Framing and Context of the Report

The ocean and cryosphere play a key role in the Earth system. Powered by the Sun’s energy, large quantities of energy, water and biogeochemical elements (predominantly carbon, nitrogen, oxygen and hydrogen) are exchanged between all components of the Earth system, including between the ocean and cryosphere (Box 1.1, Figure 1). 

During an equilibrium (stable) climate state, the amount of incoming solar energy is balanced by an equal amount of outgoing radiation at the top of Earth’s atmosphere (Hansen et al., 2011²⁹). At the Earth’s surface energy from the Sun is transformed into various forms (heat, potential, latent, kinetic, and chemical), that drive weather systems in the atmosphere and currents in the ocean, fuel photosynthesis on land and in the ocean, and fundamentally determine the climate (Trenberth et al., 2014³⁰). The ocean has a large capacity to store and release heat, and the Earth’s energy budget can be effectively monitored through the heat content of the ocean on time scales longer than one year (Palmer and McNeall, 2014³¹; von Schuckmann et al., 2016³²; Cheng et al., 2018³³). The large heat capacity of the ocean leads to different characteristics of the ocean response to external forcings compared with the atmosphere (Sections 1.3, 1.4). The reflective properties of snow and ice also play an important role in regulating climate via the albedo effect. Increased amounts of solar energy are absorbed when snow or ice are replaced by less reflective land or ocean surfaces, resulting in a climate change feedback responsible for amplified changes.

Water is exchanged between the ocean, atmosphere, land and cryosphere as part of the hydrological cycle driven by solar heating (Box 1.1, Figure 1; Trenberth et al., 2007³³; Lagerloef et al., 2010³⁴; Durack et al., 2016³⁵). Evaporation from the surface ocean is the main source of water in the atmosphere, which is moved back to the Earth’s surface as precipitation (Gimeno et al., 2012³⁶). The hydrological cycle is closed by the eventual return of water to the ocean by rivers, streams, and groundwater flow, and through ice discharge and melting of ice sheets and glaciers (Yu, 2018). Hydrological extremes related to the ocean include floods from extreme rainfall (including tropical cyclones) or ocean circulation-related droughts (Sections 6.3, 6.5), while cryosphere-related flooding can be caused by rapid snow melt and melt water discharge events (Sections 2.3, 3.4).

Ninety-two percent of the carbon on Earth that is not locked up in geological reservoirs (e.g., in sedimentary rocks or coal, oil and gas reservoirs) resides in the ocean (Sarmiento and Gruber, 2002³⁷). Most of this is in the form of dissolved inorganic carbon, some of which readily exchanges with CO2 in the overlying atmosphere. This represents a major control on atmospheric CO2 and makes the ocean and its carbon cycle one of the most important climate regulators in the Earth system, especially on time scales of a few hundred years and more (Sigman and Boyle, 2000³⁸; Berner and Kothavala, 2001³⁹). The ocean also contains as much organic carbon (mostly in the form of dissolved organic matter) as the total vegetation on land (Jiao et al., 2010⁴⁰; Hansell, 2013⁴¹). Primary production in the ocean, which is as large as that on land (Field et al., 1998⁴²), fuels complex food-webs that provide essential food for people. 

Ocean circulation and mixing redistribute heat and carbon over large distances and depths (Delworth et al., 2017⁴³). The ocean moves heat laterally from the tropics towards polar regions (Rhines et al., 2008⁴⁴). Vertical redistribution of heat and carbon occurs where warm, low-density surface ocean waters transform into cool high-density waters that sink to deeper layers of the ocean (Talley, 2013⁴⁵), taking high carbon concentrations with them (Gruber et al., 2019⁴⁶). Driven by winds, ocean circulation also brings cold water up from deep layers (upwelling) in some regions, allowing heat, oxygen and carbon exchange between the deep ocean and the atmosphere (Oschlies et al., 2018⁴⁷; Shi et al., 2018⁴⁸) and fuelling biological production (Sarmiento and Gruber, 2006⁴⁹).

The ocean and cryosphere are interconnected in a multitude of ways (Box 1.1, Figure 1). Evaporation from the ocean provides snowfall that builds and sustains the ice sheets and glaciers that store large amounts of frozen water on land (Section 4.2.1). The vast ice sheets in Antarctica and Greenland currently hold about 66 m of potential global sea level rise (Fretwell et al., 2013⁵⁰), although the loss of a large fraction of this potential would require millennia of ice sheet retreat. Ocean temperature and sea level affect ice sheet, glacier and ice shelf stability in places where the base of ice bodies are in direct contact with ocean water (Section 3.3.1). The nonlinear response of ice-melt to ocean temperature changes means that even slight increases in ocean temperature have the potential to rapidly melt and destabilise large sections of an ice sheet or ice shelf (Section 3.3.1.5).

The formation of sea ice leads to the production of dense ocean water that contributes to the deep ocean circulation (Section 3.3.3.2). Paleoclimate evidence and modelling indicates that releases of large amounts of glacier and ice sheet melt water into the surface ocean can disrupt deep overturning circulation of the ocean, causing global climate impacts (Knutti et al., 2004⁵¹; Golledge et al., 2019⁵²). Ice sheet melt water in the Antarctic may cause changes in surface ocean salinity, stratification and circulation, that feedback to generate further ocean-driven melting of marine-based ice sheets (Golledge et al., 2019⁵³) and promote sea ice formation (Purich et al., 2018⁵⁴). The cryosphere and ocean further link through the movement of biogeochemical nutrients. For example, iron accumulated in sea ice during winter is released to the ocean during the spring and summer melt, helping to fuel ocean productivity in the seasonal sea ice zone (Tagliabue et al., 2017⁵⁵). Nutrient rich sediments delivered by glaciers further connect cryosphere processes to ocean productivity (Arrigo et al., 2017⁵⁶). 

