Between 1901 and 2018, the average global sea level rose by , or 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. Between 1993 and 2018, thermal expansion of water accounted for 42% of sea level rise. Melting temperate glaciers accounted for 21%, with Greenland accounting for 15% and Antarctica 8%. 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 will depend on what happens with 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 from now by 2100. With high emissions it may accelerate. It could rise by or even by then. In the long run, sea level rise would amount to over the next 2000 years if warming amounts to . It would be if warming peaks at .
Rising seas ultimately impact every coastal and island population on Earth. This can be through flooding, higher storm surges, king tides, and tsunamis. These have many knock-on effects. They lead to loss of coastal ecosystems like mangroves. Crop production falls because of salinization of irrigation water and 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 affected by large scale migrations and economic disruption.
At the same time, local factors like tidal range or land subsidence, as well as the varying resilience and adaptive capacity of individual ecosystems, sectors, and countries will greatly affect the severity of impacts. For instance, sea level rise along the United States (particularly along the US East Coast) is already higher than the global average, and it is expected to be 2 to 3 times greater than the global average by the end of the century. Yet, out of the 20 countries with the greatest exposure to sea level rise, 12 are in Asia. Bangladesh, China, India, Indonesia, Japan, the Philippines, Thailand and Vietnam collectively account for 70% of the global population exposed to sea level rise and land subsidence. Finally, the greatest near-term impact on human populations will occur in the low-lying Caribbean and Pacific islands—many of those would be rendered uninhabitable by sea level rise later this century.
Societies can adapt to sea level rise in three ways: by managed retreat, by accommodating coastal change, or by protecting against sea level rise through hard-construction practices like seawalls or 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 quickly increasing: this is a particularly acute problem for Africa, where the population of low-lying coastal areas is projected 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, and sea level rise at some locations may be compounded by other environmental issues, such as subsidence in so-called sinking cities. Coastal ecosystems typically adapt to rising sea levels by moving inland; but may not always be able to do so, due to natural or artificial barriers.
Observations
Between 1901 and 2018, the global mean sea level rose by about 20 cm (or 8 inches). More precise data gathered from satellite radar measurements found a rise of from 1993 to 2017 (average of 2.9 mm/yr), accelerating to 4.62 mm/yr for the decade 2013–2022.
Regional variations
Sea level rise is not uniform around the globe. Some land masses are moving up or down as a consequence of subsidence (land sinking or settling) or post-glacial rebound (land rising due to the loss of weight from ice melt). Therefore, local relative sea level rise may be higher or lower than the global average. Gravitational effects of changing ice masses also add to differences in the distribution of sea water around the globe.