melt into the 21st century and beyond. Rates and magnitudes of the cryosperic changes will continue, especially under the high greenhouse gas scenario. If there are strong reductions in greenhouse gases then changes will be reduced after 2050. Currently the Greenland ice sheet is contributing more to sea level rise than Antarctica but this may increase by the end of the 21st century although this is a low confidence statement. Arctic autumn and spring snow cover is projected to decrease by 5-10% relative to 1985-2006 in the near term (2031-2050). High mountain areas projections in low elevation snow melt are likely to be 10-40% over the same time period. Widespread permafrost thaw is expected this century and beyond. High emission scenarios would see a cumulative release of gigatonnes of permafrost carbon as carbon dioxide and methane released to the atmosphere. In high areas glacial melt will lead to further glacial lakes and there will be an increased risk of flooding from glacial lake outbursts. Arctic sea ice loss is projected to continue into the middle of the century.
Low greenhouse gas scenarios will lead to smaller changes in ocean heating, upper ocean stratification, ocean acidification and oxygen decline than higher scenarios. The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken, although the extent of the weakening will depend upon the greenhouse gas emissions. The ocean will continue to warm through the twenty first century. Continuing ocean carbon take up will exacerbate the ocean acidification. The climate change since pre-industrial periods is affecting the ocean ecosystems. Extreme El Niño and La Niña events are likely to increase in frequency and intensify over the century. This will lead to drier or wetter events around the globe. There is likely to be a doubling of El Niño events under both RPC2.6 and RPC8.5. Sea levels are rising at an increasing rate and there will be more extreme events. The once a century event will become annual by 2050 in all RCP scenarios, especially in tropical regions. Seal level rises will continue beyond 2100. Coastal hazards will increase with more frequent and more intense tropical cyclones for example. There would be an increase in precipitation from these cyclones. The hazards will affect low lying cities and island states. The global mean sea level (GMSL) rise under the projection RCP2.6 is projected to be around 0.39m (in the 0.26m-0.53m likely range) for 2081-2100 and around 0.43m (in the 0.29m-0.59m likely range) in 2100 with respect to the baseline from the period 1986-2005. For RCP8.5 the corresponding rises in sea level would be 0.71m for 2081-2100 and 0.84m for 2100. It is likely to exceed 1m beyond 2100 but there is uncertainty from the melting of the ice in Antarctica. There will be regional GMSL differences. Natural and human processes effect the sea levels locally and these are not driven by a changing climate. One example is subsidence. These local processes will likely impact the relative sea level changes. Under RPC8.5 the projected sea level change is expected to be around 15mm each year. It may exceed several centimetres in the 22nd century. Several studies show a multi-meter sea level rise by 2300 unless emissions are greatly reduced. If the Antarctic ice sheet collapses there could be consequences for the sea levels but it is a complicated area.
High mountain and polar region terrestrial and freshwater ecosystems will change with shifts in species and there will be a wildfire increase in these regions too. Alpine species will decline as other species migrate up slope. There will be range contractions. Permafrost thaw and decreases in snowfall will affect mountain hydrology and wildfire with impacts on the vegetation. Cold water coral ecosystems are projected to decline. The continued loss of arctic multi year sea ice will impact polar marine ecosystems through both direct and indirect effects on their habitats, populations and viability. Risks of severe impacts on biodiversity, structure and function of coastal ecosystems are projected to increase for elevated temperatures. The risk increases to very high risk under RCP8.5. Salinisation and hypoxia are likely to increase in estuaries with warming water, increased sea levels and tidal changes. There will be associated risks to the local estuary ecosystems including local extinction. There is high confidence that warm water corals will suffer local extinctions and reductions in area even if warming could be limited to 1.5 Celsius.
There will be risks for people and ecosystem services. Future cryosphere changes will affect water use including irrigation and hydropower systems. High mountain disaster risk will increase with risks such as fire, landslips, floods, avalanches and infrastructure exposure being affected. Engineering risk calculations will need to take into account the changes in the environments. High mountain tourism, recreation and cultural assets are all likely to be negatively affected. Changes in fish distribution and fish abundance will change with the climate changing. There is a medium confidence risk of a compromise to food safety through human exposure to elevated bioaccumulation of persistent organic pollutants and mercury in plants and animals. Fishing areas will be changed with cultural impacts on those communities who rely on their marine ecosystems. Without reductions in emissions the current trends show an increased exposure and vulnerability of coastal communities, risks (e.g. erosion and land loss or flooding or salinisation) and cascading impacts due to mean sea level rise and extreme events. There are very high risks to low lying communities, people in coral reef environments, urban atoll islands and low-lying Arctic locations in the case of high emission scenarios. Some island states are likely to become uninhabitable. Overall, and at a global level, a slower rate of climate related ocean and cryosphere changes provide greater adaptation opportunities.