Increasing greenhouse gases in the atmosphere cause heat uptake in the Earth system (Section 1.2) and as reported since 1970, there is high confidence³ that the majority (more than 90%) of the extra thermal energy in the Earth’s system is stored in the global ocean (IPCC, 2013⁹⁶). Mean ocean surface temperature has increased since the 1970s at a rate of 0.11 (0.09–0.13)°C per decade (high confidence), and forms part of a long-term warming of the surface ocean since the mid-19th century. The upper ocean (0–700 m, virtually certain) and intermediate ocean (700 to 2,000 m, likely) have warmed since the 1970s. Ocean heat uptake has continued unabated since AR5 (Sections 3.2.1.2.1, 5.2), increasing the risk of marine heat waves and other extreme events (Section 6.4). During the 21st century, ocean warming is projected to continue even if anthropogenic greenhouse gas emissions cease (Sections 1.3, 5.2). The global water cycle has been altered, resulting in substantial regional changes in sea surface salinity (high confidence; Rhein et al., 2013⁹⁷), which is expected to continue in the future (Sections 5.2.2, 6.3, 6.5). 

The rate of sea level rise since the mid-19th century has been larger than the mean rate of the previous two millennia (high confidence). Over the period 1901 to 2010, global mean sea level rose by 0.19 (0.17–0.21) m (high confidence) (Church et al., 2013⁹⁸; Table SM1.1). Sea level rise continues due to freshwater added to the ocean by melting of glaciers and ice sheets, and as a result of ocean expansion due to continuous ocean warming, with a projected acceleration and century to millennial-scale commitments for ongoing rise (Section 4.2.3). In SROCC, recent developments of ice sheet modelling are assessed (Sections 1.8, 4.3, Cross-Chapter Box 8 in Chapter 3) and the projected sea level rise at the end of 21st century is higher than reported in AR5 but with a larger uncertainty range (Sections 4.2.3.2, 4.2.3.3).

By 2011, the ocean had taken up about 30 ± 7% of the anthropogenic CO2 that had been released to the atmosphere since the industrial revolution (Ciais et al., 2013⁹⁹; Section 5.2). In response, ocean pH decreased by 0.1 since the beginning of the industrial era (high confidence), corresponding to an increase in acidity of 26% (Table SM1.1) and leading to both positive and negative biological and ecological impacts (high confidence) (Gattuso et al., 2014¹⁰⁰). Evidence is increasing that the ocean’s oxygen content is declining (Oschlies et al., 2018¹⁰¹). AR5 did not come to a final conclusion with regard to potential long-term changes in ocean productivity due to short observational records and divergent scientific evidence (Boyd et al., 2014¹⁰²; Section 5.2.2). Ocean acidification and deoxygenation are projected to continue over the next century with high confidence (Sections 3.2.2.3, 5.2.2).

Changes in the cryosphere documented in AR5 included the widespread retreat of glaciers (high confidence), mass loss from the Greenland and Antarctic ice sheets (high confidence) and declining extents of Arctic sea ice (very high confidence) and Northern Hemisphere spring snow cover (very high confidence; IPCC, 2013¹⁰³; Vaughan et al., 2013¹⁰⁴).

A particularly rapid change in Earth’s cryosphere has been the decrease in Arctic sea ice extent in all seasons (Section 3.2.1.1). AR5 assessed that there was medium confidence that a nearly ice-free summer Arctic Ocean is likely to occur before mid-century under a high emissions future (IPCC, 2013¹⁰⁵), and SR15 assessed that ice-free summers are projected to occur at least once per century at 1.5oC of warming, and at least once per decade at 2oC of warming above pre-industrial levels (IPCC, 2018¹⁰⁶). Sea ice thickness is decreasing further in the Northern Hemisphere and older ice that has survived multiple summers is rapidly disappearing; most sea ice in the Arctic is now ‘first year’ ice that grows in the autumn and winter but melts during the spring and summer (AMAP, 2017¹⁰⁷).

AR5 assessed that the annual mean loss from the Greenland ice sheet very likely substantially increased from 34 (-6–74) Gt yr–1 (billion tonnes yr-1) over the period 1992–2001, to 215 (157–274) Gt yr–1 over the period 2002–2011 (IPCC, 2013). The average rate of ice loss from the Antarctic ice sheet also likely increased from 30 (-37–97) Gt yr–1 over the period 1992–2001, to 147 (72–221) Gt yr–1 over the period 2002–2011 (IPCC, 2013¹⁰⁸). The average rate of ice loss from glaciers around the world (excluding glaciers on the periphery of the ice sheets), was very likely 226 (91–361) Gt yr-1 over the period 1971–2009, and 275 (140–410) Gt yr-1 over the period 1993–2009 (IPCC, 2013). The Greenland and Antarctic ice sheets are continuing to lose mass at an accelerating rate (Section 3.3) and glaciers are continuing to lose mass worldwide (Section 2.2.3, Cross-Chapter Box 6 in Chapter 2). Confidence in the quantification of glacier and ice sheet mass loss has increased across successive IPCC reports (Table SM1.1) due to the development of remote sensing observational methods (Section 1.8.1). 

Changes in seasonal snow are best documented for the Northern Hemisphere. AR5 reported that the extent of snow cover has decreased since the mid-20th century (very high confidence). Negative trends in both snow depth and duration are also detected with station observations (medium confidence), although results depend on elevation and observational period (Section 2.2.2). AR5 assessed that permafrost temperatures have increased in most regions since the early 1980s (high confidence), and the rate of increase has varied regionally (IPCC, 2013¹⁰⁹). Methane and carbon dioxide release from soil organic carbon is projected to continue in high mountain and polar regions (Box 2.2), and SROCC has used multiple lines of evidence to reduce uncertainty in permafrost change assessments (Cross-Chapter Box 5 in Chapter 1, Section 3.4.3.1.1). 

