IPCC, Special Report, The Ocean and Cryosphere in a Changing Climate
Den här specialrapporten om klimatförändringar (SROCC) understryker brådskan av att prioritera snabba, ambitiösa och samordnade åtgärder för att ta itu med varaktiga förändringar, utan motstycke, i havet och kryosfären.
Rapporten redovisar fördelarna med ambitiös och effektiv anpassning för hållbar utveckling och omvänt de eskalerande kostnaderna och riskerna för försenad åtgärd.
Havet och kryosfären – de frysta delarna av planeten – spelar en avgörande roll för livet på jorden
Totalt är 670 miljoner människor i bergsområden och 680 miljoner människor i lågt liggande kustzoner beroende direkt av dessa system. Fyra miljoner människor bor permanent i den arktiska regionen, och ”utvecklingsstater” på små öar är hem för 65 miljoner människor.
Den globala uppvärmningen har redan nått +1°C över den för-industriella nivån orsakat av redan gjorda och nuvarande utsläpp av växthusgaser. Det finns överväldigande bevis på att detta leder till djupa konsekvenser för ekosystem och människor.
Haven är varmare, surare och mindre produktiva. Smältande glaciärer och ismassor orsakar stigande havsnivå, och extrema kustnära händelser blir allt värre.
IPCC: s särskilda rapport om havet och kryosfären i ett förändrat klimat, som godkändes 24 september 2019 av 195 IPCC: s medlemsregeringar, ger nya bevis för fördelarna med att begränsa den globala uppvärmningen till lägsta möjliga nivå – i linje med det mål som regeringarna själva har satt i Parisavtalet 2015. Minskande utsläpp av växthusgaser begränsar kraftigt förändringarna av hav och kryosfär. Ekosystem och försörjningen som är beroende av dem kan bevaras.
APPROVED REPORT OUTLINE
Chapter 1: Framing and Context
- All people on Earth depend directly or indirectly on the ocean and cryosphere.
- Sustainable development is at risk from emerging and intensifying ocean and cryosphere changes.
- Communities living in close connection with polar, mountain, and coastal environments are particularly exposed to the current and future hazards of ocean and cryosphere change.
- Ocean and cryosphere changes are pervasive and observed from high mountains, to the polar regions, to coasts, and into the deep ocean.
- Evidence and understanding of the human causes of climate warming, and of associated ocean and cryosphere changes, has increased over the past 30 years of IPCC assessments (very high confidence).
- Ocean and cryosphere changes and risks by the end-of-century (2081-2100) will be larger under high greenhouse gas emission scenarios, compared with low emission scenarios (very high confidence).
- Characteristics of ocean and cryosphere change include thresholds of abrupt change, long-term changes that cannot be avoided, and irreversibility (high confidence).
- Societies will be exposed, and challenged to adapt, to changes in the ocean and cryosphere even if current and future efforts to reduce greenhouse gas emissions keep global warming well below 2°C (very high confidence).
- The scale and cross-boundary dimensions of changes in the ocean and cryosphere challenge the ability of communities, cultures and nations to respond effectively within existing governance frameworks (high confidence). Profound economic and institutional transformations are needed if climate-resilient development is to be achieved (high confidence).
- Robust assessments of ocean and cryosphere change, and the development of context-specific governance and response options, depend on utilising and strengthening all available knowledge systems (high confidence).
- Long-term sustained observations and continued modeling are critical for detecting, understanding and predicting ocean and cryosphere change, providing the knowledge to inform risk assessments and adaptation planning (high confidence).
Chapter 2: High Mountain Areas
Observations of cryospheric changes, impacts, and adaptation in high mountain areas
- Observations show general decline in low-elevation snow cover (high confidence), glaciers (very high confidence) and permafrost (high confidence) due to climate change in recent decades.
- Glacier, snow and permafrost decline has altered the frequency, magnitude and location of most related natural hazards (high confidence). Exposure of people and infrastructure to natural hazards has increased due to growing population, tourism and development (high confidence).
