Sea level rise

Human-caused climate change is predominantly the cause, as it constantly heats (and thus expands) the ocean and melts land-based ice sheets and glaciers. Between 1993 and 2018, thermal expansion of water contributed 42% to sea level rise (SLR); melting of temperate glaciers contributed 21%; Greenland contributed 15%; and Antarctica contributed 8%. Because sea level rise lags changes in Earth temperature, it will continue to accelerate between now and 2050 purely in response to already-occurring warming; whether it continues to accelerate after that depends on human greenhouse gas emissions. If global warming is limited to 1.5 °C (2.7 °F), then sea level rise does not accelerate, but it would still amount to 2–3 m (7–10 ft) over the next 2000 years, while 19–22 metres (62–72 ft) would occur if the warming peaks at 5 °C (9.0 °F).

While the rise in sea levels ultimately impacts every coastal and island population on Earth, it does not occur uniformly due to local factors like tides, currents, storms, tectonic effects and land subsidence. Moreover, the differences in resilience and adaptive capacity of ecosystems, sectors, and countries again mean that the impacts will be highly variable. For instance, sea level rise along US coasts (and along the US East Coast in particular) 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 Asia will be the region where sea level rise would impact the most people. Eight Asian countries—Bangladesh, China, India, Indonesia, Japan, the Philippines, Thailand and Vietnam—account for 70% of the global population exposed to sea level rise and land subsidence. Out of the 20 countries with the greatest exposure to sea level rise, 12 are in Asia. Areas not directly exposed to rising sea levels could still be affected by issues such as large scale migrations and economic disruption. 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.