Hazards faced by marine and coastal organisms, and the ecosystem services they provide are generally dependent on future greenhouse gas emission pathways, with moderate likelihood under a low-emission future, but high to very high likelihood under higher emission scenarios (very high confidence) (Mora et al., 2013²²¹; Gattuso et al., 2015²²²). Hazards to marine ecosystems assessed in AR5 (IPCC, 2014²²³) included degradation of coral reefs (high confidence), ocean deoxygenation (medium confidence) and ocean acidification (high confidence). Shifts in the ranges of plankton and fish were identified with high confidence regionally, but with uncertain trends globally. SROCC provides more evidence for global shifts in the distribution of marine organisms, and in how the phenology of animals is responding to ocean change (Sections 3.2.3, 5.2). The signature of climate change is now detected in almost all marine ecosystems. Similar trends of changing habitat due to climate change are reported for the cryosphere (Sections 2.2, 3.4.3.2). The risk of irreversible loss of many marine and coastal ecosystems increases with global warming, especially at 2°C or more (high confidence; IPCC, 2018²²⁴). Risk also increases for habitat displacements, both poleward (Section 3.2.4) and to greater ocean depths (Section 5.2.4), or habitat reductions, such as that caused by glacier retreat (Section 2.2.3). 

Changes in the ocean and cryosphere bring hazards that affect the health, wellbeing, safety and security of populations in coastal, mountain and polar environments (Section 2.3.5, 3.4.3, 4.3.2). Some impacts are direct, such as sea level rise or coastal erosion that can displace coastal residents (4.3.2.3, 4.4.2.6, Box 4.1). Other effects are indirect; for example, rising ocean temperatures have led to increases in maximum wind speed and rainfall rates in tropical cyclones (Section 6.3), creating hazards with severe consequences for natural and human systems (Sections 4.3, 6.2, 6.3, 6.8). The multiple category 4 and 5 Atlantic hurricanes in 2017 caused the loss of over 3300 lives and more than 350 billion USD in economic damages (Cross-Chapter Box 9; Andrade et al., 2018²²⁵; Murakami et al., 2018²²⁶; NOAA, 2018²²⁷). In mountain regions, glacial lake outburst floods have caused severe impacts on lives, livelihoods and infrastructure that often extend beyond the directly affected areas (Section 2.3.2 and 6.2.2). Some hazards related to ocean and cryosphere change involve abrupt and irreversible changes (Section 1.3), which generate sometimes unpredictable risks, and multiple hazards can coincide to greatly elevate the total risk (Section 6.8.2). For example, combinations of thawing permafrost, sea level rise, loss of sea ice, ocean surface waves and extreme weather events (Thomson and Rogers, 2014²²⁸; Ford et al., 2017²²⁹) have damaged Arctic infrastructure (e.g., buildings, roads) (AMAP, 2015²³⁰; AMAP, 2017²³¹), impacted reindeer husbandry livelihoods for Sami and other Arctic Indigenous peoples and impeded access to hunting grounds, other communities and travel routes fundamental to the livelihoods, food security and wellbeing of Inuit and other Northern cultures (Section 3.4.3). In some Arctic regions, tipping points may have already been reached such that adaptive practices can no longer work (Section 3.5).

Climate change impacts on the ocean and cryosphere can also present opportunities, in at least the near- and medium-term. For example, in Nepal warming of high mountain environments and accelerated melting of snow and ice have extended the growing season and crop yields in some regions (Section 2.3; Gaire et al., 2015²³²; Merrey et al., 2018²³³), while tourism and shipping has increased in the Arctic with loss of sea ice (Section 3.2.4). Moreover, rising ocean temperatures redistribute the global fish population, allowing new fishing opportunities while reducing some established fisheries (Bell et al., 2011²³⁴; Fenichel et al., 2016²³⁵; Section 5.4). To gain from new opportunities, while also avoiding or mitigating new or increasing hazards, it is necessary to be aware of trade-offs between risks and benefits to understand who is and is not benefiting. For example, opportunities can involve trade-offs with mitigation and/or SDGs (Section 3.5.2), and the balance of economic costs and benefits may differ substantially between the near-term and long-term future (Section 5.4.2.2). 

Exposure to hazards in cryosphere systems occur in the immediate vicinity of cryosphere components, and at regional to global scales where cryosphere changes link to other natural systems. For example, decreasing Arctic sea ice increases exposure for organisms that depend upon habitats provided by sea ice, but also has far-reaching impacts through the resulting direct albedo feedback and amplification of Arctic climate warming (e.g., Pistone et al., 2014) that then locally increases surface melting of the Greenland ice sheet (Liu et al., 2016²³⁶; Stroeve et al., 2017²³⁷). Additionally, ice loss from ice sheets contribute to the global-scale exposure of sea level rise, and more local-scale modifications and losses of coastal habitats and ecosystems (Sections 3.2.3 and 4.3.3.5). Interactions within and between natural systems also influence the spatial reach of risks associated with cryosphere change. Permafrost degradation, for example, interacts with ecosystems and climate on various spatial and temporal scales, and feedbacks from these interactions range from local impacts on topography, hydrology and biology, to global-scale impacts via biogeochemical cycling (e.g., methane release) on climate (Sections 2.2, 2.3, 3.4; Kokelj et al., 2015²³⁸; Grosse et al., 2016²³⁹).