- Changes in snow and glaciers have changed the amount and seasonality of runoff in snow-dominated and glacier-fed river basins (very high confidence) with impacts on agriculture (medium confidence).
- Species composition and abundance have markedly changed in high-mountain ecosystems in recent decades (very high confidence), partly due to changes in the cryosphere (high confidence).
- Tourism and recreation activities such as skiing, glacier tourism and mountaineering have been negatively impacted by declining snow cover, glaciers and permafrost (medium confidence).
- Adaptation in agriculture, tourism and drinking water supply has aimed to reduce the impacts of cryosphere change (medium confidence), though there is limited evidence on their effectiveness owing to a lack of formal evaluations, or technical, financial and institutional barriers to implementation.
Future projections of cryospheric changes, their impacts and risks, and adaptation in high mountain areas.
- Snow cover, glaciers and permafrost are projected to continue to decline in almost all regions throughout the 21st century (high confidence).
- Most types of natural hazards are projected to change in frequency, magnitude and areas affected as the cryosphere continues to decline (medium confidence).
- River runoff in snow-dominated and glacier-fed river basins will change further in amount and seasonality in response to projected snow cover and glacier decline (very high confidence) with negative impacts on agriculture, hydropower and water quality in some regions (medium confidence).
- Current trends in cryosphere-related changes in high-mountain ecosystems are expected to continue and impacts to intensify (very high confidence).
- Cultural assets, such as snow- and ice-covered peaks in many UNESCO World Heritage sites, and tourism and recreation activities, are expected to be negatively affected by future cryospheric change in many regions (high confidence).
Enablers and response options to promote adaptation and sustainable development in high mountain areas
- The already committed and unavoidable climate change affecting all cryosphere elements, irrespective of the emission scenario, point to integrated adaptation planning to support and enhance water availability, access, and management (medium confidence).
- Effective governance is a key enabler for reducing disaster risk, considering relevant exposure factors such as planning, zoning, and urbanization pressures, as well as vulnerability factors such as poverty, which can challenge efforts towards resilience and sustainable development for communities (medium confidence).
- International cooperation, treaties and conventions exist for some mountain regions and transboundary river basins with potential to support adaptation action. However, there is limited evidence on the extent to which impacts and losses arising from changes in the cryosphere are specifically monitored and addressed in these frameworks.
Chapter 3: Polar Regions
The polar regions are losing ice, and their oceans are changing rapidly. The consequences of this polar transition extend to the whole planet, and are affecting people in multiple ways.
- Arctic surface air temperature has likely increased by more than double the global average over the last two decades, with feedbacks from loss of sea ice and snow cover contributing to the amplified warming.
- The Arctic and Southern Oceans are continuing to remove carbon dioxide from the atmosphere and to acidify (high confidence).
- Both polar oceans have continued to warm in recent years, with the Southern Ocean being disproportionately and increasingly important in global ocean heat increase (high confidence).
- Climate-induced changes in seasonal sea ice extent and thickness and ocean stratification are altering marine primary production (high confidence), with impacts on ecosystems (medium confidence).
- In both polar regions, climate-induced changes in ocean and sea ice, together with human introduction of non-native species, have expanded the range of temperate species and contracted the range of polar fish and-ice associated species (high confidence).
- It is very likely that Arctic sea ice extent continues to decline in all months of the year; the strongest reductions in September (-12.8 ± 2.3% per decade; 1979-2018) are likely unprecedented in at least 1000 years.
- It is very likely that Antarctic sea ice cover exhibits no significant trend over the period of satellite observations (1979 to 2018).
- Shipping activity during the Arctic summer increased over the past two decades in regions for which there is information, concurrent with reductions in sea ice extent (high confidence).
- Permafrost temperatures have increased to record high levels (very high confidence), but there is medium evidence and low agreement that this warming is currently causing northern permafrost regions to release additional methane and carbon dioxide.
- Climate-related changes to Arctic hydrology, wildfire and abrupt thaw are occurring (high confidence), with impacts on vegetation and water and food security.
