Special Report on the Ocean and Cryosphere in a Changing Climate

The report says that changes in the oceans and the cryosphere affect our ability to achieve the United Nations' Sustainable Development Goals (SDGs). Progress on the SDGs can support climate action and help reduce the exposure or vulnerability of communities to ocean and cryosphere changes.

The extent of the snow cover on land in the Arctic in June decreased by 13.4 ± 5.4 per cent per decade from 1967 to 2018 – a total loss of approximately 2.5 million km2 of snow cover – due to rise in the surface air temperature. In mountain areas, the depth, mass and duration of snow cover at lower elevations have declined since the middle of the 20th century.

Globally, marine heat-related events have increased. Between 1982 and 2016, marine heatwaves (defined by the report as periods of extremely warm near-sea surface temperature that persists for days or months and can extend up to thousands of kilometres) very likely doubled, and projections say they will increase in frequency, duration, intensity and spread, even under lower levels of global warming. It is very likely that between 84 and 90 per cent of the marine heat waves that occurred between 2006 and 2015 can be attributed to anthropogenic temperature rise.

The report says that there is evidence for several regional changes in tropical cyclone behaviour. These include an increase in the annual global proportion of Category 4 or 5 tropical cyclones in recent decades, severe tropical cyclones occurring in the Arabian Sea and making landfall in East and Southeast Asia, the increasing frequency of moderately large U.S. storm surge events since 1923, and the decreasing frequency of severe tropical cyclones making landfall in eastern Australia since the late 1800s. However, the report concludes that these are not necessarily signals of anthropogenic activity.

Carbon emissions from human activities are causing ocean warming, acidification and oxygen loss, and there is some evidence of changes in the ocean’s nutrient cycles and its primary producers (microorganisms known as phytoplankton). The warming ocean is affecting marine organisms at multiple nutritional levels and impacting fisheries, which has implications for food production and human communities.

Coastal ecosystems are under stress from ocean warming and sea level rise, which are exacerbated by human activity in oceans and on land. The global wetland area has declined by nearly 50 per cent from pre-industrial levels (1850–1900) as a result of warming, sea level rise, extreme climate events and other human activities.

Ocean warming, acidification and deoxygenation, ice sheet and glacier mass loss, and permafrost degradation are expected to be irreversible. The oceans and the cryosphere will undergo long-term change (lasting from decades to millennia), even after atmospheric greenhouse gas concentrations and 'radiative forcing' stabilise.

The Arctic and the Antarctic oceans have continued to become warmer in recent years. The Antarctic ocean (also known as the Southern Ocean) has been disproportionately and increasingly important in the global ocean heat increase, which can be attributed to the transfer of heat from the atmosphere (including heat from anthropogenic warming) to the world’s oceans. In recent years (2005-2017), the Southern Ocean was responsible for 45-62 per cent of the global ocean heat increase.

Permafrost temperatures (or the temperatures of thick, subsurface layers of soil mostly in polar regions that remain frozen at 0°C or less for at least two years) have increased to record high levels. However, there is limited evidence for and not much agreement on the inference that this warming is causing northern permafrost regions to release additional methane and carbon dioxide.

Snow and ice-covered peaks and high mountain tourism are likely to be negatively affected by future changes in the cryosphere. Current adaptation strategies, like snowmaking to support ski tourism, will be less effective in most parts of Europe, North America and Japan at a global warming of 1.5°C. They will be even less effective at a global warming of 2°C.

Focus and Factoids by Vaishnavi Iyer.

PARI Library's health archive project is part of an initiative supported by the Azim Premji University to develop a free-access repository of health-related reports relevant to rural India.