Exposure to climate change risk exists for virtually all coastal organisms, habitats and ecosystems (Section 5.2), through processes such as inundation and salinisation (Section 4.3), ocean acidification and deoxygenation (Sections 3.2.3, 5.2.3), increasing marine heatwaves (Section 6.4.1.2), and increases in harmful algal blooms and invasive species (Glibert et al., 2014²⁴⁰; Gobler et al., 2017²⁴¹; Townhill et al., 2017²⁴²; Box 5.3). Aggregate impacts of multiple drivers are dramatically altering ecosystem structure and function in the coastal and open ocean (Boyd et al., 2015²⁴³; Deutsch et al., 2015²⁴⁴; Przeslawski et al., 2015²⁴⁵), such as coral reefs under increasing pressure from both rising ocean temperature and acidification (Section 5.3.4). Increasing exposure to climate change hazards in open ocean natural systems includes ocean acidification (O’Neill et al., 2017²⁴⁶; Section 5.2.3), changes in ocean ventilation, deoxygenation (Shepherd et al., 2017²⁴⁷; Breitburg et al., 2018²⁴⁸; Section 5.2.2.4), increased cyclone and flood risk (Section 6.3.3) and an increase in extreme El Niño and La Niña events (Section. 6.5.1). Heat content is rapidly increasing within the ocean (Section 5.2.2) and marine heat waves are becoming more frequent across the world ocean (Section 6.4.1).

People who live close to the ocean and/or cryosphere, or depend directly on their resources for livelihoods, are particularly exposed to climate change impacts and hazards (very high confidence) (Barange et al., 2014²⁴⁹; Romero-Lankao et al., 2014²⁵⁰; AMAP, 2015²⁵¹). These exposures can result in infrastructure damage and failure (Sections 2.3.1.3, 3.4.3, 3.5., 4.3.2), loss of habitability (Sections 2.3.7, 3.4.3, 3.5, 4.3.3), changes in air quality (Section 6.5.2), proliferation of disease vectors (Sections 3.4.3.2.2, 5.4.2.1.1), increased morbidity and mortality due to injury, infectious disease, heat stress, and mental health and wellness challenges (Section 3.4.3.3), compromised food and water security (Sections 2.3.1, 3.4.3.3, 4.3.3.6, 5.4.2.1, 6.8.4), degradation of ecosystem services (Sections 2.3.1.2, 2.3.3.4, 4.3.3, 5.4.1, 6.4.2.3), economic and non-economic impacts due to reduced production and social network system disruption (Section 2.3.7), conflict (Sections 2.3.1.14, 3.5) and widespread human migration (Sections 2.3.7, 4.4.3.5; Oppenheimer et al., 2014²⁵²; van Ruijven et al., 2014²⁵³; AMAP, 2015²⁵⁴; Cunsolo and Ellis, 2018²⁵⁵). 

This report documents how people residing in coastal and cryosphere regions are already exposed to climate change hazards, and which of these hazards are projected to increase in the future. For example, mountain communities have been exposed to increased rockfall, rock avalanches and landslides due to permafrost degradation and glacier shrinkage, and to changes in snow avalanche type and seasonal timing (Section 2.3.1). Cryosphere changes that can impact water availability in mountain regions and for downstream populations (Sections 2.3.1, 2.3.4, 2.3.5) have implications for drinking water, irrigation, livestock grazing, hydropower production and tourism (Section 2.3). Some declining mountain glaciers hold sacred and symbolic meanings for local communities who will experience spiritual losses (Section 2.3.4, 2.3.5, and 2.3.6). Exposures to extreme warming, and continued sea ice and permafrost loss in the Arctic, challenge Indigenous communities with close interdependent relationships of economy, lifestyles, cultural identity, self-sufficiency, Indigenous knowledge, health and wellbeing with the Arctic cryosphere (Section 3.4.3, 3.5). 

The population living in low elevation coastal zones (land less than 10 m above sea level) is projected to increase to more than one billion by 2050 (Section 4.3.2.2). These people and communities are particularly exposed to future sea level rise, rising ocean temperature (including marine heat waves; Section 6.4), enhanced coastal erosion, increasing wind, wave height, storm intensity and ocean acidification (Section 4.3.4). These exposures bring associated risks for livelihoods linked to fisheries, tourism and trade, as well as loss of life, damaged assets, and disruption of basic services including safe water supplies, sanitation, energy and transportation networks (Chapters 4, 5, and 6; Cross-Chapter Box 9). 

Direct and indirect risks to natural systems are influenced by vulnerability to climate change as well as deterioration of ecosystem services. For example, about half of species assessed on the northeast United States continental shelf exhibited high to very high climate vulnerability due to temperature preferences and changes in habitat space (Hare et al., 2016²⁵⁶), with corresponding northward range shifts for many species (Kleisner et al., 2017²⁵⁷) and increased vulnerability for organisms or ecosystems unable to migrate or evolve at the rate required to adapt to ocean and cryosphere changes (Miller et al., 2018²⁵⁸). Non-climatic pressures also magnify the vulnerability of ocean and cryosphere ecosystems to climate-related changes, such as overfishing, coastal development, and pollution, including plastic pollution (Halpern et al., 2008²⁵⁹; Halpern et al., 2015²⁶⁰; IPBES, 2018a²⁶¹; IPBES, 2018b²⁶²; IPBES, 2018c²⁶³; IPBES, 2018d²⁶⁴). Conventional (fossil fuel-based) plastics produced in 2015 accounted for 3.8% of global CO2 emissions and could reach up to 15% by 2050 (Zheng and Suh, 2019²⁶⁵). 

The vulnerability of mountain, Arctic and coastal communities is affected by social, political, historical, cultural, economic, institutional, environmental, geographical and/or demographic factors such as gender, age, race, class, caste, Indigeneity and disability (Thomas et al., 2019²⁶⁶; Sections 2.3.6 and 3.5; Cross-Chapter Box 9). Disparities and inequities in such factors may result in social exclusion, inequalities and non-climatic challenges to health and wellbeing, economic development and basic human rights (Adger et al., 2014²⁶⁷; Olsson et al., 2014²⁶⁸; Smith et al., 2014²⁶⁹). Those less advantaged often also have reduced access to and control over the social, financial, technological and environmental resources that are required for adaptation and transformation (Oppenheimer et al., 2014²⁷⁰; AMAP, 2015²⁷¹), thus limiting options for coping and adapting to change (Hijioka et al., 2014²⁷²). However, even populations with greater wealth and privilege can be vulnerable to some climate change risks (Cardona et al., 2012²⁷³; Smith et al., 2014²⁷⁴), especially if sources of wealth and wellbeing depend upon established infrastructure that is poorly suited to ocean or cryosphere change. 