- Limited knowledge, financial resources, human capital and organisational capacity are constraining adaptation in many human sectors in the Arctic (high confidence).
- It is extremely likely that the rapid ice loss from the Greenland and Antarctic ice sheets during the early 21st century has increased into the near present-day, adding to the ice sheet contribution to global sea level rise.
- Mass loss from Arctic glaciers (-212 ± 29 Gt yr-1) during 2006-2015 contributed to sea level rise at a similar rate (0.6 ± 0.1 mm yr-1) to the Greenland Ice Sheet (high confidence).
- There is limited evidence and high agreement that recent Antarctic Ice Sheet mass losses could be irreversible over decades to millennia.
The polar regions will be profoundly different in future compared with today, and the degree and nature of that difference will depend strongly on the rate and magnitude of global climatic change3. This will challenge adaptation responses regionally and worldwide.
- It is very likely that projected Arctic warming will result in continued loss of sea ice and snow on land, and reductions in the mass of glaciers. Important differences in the trajectories of loss emerge from 2050 onwards, depending on mitigation measures taken (high confidence).
- Both polar oceans will be increasingly affected by CO2 uptake, causing conditions corrosive for calcium carbonate shell-producing organisms (high confidence), with associated impacts on marine organisms and ecosystems (medium confidence).
- Future climate-induced changes in the polar oceans, sea ice, snow and permafrost will drive habitat and biome shifts, with associated changes in the ranges and abundance of ecologically-important species (medium confidence).
- The projected effects of climate-induced stressors on polar marine ecosystems present risks for commercial and subsistence fisheries with implications for regional economies, cultures and the global supply of fish, shellfish, and Antarctic krill (high confidence). Future impacts for linked human systems depend on the level of mitigation and especially the responsiveness of precautionary management approaches (medium confidence).
- Widespread disappearance of Arctic near-surface permafrost is projected to occur this century as a result of warming (high confidence), with important consequences for global climate.
- Projected permafrost thaw and decrease in snow will affect Arctic hydrology and wildfire, with impacts on vegetation and human infrastructure (medium confidence).
Response options exist that can ameliorate the impacts of polar change, build resilience and allow time for effective mitigation measures. Institutional barriers presently limit their efficacy.
- Responding to climate change in polar regions will be more effective if attention to reducing immediate risks (short-term adaptation) is concurrent with long-term planning that builds resilience to address expected and unexpected impacts (high confidence).
- Innovative tools and practices in polar resource management and planning show strong potential in improving society’s capacity to respond to climate change (high confidence).
- Institutional arrangements that provide for strong multiscale linkages with Arctic local communities can benefit from including indigenous knowledge and local knowledge in the formulation of adaptation strategies (high confidence).
- The capacity of governance systems in polar regions to respond to climate change has strengthened recently, but the development of these systems is not sufficiently rapid or robust to address the challenges and risks to societies posed by projected changes (high confidence).
Chapter 4: Sea Level Rise and Implications for Low Lying Islands, Coasts and Communities
- Global mean sea level (GMSL) is rising (virtually certain) and accelerating (high confidence). The sum of glacier and ice sheet contributions is now the dominant source of GMSL rise (very high confidence)
- Global mean sea level was considerably higher than today during past climate states that were slightly warmer than preindustrial, including the Last Interglacial (LIG; 129–116 thousand years ago), when global mean surface temperature was 0.5oC to 1.0oC warmer, and the mid-Pliocene Warm Period (mPWP; ~3.3 to 3.0 million years ago), 2o-4o C warmer. Despite the modest global warmth of the Last Interglacial, Greenland and Antarctic ice sheets contributed 6-9 m to GMSL (high confidence), but not more than 10 m (medium confidence). Based on new understanding about geological constraints since AR5, 25 m is a plausible upper bound on GMSL during the mid-Pliocene Warm Period (low confidence).