Part C considers implementing responses to ocean and cryosphere change. Where governments and others who have an ambitious adaptation policy, which includes governance for transformative change, will be better at managing the risks. The greatest vulnerabilities exist where those people have the lowest capacity to respond. Temporal scales of climate change will be longer than those of government arrangements such as planning cycles, public and corporate decision making cycles and financial instruments. These temporal differences lead to challenges of how to best prepare and respond to long-term changes. Often governance arrangements are too fragmented either across nations or within nations and between departments or agencies. There is often a slow response to the rapid changes are going to occur, one example being shifting ecosystems. There needs to be rapid and robust governance systems in place to respond to climate impacts. Limitations may include the space that ecosystems need, non-climatic drivers and human impacts that need to be addressed as part of the adaptation response. Adaptive capacity differs between as well as within communities and societies. Those with the highest exposure and vulnerability to current and future ocean and cryosphere changes are often those with the lowest adaptive capacity especially in low lying areas, islands or coasts.
In order to strengthen responses to climate change there needs to be a shift towards protection, restoration and precautionary ecosystem-based management of renewable resource use, reductions in pollution and other stressors. Integration of water management and ecosystem-based adaptation lower the climate change risk locally and provide further multiple societal benefits. There are also constraints to such actions. Networks of protected areas help to maintain ecosystem services including carbon take up and storage. Terrestrial and marine habitat restoration and ecosystem management tools can be locally effective in enhancing ecosystem based adaptation: These actions are most successful when they have community support that is long-term support and are based upon good science. Coastal communities face challenging choices to create appropriate local integrated responses to sea level rises that balance the cost, benefits and trade offs of available options. Higher sea levels mean further challenges to protection of the coastline mainly due to economic, financial and social barriers. Reducing drivers of local exposure and vulnerability such as urbanising coastal areas and human induced subsidence are effective responses over the next decades. In areas of high assets such as cities the hard protection measures, such as dykes or sea walls, are likely to be positive responses. Resource limited areas may not be able to afford such measures. Where there is space ecosystem measures are likely to be a good adaptation method with benefits of coastal protection, carbon storage, improved water quality and biodiversity and livelihood support. Early warning systems are currently an effective coastal accommodation measure although there may have to be further measures introduced with the sea level rises predicted. Responses to sea level rise and associated risks will present society with profound governance shifts: there will need to be locally appropriate combinations of decision analysis, land-use planning, public participation, diverse knowledge systems and conflict resolution. Long term coastal plans need to account for sea level rises over decades over the next century.
Enabling conditions have been highlighted in the report. Enabling climate resilience and sustainable development depends upon urgent and ambitious emissions reduction targets combined with ambitious adaptation actions. There will need to be greater cooperation and coordination among governing authorities across spatial scales and planning horizons. It will be essential to have education, climate literacy, monitoring and forecasting, use of knowledge sources, sharing of data, information and knowledge, finance, addressing social vulnerability and equity and institutional support are essential. These investments enable capacity-building, social learning and participation in context specific adaptation. There needs be a move to building long-term resilience and sustainability.
Many nations will face challenges to adapt, even with ambitious mitigation. Many ocean and cryosphere-dependent communities are projected to face adaptation limits (e.g. biophysical, geographical, financial, technical, social and political) during the second half of the twenty first century. Low emission pathways will limit these risks from ocean and cryosphere changes. The low emissions scenarios will allow more effective responses and create co-benefits. Regional cooperation can support adaptation actions. Taking long term perspectives when making short-term decisions help to enable longer term risk taking beyond 2050 where there will be greater uncertainty. Context-specific monitoring and forecasting of changes in the ocean and the cryosphere informs adaptation planning and implementation, and helps with decision making over a longer time period. Measures should be prioritised to address social vulnerability and equity and this underpins efforts to promote fair and just climate resilience and sustainable development.
Conclusions: An Urgency For Climate Action
This assessment of the ocean and cryosphere illustrates the benefits of ambitious mitigation and effective adaptation for sustainable development and, conversely, the escalating costs and risks of delayed action. It highlights the urgency of taking climate action.