Institutions and governance can shape vulnerability and adaptive capacity, and it can be challenging for weak governance structures to respond effectively to extreme or persistent climate change hazards (Sections 6.4 and 6.9; Cross-Chapter Box 3 in Chapter 1; Berrang-Ford et al., 2014²⁷⁵; Hijioka et al., 2014²⁷⁶). Furthermore, populations can be negatively impacted by inappropriate climate change mitigation and/or adaptation policies, particularly ones that further marginalise their knowledge, culture, values and livelihoods (Field et al., 2014²⁷⁷; Cross-Chapter Box 4 in Chapter 1). 

Vulnerability is not static in place and time, nor homogeneously experienced. The vulnerabilities of individuals, groups, and populations to climate change is dynamic and diverse, and reflects changing societal and environmental conditions (Thomas et al., 2019²⁷⁸). SROCC examines vulnerability following the conceptual definition presented in Cross-Chapter Box 2 in Chapter 1, and vulnerability in human systems is treated in relative rather than absolute terms.

Mitigation and adaptation pathways to avoid dangerous anthropogenic interference with the climate system (United Nations, 1992) are considered in SR15 (IPCC, 2018). SROCC assesses several ocean and cryosphere-specific measures for mitigation and adaptation including options for to address the causes of climate change, support biological and ecological adaptation, and enhance societal adaptation (Figure 1.2). Other measures have been proposed, including solar radiation management and several other forms of carbon dioxide removal, but these are not addressed in SROCC as they are covered in other products of the IPCC Sixth Assessment Cycle (SR15 and AR6 Working Group III) and are outside the scope of SROCC. SROCC does assess indirect mitigation measures that involve the ocean and the cryosphere (Figure 1.2) by supporting biological and ecological adaptation, such as through reducing nutrient and organic carbon pollution (which moderates ocean acidification in eutrophied areas) and conservation (which preserves biodiversity and habitats) in coastal regions (Billé et al., 2013).

A literature-based expert assessment shows that ocean-related mitigation measures have trade-offs, with the greatest benefits derived by combining global and local measures (high confidence; Gattuso et al., 2018). Local measures, such as pollution reduction and conservation, provide significant co-benefits and few adverse side effects (high confidence; Sections 5.5.1, 5.5.2). They can be relatively rapidly implemented, but are generally less effective in addressing the global problem (high confidence; Sections 5.5.1, 5.5.2). Likewise, local efforts to decrease air pollution near mountain glaciers and other cryosphere components, for example reducing black carbon emissions, can bring regional-scale benefits for health and in reducing snow and ice-melt (Shindell et al., 2012; Box 2.2).

Well-chosen human interventions can enhance the adaptive capacity of natural systems to climate change. Such interventions through manipulating an ecosystem’s structural or functional properties (e.g., restoration of mangroves) may minimise climate change pressures, enhance natural resilience and/or re-direct ecosystem responses to reduce cascading risks on societies. In human systems, adaptation can involve both infrastructure (e.g., enhanced sea defences) and community-based action (e.g., changes in policies and practices). Adaptation options to ongoing climate change are most effective when considered together with mitigation strategies because there are limits to effective adaptation, mitigation actions can make adaptation more difficult, and some adaptation measures may increase greenhouse gas emissions.

Adaptation and mitigation decisions are connected with economic concerns. In SROCC, two main economic approaches are used. The first comprises the Total Economic Value method and the valuation of ecosystem services. SROCC considers the paradigm of sustainable development, and the linkages between climate impacts on ecosystem services (Section 5.4.1) and the consequences on SDGs including food security or poverty eradication (Section 5.4.2). The second economic approach used are formal decision analysis methods, which help to identify options (also called alternatives) that perform best or well with regards to given objectives. These methods include cost-benefit analysis, multi-criteria analysis and robust decision-making and are specifically relevant for appraising long-term investment decisions in the context of coastal adaptation (Section 4.4.4.6).

In AR5, a range of changes in ocean and cryosphere natural systems were linked with medium to high confidence to pressures associated with climate change (Cramer et al., 2014²⁸⁷). Climate change impacts on natural ecosystems are variable in space and time. The multiplicity of pressures these natural systems experience impedes attribution of population or ecosystem responses to a specific ocean and/or cryosphere change. Moreover, the interconnectivity of populations within ecosystems means that a single ‘adaptive response’ of a population, or the aggregate response of an ecosystem (the adaptive responses of the interconnected populations), is influenced not just by direct pressures of climate change, but occurs in concert with the adaptive responses of other species in the ecosystem, further complicating efforts to disentangle specific patterns of adaptation.

Notwithstanding the network of pressures and adaptations, much effort has gone into resolving the mechanisms, interactions and feedbacks of natural systems associated with the ocean and cryosphere. Chapters 4, 5 and 6, as well as Cross-Chapter Box 9, assess new knowledge on the adaptive responses of wetlands, coral reefs, other coastal habitats, and the populations of marine organisms encountering ocean-based risks, including. Likewise, Chapters 2 and 3 describe emerging knowledge on how ecosystems in high-mountain and polar areas are adapting to cryosphere decline. 