- Non-climatic anthropogenic drivers, including recent and historical demographic and settlement trends and anthropogenic subsidence, have played an important role in increasing low-lying coastal communities’ exposure and vulnerability to sea level rise and extreme sea level events (very high confidence).
- Coastal ecosystems are already impacted by the combination of sea level rise, other climate-related ocean changes, and adverse effects from human activities on ocean and land (high confidence). Attributing such impacts to sea level rise, however, remains challenging due to the influence of other climate-related and non-climatic drivers such as infrastructure development and human-induced habitat degradation (high confidence).
- Coastal risk is dynamic and increased by widely observed changes in coastal infrastructure, community livelihoods, agriculture and habitability (high confidence). As with coastal ecosystems, attribution of observed changes and associated risk to sea level rise remains challenging.
- A diversity of adaptation responses to coastal impacts and risks have been implemented around the world, but mostly as a reaction to current coastal risk or experienced disasters (high confidence).
- Future rise in global mean sea level caused by thermal expansion, melting of glaciers and ice sheets, and land water storage changes, is strongly dependent on which RCP emission scenario is followed. Sea level rise at the end of the century is projected to be faster under all scenarios, including those compatible with achieving the long-term temperature goal set out in the Paris Agreement. GMSL will rise between 0.43 m (0.29–0.59 m, likely range) (RCP2.6) and 0.84 m (0.61–1.10 m, likely range) (RCP8.5) by 2100 (medium confidence) relative to 1986-2005.
- Under RCP8.5, the rate of sea level rise will be 15 mm yr–1 (10–20 mm yr–1, likely range) in 2100, and could exceed several cm yr–1 in the 22nd century.
- Processes controlling the timing of future ice-shelf loss and the extent of ice sheet instabilities could increase Antarctica’s contribution to sea level rise to values higher than the likely range on century and longer time-scales (low confidence).
- Sea level does not and will not rise uniformly. Thermal expansion, ocean dynamics and land ice loss contributions will generate regional departures of about ±30% around the GMSL rise. Local anthropogenic subsidence and change in wave height and period are important contributors to future changes in relative sea level (RSL) at the coast (high confidence).
- Due to projected global mean sea level rise, extreme sea level events (ESLs) that are historically rare (for example, today’s hundred-year event) will become common by 2100 under all RCPs (high confidence)
- Non-climatic anthropogenic drivers will continue to increase the exposure and vulnerability of coastal communities to future sea level rise and extreme sea level events in the absence of major adaptation efforts compared to today (high confidence).
- The expected impacts of sea level rise on coastal ecosystems over the course of the century include habitat contraction, loss of functionality and biodiversity, and lateral and inland migration. Impacts will be exacerbated in cases of land reclamation and where anthropogenic barriers prevent inland migration of marshes and mangroves and limit the availability and re-location of sediment (high confidence).
- In the absence of adaptation, more intense and frequent extreme sea level events, together with trends in coastal development will increase expected annual flood damages by 2-3 orders of magnitude by 2100 (high confidence). However, well-designed coastal protection is very effective in reducing expected damages and cost efficient for urban and densely populated regions, but generally unaffordable for rural and poorer areas (high confidence).
- Risk related to sea level rise (including erosion, flooding and salinization) is expected to significantly increase by the end of this century along all low-lying coasts in the absence of major additional adaptation efforts (very high confidence).
Choosing and Implementing Responses.
- All types of responses to sea-level rise, including protection, accommodation, ecosystem-based adaptation, advance and retreat, have important and synergistic roles to play in an integrated and sequenced response to sea-level rise (high confidence).
- Technical limits to hard protection are expected to be reached under high emission scenarios (RCP8.5) beyond 2100 (high confidence) and biophysical limits to ecosystem-based adaptation may arise during the 21st century, but economic and social barriers arise well before (medium confidence).
- Choosing and implementing responses to sea level rise presents society with profound governance challenges and difficult social choices, which are inherently political and value-laden (high confidence).
- Despite the large uncertainties about post 2050 sea level rise, adaptation decisions can be made now, facilitated by using decision analysis methods specifically designed to address uncertainty (high confidence).
