Sea level rise

Between 1901 and 2018, average global sea level rose by 15–25 cm (6–10 in), an average of 1–2 mm per year. This rate accelerated to 4.62 mm/yr for the decade 2013–2022. Climate change due to human activities is the main cause.^: 5, 8 Between 1993 and 2018, thermal expansion of water accounted for 42% of sea level rise. Melting temperate glaciers accounted for 21%. Within this, Greenland accounted for 15% and Antarctica 8%.^: 1576 Sea level rise lags changes in the Earth's temperature. So sea level rise will continue to accelerate between now and 2050 in response to warming that is already happening. What happens after that depends on human greenhouse gas emissions. Sea level rise may slow down between 2050 and 2100 if there are deep cuts in emissions. It could then reach a little over 30 cm (1 ft) from now by 2100. With high emissions it may accelerate. It could rise by 1 m (3+1⁄2 ft) or even 2 m (6+1⁄2 ft) by then. In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming amounts to 1.5 °C (2.7 °F). It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F).^: 21

Rising seas affect every coastal and island population on Earth. This can be through flooding, higher storm surges, king tides, and tsunamis. There are many knock-on effects. They lead to loss of coastal ecosystems like mangroves. Crop production falls because of salinization of irrigation water. Damage to ports disrupts sea trade. The sea level rise projected by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. Without a sharp reduction in greenhouse gas emissions, this may increase to hundreds of millions in the latter decades of the century. Areas not directly exposed to rising sea levels could be vulnerable to large-scale migration and economic disruption.

Local factors like tidal range or land subsidence will greatly affect the severity of impacts. The varying resilience and adaptive capacity of individual ecosystems, sectors, and countries are also factors. For instance, sea level rise in the United States (particularly along the US East Coast) is already higher than the global average. It is likely to be 2 to 3 times greater than the global average by the end of the century. Yet, of the 20 countries with the greatest exposure to sea level rise, 12 are in Asia. Eight of them collectively account for 70% of the global population exposed to sea level rise and land subsidence. These are Bangladesh, China, India, Indonesia, Japan, the Philippines, Thailand and Vietnam. The greatest impact on human populations in the near term will occur in the low-lying Caribbean and Pacific islands. Sea level rise will make many of them uninhabitable later this century.

Societies can adapt to sea level rise in three ways. Managed retreat, accommodating coastal change, or protecting against sea level rise through hard-construction practices like seawalls are hard apporaches. There are also soft approaches such as dune rehabilitation and beach nourishment. Sometimes these adaptation strategies go hand in hand. At other times choices must be made among different strategies. A managed retreat strategy is difficult if an area's population is increasing rapidly. This is a particularly acute problem for Africa. There the population of low-lying coastal areas is likely to increase by around 100 million people within the next 40 years. Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states. Sea level rise at some locations may be compounded by other environmental issues. One example is subsidence in sinking cities. Coastal ecosystems typically adapt to rising sea levels by moving inland. Natural or artificial barriers may make that impossible.