IPCC special report on the ocean and cryosphere in a changing climate (SROCC) was formally approved at the second joint session of working groups I and II of the IPCC and accepted by the 51st session of the IPCC on 24 September 2019. Assessment content related to extreme event, abrupt change and its impact and risk of ocean and cryosphere in SROCC is synthesized in this paper. Based on new assessment results in SROCC, it is indicated that extreme events of ocean and cryosphere have become more frequently, and their intensity has increased. The frequency of landslides, snow avalanches and floods events caused by changes in cryosphere has increased. Marine heat wave happens more frequently. Extreme El Niño events become more intense. Atlantic Meridional Overturning Circulation (AMOC) has weakened. In coastal areas, extreme sea level and extreme wave height have increased, and the impact of extreme tropical cyclones has amplified. Those changes, such as marine heat wave, can be attributed to anthropogenic temperature increase. Furthermore, changes in ocean and cryosphere related extreme events are projected to be exacerbated. Those changes can have severe impacts on lives and livelihoods in high mountain, polar and coastal community, and on ocean and cryosphere ecosystem service function. Better prediction and early warning systems, including seasonal prediction system, annual to decadal prediction system of extreme events and abrupt change, are needed to be well-prepared to reduce extreme events risk. What’s more, improved scientific understanding and technical capacities of extreme events and suitable post-recovery and reconstruction are also key points in risk management.

The IPCC special report on the ocean and cryosphere in a changing climate (SROCC) presents an assessment of the changes in the ocean climate, their impacts on ecosystems and human society, related risks, and responses. The results show that the physical and chemical properties of the ocean, such as warming, acidification, deoxygenation, and reduction of nutrients referred to ocean climate hazards, have changed significantly over the past decades (high confidence). This is affecting the marine ecosystems from the surface to sea floor and having negative consequences on human society and sustainable development (high confidence), e.g., the changes in biogeography of organisms ranging from phytoplankton to benthonic organism, consequently reduction in primary production and maximum catch potential of fish stocks as well as food provision etc. It is obvious that, globally, almost all marine ecosystems are expected to be at high or very high risk by the end of the 21st century under compound impacts of climate change and non-climatic anthropogenic factors, with increasing greenhouse gas emission (from RCP2.6 to RCP8.5 scenario) (high confidence). In particular, the warm water coral reefs are projected to disappear by 70%-90% and larger than 99% at global warming of 1.5℃ and 2℃, respectively (very high confidence). However, the effectiveness of many ocean-based climate mitigation is smaller or have higher uncertainties, and the capacity of many adaptation approaches for risk reduction, especially under the RCP8.5 scenario, are also limited. It is noted that RCP2.6 has a lower level of risk for all types of marine ecosystems than RCP8.5 (very high confidence). These highlight the critical importance of mitigation and the increasing effectiveness of the different type portfolios of mitigation and adaptation to climate change.

The greenhouse gases emitted by human activities into the atmosphere produce an Earth’s energy imbalance that leads to global heating. IPCC AR5 indicated that the ocean has taken up more than 90% of the excess heat in the climate system. It is virtually certain that the global ocean has warmed unabated since 1970, which is attributed to anthropogenic forcing. Since 1993, the rate of ocean warming has likely more than doubled. The deep ocean below 2000 m has likely warmed since 1992, especially in the Southern Ocean. The Southern Ocean accounted for 35%-43% of the total heat gain in the upper 2000 m global ocean between 1970 and 2017 (high confidence). Its share increased to 45%-62% between 2005 and 2017. It is virtually certain that the ocean will continue warming throughout the 21st century. By 2100, the top 2000 m of the ocean is projected to take up 5-7 times more heat under RCP8.5 (or 2-4 times more heat under RCP2.6) than the observed accumulated ocean heat uptake within 1970-2017. The thermal expansion due to ocean warming contributes to ~43% of the global sea level rise since 1993.

The IPCC special report on the ocean and cryosphere in a changing climate (SROCC) presents an assessment of past and future contributions of climate change to global, regional and extreme sea level changes, the associated risk to low-lying islands, and response options and pathways to resilience as reported in the sea level chapter. The special report covers the field from observational changes, improved physical insights and projections to impacts and risk, and response options. The results indicate that, in the context of global warming, the global mean sea level (GMSL) is rising (virtually certain) and accelerating (high confidence), and the height of extreme sea level (ESL) is increasing significantly, attributed to the contribution of land glaciers and ice sheets that has exceeded the effect of the ocean thermal expansion since 2006. Meanwhile, the rate of ocean warming has more than doubled since 1993, strong tropical cyclones and storm surges have increased and the return period of ESL has greatly decreased. By 2100, GMSL will rise by about 0.43 m (low greenhouse gas emission scenario, RCP2.6) and 0.84 m (high emission scenario, RCP8.5) (medium confidence), and the currently rare ESL, e.g., today’s hundred-year event at many coastal locations, will become annually or more frequently, which will even happen by the mid-century for many low-lying coastal areas. The results also indicate that, rising GMSL, frequent ESL and anthropogenic drivers such as reclamation and subsidence, increase the exposure and vulnerability of coastal social-ecological systems. The sea level related hazards (such as submergence of land, enhanced coastal erosion, more frequent or intense flooding, salinization of soils, groundwater and surface waters, loss of and change of coastal ecosystems) will increase (high confidence). At the century scale and without adaptation, the vast majority of low-lying islands, coasts and communities, e.g., resources-rich coastal cities, urban atoll islands, tropical agriculture deltas, and Arctic communities, face substantial risk from these coastal hazards (high confidence).