- Adaptation experience to date demonstrates that using a locally appropriate combination of decision analysis, land-use planning, public participation and conflict resolution approaches can help to address the governance challenges faced in responding to SLR (high confidence).
Achieving the United Nations Sustainable Development Goals and charting Climate Resilient Development Pathways depends in part on ambitious and sustained mitigation efforts to contain SLR coupled with effective adaptation actions to reduce SLR impacts and risk (medium evidence, high agreement).
Chapter 5: Changing Ocean, Marine Ecosystems, and Dependent Communities
Observations: Climate-related trends, impacts, adaptation.
- Carbon emissions from human activities are causing ocean warming, acidification, and oxygen loss with some evidence of changes in nutrient cycling and primary production. The warming ocean is affecting marine organisms at multiple trophic levels, impacting fisheries with implications for food production and human communities. Concerns regarding the effectiveness of existing ocean and fisheries governance have already been reported, highlighting the need for timely mitigation and adaptation responses.
- The ocean has warmed unabated since 2005, continuing the clear multi-decadal ocean warming trends documented in the IPCC Fifth Assessment Report (AR5).
- It is likely that the rate of ocean warming has increased since 1993.
- The upper ocean is very likely to have been stratifying since 1970.
- Multiple datasets and models show that the rate of ocean uptake of atmospheric CO2 has continued to strengthen in the recent two decades in response to the increasing concentration of CO2 in the atmosphere.
- The ocean is continuing to acidify in response to ongoing ocean carbon uptake.
- There is a growing consensus that the open ocean is losing oxygen overall with a very likely loss of 0.5 to 3.3% between 1970-2010 from the ocean surface to 1000 m (medium confidence).
- In response to ocean warming and increased stratification, open ocean nutrient cycles are being perturbed and there is high confidence that this is having a regionally variable impact on primary producers.
- Ocean warming has contributed to observed changes in biogeography of organisms ranging from phytoplankton to marine mammals (high confidence), consequently changing community composition (high confidence), and in some cases, altering interactions between organisms (medium confidence).
- Warming-induced range expansion of tropical species to higher latitudes has led to increased grazing on some coral reefs, rocky reefs, seagrass meadows and epipelagic ecosystems, leading to altered ecosystem structure (medium confidence).
- Fisheries catches and their composition in many regions are already impacted by the effects of warming and changing primary production on growth, reproduction and survival of fish stocks (high confidence).
- Warming-induced changes in spatial distribution and abundance of fish stocks have already challenged the management of some important fisheries and their economic benefits (high confidence).
- Coastal ecosystems are observed to be under stress from ocean warming and sea level rise that are exacerbated by non-climatic pressures from human activities on ocean and land (high confidence).
- Coastal and near-shore ecosystems including saltmarshes, mangroves, and vegetated dunes in sandy beaches have a varying capacity to build vertically and expand laterally in response to sea-level rise.
- Three out of the four major Eastern Boundary Upwelling Ecosystems (EBUS) have shown large-scale wind intensification in the past 60 years (high confidence).
- Since the early 1980s, the occurrence of harmful algal blooms (HABs) and pathogenic organisms (e.g. Vibrio) has increased in coastal areas in response to warming, deoxygenation and eutrophication, with negative impacts on food provisioning, tourism, the economy and human health (high confidence).
- Many frameworks for climate-resilient coastal adaptation have been developed since AR5, with substantial variations in approach between and within countries, and across development status (high confidence).
Projections: scenarios and time horizons
- Climate models project significant changes in the ocean state over the coming century. Under the high emissions scenario (RCP8.5) the impacts by 2090 are substantially larger and more widespread than for the low emissions scenario (RCP2.6) throughout the surface and deep ocean, including: warming (virtually certain); ocean acidification (virtually certain); decreased stability of mineral forms of calcite (virtually certain); oxygen loss (very likely); reduced near- surface nutrients (likely as not); decreased net-primary productivity (high confidence); reduced fish production (likely) and loss of key ecosystems services (medium confidence) that are important for human wellbeing and sustainable development.