AR5 and SR15 have highlighted the importance of evolutionary adaptation as a component of how populations adapt to climate change pressures (e.g., Pörtner et al., 2014; Hoegh-Guldberg et al., 2018²⁸⁸). Acclimatisation (variation in morphology, physiology or behaviour) can result from changes in gene expression but does not involve change in the underlying DNA sequence. Responses related to acclimatisation can occur both within single generations and over several generations. In contrast, evolution requires changes in the genetic composition of a population over multiple generations; for example, by differential survival or fecundity of different genotypes (Sunday et al., 2014²⁸⁹). Adaptive evolution is the subset of evolution attributable to natural selection, and natural selection may lead to populations becoming more fit (Sunday et al., 2014) or extend the range of environments where populations persist (van Oppen et al., 2015²⁹¹). The efficacy of natural selection is affected by population size (Charlesworth, 2009²⁹²), standing genetic variation, the ability of a population to generate novel genetic variation, migration rates and the frequency of genetic recombination (Rice, 2002²⁹³). Many studies have shown evolution of traits within and across life stages of populations (Pespeni et al., 2013²⁹⁴; Hinners et al., 2017²⁹⁵), but there are fewer studies on how evolutionary change can impact ecosystem or community function, and whether trait evolution is stable (Schaum and Collins, 2014²⁹⁶). Although acclimatisation and evolutionary adaptation are separate processes, they influence each other, and both adaptive and maladaptive variation of traits can facilitate evolution (Schaum and Collins, 2014²⁹⁷; Ghalambor et al., 2015²⁹⁸). Natural evolutionary adaptation may be challenged by the speed and magnitude of current ocean and cryosphere changes, but emerging studies investigate how human actions may assist evolutionary adaptation and thereby possibly enhance the resilience of natural systems to climate change pressures (e.g., Box 5.4 in Section 5.5.2). Through acclimatisation and evolutionary adaptation to the pressures from climate change (and all other persistent pressures), populations, species and ecosystems present a constantly changing context for the adaptation of human systems to climate change. 

There are several human adaptation options for climate change impacts on the ocean and cryosphere. Adaptive responses include nature- and ecosystem-based approaches (Renaud et al., 2016²⁹⁹; Serpetti et al., 2017³⁰⁰). Additionally, more social-based approaches for human adaptation range from community-based and infrastructure-based approaches to managed retreat, along with other forms of internal migration (Black et al., 2011³⁰¹; Hino et al., 2017³⁰²). Building on AR5 (Wong et al., 2014³⁰³), Chapter 4 describes four main modes of adaptation to mean and extreme sea level rise: protect, advance, accommodate, and retreat. This report demonstrates that all modes of adaptation include mixes of institutional, individual, socio-cultural, engineering, behavioural and/or ecosystem-based measures (e.g., Section 4.4.2). 

The effectiveness and performance of different adaptation options across spatial and social scales is influenced by their social acceptance, political feasibility, cost-efficiency, co-benefits and trade-offs (Jones et al., 2012³⁰⁴; Adger et al., 2013³⁰⁵; Eriksen et al., 2015³⁰⁶). Scientific evaluation of past successes and future options, including understanding barriers, limits, risks and opportunities, are complex and inadequately researched (Magnan and Ribera, 2016³⁰⁷). In the end, adaptation priorities will depend on multiple parameters including the extent and rate of climate change, the risk attitudes and social preferences of individuals and institutions (and the returns they may gain) (Adger et al., 2009³⁰⁸; Brügger et al., 2015³⁰⁹; Evans et al., 2016³¹⁰; Neef et al., 2018³¹¹) and access to finances, technology, capacity and other resources (Berrang-Ford et al., 2014³¹²; Eisenack et al., 2014³¹³).

Since AR5, transformational adaptation (i.e., the need for fundamental changes in private and public institutions and flexible decision-making processes to face climate change consequences) has been increasingly studied (Cross-Chapter Box 2 in Chapter 1). The recent literature documents how societies, institutions, and/or individuals increasingly assume a readiness to engage in transformative change, via their acceptance and promotion of fundamental alterations in natural or human systems (Klinsky et al., 2016³¹⁴). People living in and near coastal, mountain and polar environments often pioneer these types of transformations, since they are at the forefront of ocean and cryosphere change (e.g., Solecki et al., 2017). Community led and indigenous led adaptation research continues to burgeon (Ayers and Forsyth, 2009³¹⁵; David-Chavez and Gavin, 2018³¹⁶), especially in many mountain (Section 2.3.2.3), Arctic (Section 3.5), and coastal (Section 4.4.4.4, 4.4.5.4, Cross-Chapter Box 9) areas, and demonstrate potential for enabling transformational adaptation (Dodman and Mitlin, 2013³¹⁷; Chung Tiam Fook, 2017³¹⁸). Similarly, the concepts of scenario planning and ‘adaptation pathway’ design have expanded since AR5, especially in the context of development planning for coastal and delta regions (Section 4.4, Cross-Chapter Box 9; Wise et al., 2014³¹⁹; Maier et al., 2016³²⁰; Bloemen et al., 2018³²¹; Flynn et al., 2018³²²; Frame et al., 2018³²³; Lawrence et al., 2018³²⁴).

Humans create, use, and adapt knowledge systems to interact with their environment (Agrawal, 1995⁴³⁶; Escobar, 2001⁴³⁷; Sillitoe, 2007⁴³⁸), and to observe and respond to change (Huntington, 2000⁴³⁹; Gearheard et al., 2013⁴⁴⁰; Maldonado et al., 2016⁴⁴¹; Yeh, 2016⁴⁴²). Indigenous knowledge (IK) refers to the understandings, skills, and philosophies developed by societies with long histories of interaction with their natural surroundings. It is passed on from generation to generation, flexible, and adaptive in changing conditions, and increasingly challenged in the context of contemporary climate change. Local knowledge (LK) is what non-Indigenous communities, both rural and urban, use on a daily and lifelong basis. It is multi-generational, embedded in community practices and cultures and adaptive to changing conditions (FAO, 2018). Each chapter of SROCC cites examples of IK and LK related to ocean and cryosphere change.

