Sea level rise

Sea level rise refers to the increase in the average height of the world's oceans. This phenomenon is a critical indicator of global environmental change. It affects every coastal population on Earth through various mechanisms. These include increased flooding, higher storm surges, and more frequent king tides. Rising seas can also make tsunamis more dangerous for coastal residents.

The process of rising sea levels is driven by two primary physical mechanisms. The first is the melting of land-based ice, such as glaciers and ice sheets. Between 1993 and 2018, this melting accounted for 44% of the observed rise. The second mechanism is thermal expansion. This occurs because water expands as it absorbs heat. Thermal expansion was responsible for 42% of the rise during that same period.

Scientists use several different approaches to project future sea level changes. Process-based modeling uses ice-sheet models to compute melting. It also uses general circulation models to track rising sea temperatures and expansion. Another method is the semi-empirical approach. This uses statistical techniques and historical paleoclimate data to predict trends. Some researchers also use structured expert judgement (SEJ). This method combines the informed opinions of many different scientists to reach a consensus.

Historical data shows that sea level rise is accelerating. Between 1901 and 2018, the average sea level rose by approximately 20 centimeters. This rate has increased significantly in recent decades. For the decade between 2013 and 2022, the rate reached 4.5 millimeters per year. This is the fastest rate of rise in at least the last 3,000 years. Some recent studies suggest it is the fastest in at least 4,000 years.

Future projections depend heavily on human greenhouse gas emissions. The Intergovernmental Panel on Climate Change (IPCC) provides various scenarios for the year 2100. In a low-emission scenario, like SSP1-1.9, sea level rise might be limited to a range of 0.28 to 0.55 meters. However, high-emission scenarios could lead to much higher levels. Under these conditions, the sea could rise by 0.5 meters or even 1.9 meters by 2100. In the very long run, warming could cause a rise of over 20 meters in the next 2,000 years.

Rising seas create significant challenges for ecosystems and human infrastructure. Coastal ecosystems, such as mangrove swamps, may be lost to the water. In agriculture, rising salt levels in irrigation water can reduce crop yields. Damage to ports can also disrupt global sea trade. By 2050, tens of millions of people may face annual flooding. This number could grow to hundreds of millions later in the century.

Different regions experience sea level rise in different ways. Local factors like land subsidence, or sinking land, can make the rise feel much worse. For example, sea level rise in the United States may be two to three times the global average. Of the twenty countries most exposed to these changes, twelve are in Asia. These include Indonesia, Bangladesh, and the Philippines. Low-lying Pacific and Caribbean islands are also at risk of becoming uninhabitable.

Societies use various strategies to adapt to these changing conditions. Hard approaches include building seawalls to block the water. Soft approaches include beach nourishment, which adds sand to shores, or rehabilitating dunes. Some communities may practice managed retreat, which involves moving away from the coast.

These choices often depend on a nation's wealth and resources. While richer states may build large defenses, poorer nations may face greater difficulties in adapting.