- By 2100 the ocean is very likely to warm by 2 to 4 times as much for low emissions (RCP2.6) and 5 to 7 times as much for the high emissions scenario (RCP8.5) compared with the observed changes since 1970.
- The upper ocean will continue to stratify.
- It is very likely that the majority of coastal regions will experience statistically significant changes in tidal amplitudes over the course of the 21st century.
- It is virtually certain that surface ocean pH will decline, by 0.036-0.042 or 0.287-0.29 pH units by 2081-2100, relative to 2006-2015, for the RCP2.6 or RCP8.5 scenarios, respectively.
- Oxygen is projected to decline further.
- Overall, nitrate concentrations in the upper 100m are very likely to decline by 9-14 % across CMIP5 models by 2081-2100, relative to 2006-2015, in response to increased stratification for RCP8.5, with medium confidence in these projections due to the limited evidence of past changes that can be robustly understood and reproduced by models.
- Climate models project that net primary productivity will very likely decline by 4-11% for RCP8.5 by 2081-2100, relative to 2006-2015.
- New ocean states for a broad suite of climate indices will progressively emerge over a substantial fractions of the ocean in the coming century (relative to past internal ocean variability), with ESMs showing an ordered emergence of first pH, followed by SST, interior oxygen, upper ocean nutrient levels and finally NPP.
- Simulated ocean warming and changes in net primary production during the 21st century are projected to alter community structure of marine organisms (high confidence), reduce global marine animal biomass (medium confidence) and the maximum potential catches of fish stocks (medium confidence) with regional differences in the direction and magnitude of changes (high confidence).
- Projected decreases in global marine animal biomass and fish catch potential could elevate the risk of impacts on income, livelihood and food security of the dependent human communities (medium confidence).
- Projected decrease in upper ocean export of organic carbon to the deep seafloor is expected to result in a loss of animal biomass on the deep seafloor by 5.2% to 17.6% by 2090-2100 compared to the present (2006-2015) under RCP8.5 with regional variations (medium confidence).
- Structure and functions of all types of coastal ecosystems will continue to be at moderate to high risk under the RCP2.6 scenario (medium confidence) and will face high to very high risk under the RCP8.5 scenario (high confidence) by 2100.
- Expected coastal ecosystem responses over the 21st century are habitat contraction, migration and loss of biodiversity and functionality.
- Almost all coral reefs will degrade from their current state, even if global warming remains below 2°C (very high confidence), and the remaining shallow coral reef communities will differ in species composition and diversity from present reefs (very high confidence).
- Multiple hazards of warming, deoxygenation, aragonite under-saturation and decrease in flux of organic carbon from the surface ocean will decrease calcification and exacerbate the bioerosion and dissolution of the non-living component of cold-water coral.
- Anthropogenic changes in Eastern Boundary Upwelling Ecosystems (EBUS) will emerge primarily in the second half of the 21st century (medium confidence).
- Climate change impacts on ecosystems and their goods and services threatens key cultural dimensions of lives and livelihoods.
- Climate change increases the exposure and bioaccumulation of contaminants such as persistent organic pollutants and mercury (medium confidence), and their risk of impacts on marine ecosystems and seafood safety (high agreement, medium evidence, medium confidence).
- B1.18 Shifting distributions of fish stocks between governance jurisdictions will increase the risk of potential conflicts among fishery area users and authorities or between two different communities within the same country (medium confidence).
Response options to enhance resilience
- There is clear evidence for observed climate change impacts throughout the ocean with consequences for human communities and require options to reduce risks and impacts. Coastal blue carbon can contribute to mitigation for many nations but its global scope is modest (offset of < 2% of current emissions) (likely). Some ocean indices are expected to emerge earlier than others (e.g., warming, acidification and effects on fish stocks) and could therefore be used to prioritise planning and building resilience. The survival of some keystone ecosystems (e.g., coral reefs) are at risk, while governance structures are not well- matched to the spatial and temporal scale of climate change impacts on ocean systems. Ecosystem restoration may be able to locally reduce climate risks (medium confidence) but at relatively high cost and effectiveness limited to low emissions scenarios and to less sensitive systems (high confidence).