IK and LK stand on their own, and also enrich and complement each other and scientific knowledge. For example, Australian Aboriginal groups’ Indigenous oral history provides empirical corroboration of the sea level rise 7,000 years ago (Nunn and Reid, 2016⁴⁴³), and their seasonal calendars direct hunting, fishing, planting, conservation and detection of unusual changes today (Green et al., 2010⁴⁴⁴). LK works in tandem with scientific knowledge, for example, as coastal Australian communities consider the impacts and trade-offs of sea level rise (O’Neill and Graham, 2016⁴⁴⁵).

Both IK and LK are increasingly used in climate change research and policy efforts to engage affected communities to facilitate site-specific understandings of, and responses to, the local effects of climate change (Hiwasaki et al., 2014⁴⁴⁶; Hou et al., 2017⁴⁴⁷; Mekonnen et al., 2017⁴⁴⁸). IK and LK enrich CRDPs particularly by engaging multiple stakeholders and the diversity of socioeconomic, cultural and linguistic contexts of populations affected by changes in the ocean and cryosphere (Cross-Chapter Box 4 in Chapter 1). 

Global environmental assessments increasingly recognise the importance of IK and LK (Thaman et al., 2013⁴⁴⁹; Beck et al., 2014⁴⁵⁰; Díaz et al., 2015⁴⁵¹). References to IK in IPCC assessment reports increased 60% from AR4 to AR5, and highlighted the exposures and vulnerabilities of Indigenous populations to climate change risks related to socioeconomic status, resource-based dependence and geographic location (Ford et al., 2016a⁴⁵²). All four IPBES assessments in 2018 (IPBES, 2018a⁴⁵³; IPBES, 2018b⁴⁵⁴; IPBES, 2018c⁴⁵⁵; IPBES, 2018d⁴⁵⁶) engaged IK and LK (Díaz et al., 2015⁴⁵⁷; Roué and Molnar, 2017⁴⁵⁸; Díaz et al., 2018⁴⁵⁹). Peer-reviewed research on IK and LK is burgeoning (Savo et al., 2016⁴⁶⁰), providing information that can guide responses and inform policy (Huntington, 2011⁴⁶¹; Nakashima et al., 2012⁴⁶²; Lavrillier and Gabyshev, 2018⁴⁶³). However, most global assessments still fail to incorporate ‘the plurality and heterogeneity of worldviews’ (Obermeister, 2017⁴⁶⁴), resulting ‘in a partial understanding of core issues that limits the potential for locally and culturally appropriate adaptation responses’ (Ford et al., 2016b⁴⁶⁵).

IK and LK provide case specific information that may not be easily extrapolated to the scales of disturbance that humans exert on natural systems (Wohling, 2009⁴⁶⁶). Some forms of IK and LK are also not amenable to being captured in peer-reviewed articles or published reports, and efforts to translate IK and LK into qualitative or quantitative data may mute the multidimensional, dynamic and nuanced features that give IK and LK meaning (DeWalt, 1994⁴⁶⁷; Roncoli et al., 2009⁴⁶⁸; Goldman and Lovell, 2017⁴⁶⁹). Nonetheless, efforts to collaborate with IK and LK knowledge holders (Baptiste et al., 2017⁴⁷⁰; Karki et al., 2017⁴⁷¹; Lavrillier and Gabyshev, 2017⁴⁷²; Roué et al., 2017⁴⁷³; David-Chavez and Gavin, 2018⁴⁷⁴) and to systematically assess published IK and LK literature in parallel with scientific knowledge result in increasingly effective usage of the multiple knowledge systems to better characterise and address ocean and cryosphere change (Huntington et al., 2017⁴⁷⁵; Nalau et al., 2018⁴⁷⁶; Ford et al., 2019⁴⁷⁷).

To hold global average temperature to well below 2°C above pre-industrial levels, substantial changes in the day-to-day activities of individuals, families, communities, the private sector, and governance bodies will be required (Ostrom, 2010⁴⁷⁸; Creutzig et al., 2018⁴⁷⁹). Enabling these changes at a meaningful societal scale requires sensitivity to communities and their use of multiple knowledge systems to best motivate effective responses to the risks and opportunities posed by climate change (medium confidence) (1.8.2, Cross-Chapter Box 4 in Chapter 1). Meaningful engagement of people and communities with climate change information depends on that information cohering with their perception of how the world works (Crate and Fedorov, 2013⁴⁸⁰). The values and identities people hold affect how acceptable they find the behavioural changes, technological solutions and governance that climate change action requires (Moser, 2016⁴⁸¹). 

Education and climate literacy contribute to climate change action and adaptation (high confidence). Although public understanding of humanity’s role in both causing and abating climate change has increased in the last decade (Milfont et al., 2017⁴⁸²), levels of climate concern vary greatly globally (Lee et al., 2015⁴⁸³). Educational attainment has the strongest effect on raising climate change awareness (Lee et al., 2015⁴⁸⁴), and research documents the value of evidence-based climate change education, particularly during formal schooling (Motta, 2018⁴⁸⁶). People further understand climate change as a serious threat when they experience it in their lives and have knowledge of its human causes (Lee et al., 2015⁴⁸⁷; Shi et al., 2016⁴⁸⁸). Education and tailored climate communication strategies that are respectful of people’s values and identity can aid acceptance and implementation of the local to global-scale approaches and policies required for effective climate change mitigation and adaptation (Shi et al., 2016; Anisimov and Orttung, 2018⁴⁸⁹; Sections 3.5.4, 4.4), while also supporting CRDPs (see also Cross-Chapter Box 2 in Chapter 1, and FAQ1.2). 