The Arctic and part of Antarctic are experiencing significant changes characterized by warming and cryosphere’s shrinkage, which not only profoundly affect the local eco-environment and socio-economy, but also expand to the hemisphere and global scale. The special report on the ocean and cryosphere in a changing climate (SROCC) was issued on 25 September 2019, which systematically presented basic scientific understanding of changes in ocean, cryosphere, ecosystem and their interactions, analyzed the pathway to reduce vulnerability and risk, enhance adaptability and resilience. This article is a brief interpretation of the third chapter of the SROCC, including: (1) the changes in the polar system (such as polar oceans, sea ice, snow cover, freshwater ice, permafrost, polar ice sheets and glaciers) and their impacts on socio-ecological systems, and the links between polar systems and mid-latitude weather; (2) human response to polar changes, such as consequence of climate change, documented responses, key assets and strategies of adaptive and transformative capacity, anticipated future conditions and uncertainty, etc.; (3) assessment of practices and strategies for resilience building.

The cryosphere is an integral element of high mountain regions, which are home to roughly 10% of the global population. In recent decades, widespread cryosphere changes affect physical and human systems in the mountains and surrounding lowlands, with impacts evident even in the ocean. The IPCC special report on the ocean and cryosphere in a changing climate (SROCC) was launched on 25 September 2019. The SROCC assesses new evidence on observed recent and projected changes in the mountain cryosphere as well as associated impacts and adaption measures related to natural and human systems. The SROCC reports a dramatic air temperature increase in the high mountains during the last decades, which has led to a significant shrinkage of the mountain cryosphere. Observations show snow cover duration, snow depth and extent have declined, especially in the low elevations. Glacier mass balances are likely most negative in most mountain areas. Regionally averaged mass budgets are likely most negative in the southern Andes, Caucasus and central Europe, and least negative in High Mountain Asia. Observation suggests an increase in permafrost temperature and a decrease in permafrost thickness as well as loss of ice in the ground. The trends in durations of lake and river ice also shorten. Snow cover, glaciers and permafrost are projected to continue to decline in almost all regions throughout the 21st century due to persistent warming. Cryospheric changes have or will alter the frequency, magnitude and location of most related natural hazards, and have important impacts on hydrology, ecosystems and socio-economic systems in high mountain areas. The SROCC also points out that the integrated management approaches, effective governance, international cooperation and treaties can be effective at mitigating impacts from changes in the cryosphere and reducing disaster risk, then to promote adaption and sustainable development in high mountain areas.

In September 2019, IPCC released the special report on the ocean and cryosphere in a changing climate (SROCC), as the third special report of the IPCC’s sixth assessment cycle, and the IPCC’s first assessment report on the theme of ocean and cryosphere in high mountain areas and polar regions. SROCC provides comprehensive assessments on the physical changes of the ocean and cryosphere in the context of climate change and worldwide impacts and risks, the core findings are as follows: global warming has led to widespread shrinking of the cryosphere, which projected to continue, and high mountain areas and polar regions will face increased disaster risks. Global ocean has warmed unabated since 1970, and the ocean is projected to unprecedented conditions with increased temperature and further acidification, which has been impacting marine biodiversity and threatening marine ecosystem services and human systems. Global mean sea level is rising, with acceleration in recent decades, and projected to continue to rise in the coming hundreds of years, which will worsen the risk of coastal ecosystems, coasts and communities together with frequent occurrence of extreme sea level events. SROCC calls for urgent, active, coordinated and enduring adaptation and mitigation actions that are keys to effectively addressing ocean and cryospheric changes, achieving climate resilient development pathways and sustainable development goals. It is recommended to attach great importance to long-term and irreversible impacts of ocean and cryosphere in climate system change, strengthen the urgency of tackling climate change, promote adaptive capabilities in the terrestrial cryosphere and coastal areas over China, and strengthen interdisciplinary collaborative innovation to continuously enhance China’s international influences and sci-tech support in related fields, achieving the international “big science” plan led by China.