- Coastal blue carbon ecosystems, such as mangroves, salt marshes and seagrasses, can help reduce the risks and impacts of climate change, with multiple co-benefits.
- The potential climatic benefits of blue carbon ecosystems can only be a very modest addition to, and not a replacement for, the very rapid reduction of greenhouse gas emissions.
- Socio-institutional adaptation responses are more frequently reported in the literature than ecosystem-based and built-infrastructure approaches.
- Ecosystem Based Adaptation is a cost-effective coastal protection tool that can have many co- benefits, including supporting livelihoods, contributing to carbon sequestration and the provision of a range of other valuable ecosystem services (high confidence).
- Socio-institutional adaptation responses, including community-based adaptation, capacity- building, participatory processes, institutional support for adaptation planning and support mechanisms for communities are important tools to address climate change impacts (high confidence).
- Observed widespread decline in warm water corals has led to the consideration of alternative restoration approaches to enhance climate-resilience.
- Existing ocean governance structures are already facing multi-dimensional, scale-related challenges because of climate change. This trend of increasing complexity will continue (high confidence).
- There are a broad range of identified barriers and limits for adaptation to climate change in ecosystems and human systems (high confidence).
Chapter 6: Extremes, Abrupt Changes and Managing Risks
Ongoing and Emerging Changes in the Ocean and Cryosphere, and their Impacts on Ecosystems and Human Societies
- Anthropogenic climate change has increased precipitation, winds and extreme sea level events associated with a number of observed tropical- and extra-tropical cyclones (high confidence).
- Anthropogenic climate change may have contributed to a poleward migration of maximum tropical cyclone intensity in the western North Pacific in recent decades related to anthropogenically-forced tropical expansion (low confidence)
- Extreme wave heights across the globe have increased by around 5% over the past three decades (medium confidence).
- Marine heatwaves, periods of extremely high ocean temperatures, have negatively impacted marine organisms and ecosystems in all ocean basins over the last two decades, including critical foundation species such as corals, seagrasses and kelps (very high confidence).
- Both paleoclimate and modern observations suggest that the strongest El Niño and La Niña events since the pre-industrial have occurred during the last fifty years (medium confidence).
- The equatorial Pacific trade wind system experienced an unprecedented intensification during 2001- 2014, resulting in enhanced ocean heat transport from the Pacific to the Indian Ocean, influencing the rate of global temperature change (medium confidence).
- Modern observations, climate simulations and paleoclimate reconstructions suggest that the Atlantic Meridional Overturning Circulation (AMOC) has weakened since the preindustrial (medium confidence).
- Climate change is modifying multiple types of climate-related events or hazards in terms of occurrence, intensity and periodicity. It increases the likelihood of compound hazards that comprise simultaneously or sequentially occurring events to cause extreme impacts in natural and human systems. Compound events in turn trigger cascading impacts (high confidence).
Projections of Ocean and Cryosphere Change and Hazards to Ecosystems and Human Society Under Low and High Emission Futures
- An increase in the average intensity of tropical cyclones, and the associated average precipitation rates is projected for a 2°C global temperature rise (medium confidence), although there is low confidence in future frequency changes at the global scale. Rising sea levels will contribute to higher extreme sea levels associated with tropical cyclones in the future (very high confidence).
- Wave heights are projected to increase across the Southern Ocean, tropical eastern Pacific and Baltic Sea and decrease over the North Atlantic and Mediterranean Sea under RCP 4.5 and RCP 8.5 (high confidence).
- Marine heatwaves will further increase in frequency, duration, spatial extent and intensity under future global warming (very high confidence) pushing some marine organisms, fisheries and ecosystems beyond the limits of their resilience, with cascading impacts on economies and societies (high confidence).