Human psychology complicates engagement with climate change, due to complex social factors, including values (Corner et al., 2014⁴⁹⁰), identity (Unsworth and Fielding, 2014⁴⁹¹), ideology (Smith and Mayer, 2019⁴⁹²) and the framing of climate messaging. Additionally, psychology effects adaptation actions, motivated by perceptions that others are already adapting, avoidance of an unpleasant state of mind, feelings of self-efficacy and belief in the efficacy of the adaptation action (van Valkengoed and Steg, 2019⁴⁹³). Better understandings of the psychological implications across diverse communities and social and political contexts will facilitate a just transition of both emissions reduction and adaptation (Schlosberg et al., 2017⁴⁹⁴). Impacts of climate change on natural and human environments (e.g., extreme weather) or human-caused modifications to the environment (e.g., adaptation) will raise further psychological challenges. This includes psychological impacts to the emotional wellbeing of people adversely affected by climate change (Ogunbode et al., 2018⁴⁹⁵), resulting in solastalgia (Albrecht et al., 2007⁴⁹⁶), a distress akin to homesickness while in their home environment (McNamara and Westoby, 2011⁴⁹⁷).

SROCC assesses literature on ocean and cryosphere change and associated impacts and responses, focusing on advances in knowledge since AR5. The literature used is primarily published, peer-reviewed scientific, social science and humanities research. In some cases, grey literature sources (for example, published reports from governments, industry, research institutes and non-government organisations) are used where there are important gaps in available peer-reviewed literature. It is recognised that published knowledge from many parts of the world most vulnerable to ocean and cryosphere change is still limited (Czerniewicz et al., 2017⁵³⁸). 

Where possible, SROCC draws upon established methodologies and/or frameworks. Cross-Chapter Boxes in Chapter 1 address methodologies used for projections of future change (Cross-Chapter Box 1 in Chapter 1), for assessing and reducing risk (Cross-Chapter Box 2 in Chapter 1), for governance options relevant to a problem or region (Cross-Chapter Box 3 in Chapter 1), and for using IK and LK (Cross-Chapter Box 4 in Chapter 1). It is recognised in the assessment process that multiple and non-static factors determine human vulnerabilities to climate change impacts, and that ecosystems provide essential services that have both commercial and non-commercial value (Section 1.5). Economic methods are also important in SROCC, for estimating the economic value of natural systems, and for aiding decision-making around mitigation and adaptation strategies (Section 1.6).

SROCC uses calibrated language for the communication of confidence in the assessment process (Mastrandrea et al., 2010⁵³⁹; Mach et al., 2017⁵⁴⁰). Calibrated language is designed to consistently evaluate and communicate uncertainties that arise from incomplete knowledge due to a lack of information, or from disagreement about what is known or even knowable. The IPCC calibrated language uses qualitative expressions of confidence based on the robustness of evidence for a finding, and (where possible) uses quantitative expressions to describe the likelihood of a finding (Figure 1.4).

Qualitative expressions (confidence scale) describe the validity of a finding based on the type, amount, quality and consistency of evidence, and the degree of agreement between different lines of evidence (Figure 1.4, step 2). Evidence includes all knowledge sources, including IK and LK where available. Very high and high confidence findings are those that are supported by multiple lines of robust evidence with high agreement. Low or very low confidence describe findings for which there is limited evidence and/or low agreement among different lines of evidence, and are only presented in SROCC if they address a major topic of concern. 

Quantitative expressions (likelihood scale) are used when sufficient data and confidence exists for findings to be assigned a quantitative or probabilistic estimate (Figure 1.4, step 3). In the scientific literature, a finding is often said to be significant if it has a likelihood exceeding 95% confidence. Using calibrated IPCC language, this level of statistical confidence would be termed extremely likely. Lower levels of likelihood than those derived numerically can be assigned by expert judgement to take into account structural or measurement uncertainties within the products or data used to determine the probabilistic estimates (e.g., Table CB1.1). Likelihood statements may be used to describe how climate changes relate to the ends of distribution functions, such as in detection and attribution studies that assess the likelihood that an observed climate change or event is different to a reference climate state (Section 1.3). In other situations, likelihood statements refer to the central region across a distribution of possibilities. Examples are the estimates of future changes based on large ensembles of climate model simulations, where the central 66% of estimates across the ensemble (i.e., the 17–83% range) would be termed a likely range (Figure 1.4, step 3). 

It is increasingly recognised that effective risk management requires assessments not just of ‘what is most likely’ but also of ‘how bad things could get’ (Mach et al., 2017⁵⁴¹; Weaver et al., 2017⁵⁴⁴; Xu and Ramanathan, 2017⁵⁴⁵; Spratt and Dunlop, 2018⁵⁴⁶; Sutton, 2018⁵⁴⁷). In response to the need to reframe policy relevant assessments according to risk (Section 1.5; Mach et al., 2016⁵⁴⁸; Weaver et al., 2017⁵⁴⁹; Sutton, 2018⁵⁵⁰), an effort is made in SROCC to report on potential changes for which there is low scientific confidence or a low likelihood of occurrence, but that would have large impacts if realised (Mach et al., 2017⁵⁵¹). In some cases where evidence is limited or emerging, phenomena may instead be discussed according to physically plausible scenarios of impact (e.g., Table 6.1). 

In some cases, deep uncertainty (Cross-Chapter Box 5 in Chapter 1) may exist in current scientific assessments of the processes, rate, timing, magnitude, and consequences of future ocean and cryosphere changes. This includes physically plausible high-impact changes, such as high-end sea level rise scenarios that would be costly if realised without effective adaptation planning and even then may exceed limits to adaptation. Means such as expert judgement, scenario building, and invoking multiple lines of evidence enable comprehensive risk assessments even in cases of uncertain future ocean and cryosphere changes.