- Extreme El Niño and La Niña events are likely to occur more frequently with global warming and are likely to intensify existing impacts, with drier or wetter responses in several regions across the globe, even at relatively low levels of future global warming (medium confidence).
- Lack of long-term sustained Indian and Pacific ocean observations, and inadequacies in the ability of climate models to simulate the magnitude of trade wind decadal variability and the inter-ocean link, mean there is low confidence in future projections of the trade wind system.
- The AMOC will very likely weaken over the 21st century (high confidence), although a collapse is very unlikely (medium confidence). Nevertheless, a substantial weakening of the AMOC remains a physically plausible scenario.
- Impacts from further changes in tropical and extra-tropical cyclones, marine heatwaves, extreme El Niño and La Niña events and other extremes will exceed the limits of resilience and adaptation of ecosystems and people, leading to unavoidable loss and damage (medium confidence).
Strengthening the Global Responses in the Context of Sustainable Development Goals and Charting Climate Resilient Development Pathways for Oceans and Cryosphere
- There is medium confidence that including extremes and abrupt changes, such as AMOC weakening, ice-sheet collapse (WAIS and GIS), leads to a several-fold increase in the cost of carbon emissions (medium confidence). If carbon emissions decline, the risk of extremes and abrupt changes are reduced, creating co-benefits.
- For tropical and extratropical cyclones, investment in disaster risk reduction, flood management (ecosystem and engineered) and early warning systems decreases economic loss (medium confidence), but such investments may be hindered by limited local capacities, such as increased losses and mortality from extreme winds and storm surges in less-developed countries despite adaptation efforts.
- Limiting global warming would reduce the risk of impacts of marine heatwaves, but critical thresholds for some ecosystems (e.g. kelp forests, coral reefs) will be reached at relatively low levels of future global warming (high confidence).
- Sustained long-term monitoring and improved forecasts can be used in managing the risks of extreme El Niño and La Niña events associated with human health, agriculture, fisheries, coral reefs, aquaculture, wildfire, drought and flood management (high confidence).
- Extreme change in the trade wind system and its impacts on global variability, biogeochemistry, ecosystems as well as society have not been adequately understood and represent significant knowledge gaps.
- By 2300, an AMOC collapse is as likely as not for high emission pathway and very unlikely for lower ones, highlighting that an AMOC collapse can be avoided in the long term by CO2 mitigation (medium confidence).
- The ratio between risk reduction investment and reduction of damages of extreme events varies. Investing in preparation and prevention against the impacts from extreme events is very likely less than the cost of impacts and recovery (medium confidence).
- Climate change adaptation and disaster risk reduction require capacity building and an integrated approach to ensure trade-offs between short- and long-term gains in dealing with the uncertainty of increasing extreme events, abrupt changes, and cascading impacts at different geographic scales (high confidence).
- Limiting the risk from the impact of extreme events and abrupt changes leads to successful adaptation to climate change with the presence of well-coordinated climate-affected sectors and disaster management relevant agencies (high confidence). Transformative governance inclusive of successful integration of disaster risk management and climate change adaptation, empowerment of vulnerable groups, and accountability of governmental decisions promotes climate-resilient development pathways (high confidence).
Cross-Chapter Box 9: Integrative Cross-Chapter Box on Low-lying Islands and Coasts
Ocean and cryosphere changes already impact Low-Lying Islands and Coasts (LLIC), including Small Island Developing States, with cascading and compounding risks. Disproportionately higher risks are expected in the course of the 21st century. Reinforcing the findings of the IPCC Special Report on Global Warming of 1.5°C, vulnerable human communities, especially those in coral reef environments and polar regions, may exceed adaptation limits well before the end of this century and even in a low greenhouse gas emission pathway (high confidence). Depending on the effectiveness of 21st century mitigation and adaptation pathways under all emission scenarios, most of the low-lying regions around the world may face adaptation limits beyond 2100, due to the long-term commitment of sea level rise (medium confidence).
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