Explainer: How sea level rise poses an ‘existential threat’ to humans, heritage and nature

For millions of people around the world, rising sea levels are already reshaping economies, livelihoods and cultures. The world’s oceans are currently rising at a faster rate than at any time in at least the past three millennia, with human influence the “dominant cause” of sea level rise since at least 1970. At the UN general assembly in New York this week, world leaders are set to adopt a high-level declaration on the “existential threats” posed .  The declaration notes: “Sea level rise is not a distant scenario, but a real and lived experience for many.” On average, global sea levels rose by 20 centimetres (cm) between 1901 and 2018.  This is due to both the melting of glaciers and ice sheets and the expansion of seawater as it warms, as well as changes in land-water storage. It also encompasses the entire populations of many island nations. (See: Small island developing states.) There are two ways to consider sea level rise.  Global-average sea level rise is the amount the ocean surface has moved upwards, on average, relative to a baseline.  Relative, or local, sea level rise, is how much the ocean has risen in a given place. This can vary from the global average due to a number of factors, including land motion and ocean circulation, as well as changes to the Earth’s surface, rotation and gravitational pull due to the melting of the ice sheets. Higher sea levels bring with them myriad dangers for coastal communities: they can increase persistent flooding and inundation, strengthen dangerous storm surges and erode beaches and cliffs. Sea level rise also causes the water table of coastal land to rise, which exacerbates flood risk. The frequency of 100-year “extreme sea level events” has already increased 12-fold since 1900. These are events where high tides, storm surges and relative sea level rise combine to produce exceptionally high sea levels. Under a moderate-emissions scenario, these events are likely to occur at least annually – and, potentially, even more frequently – in many places . In addition to damage to homes and other buildings, critical infrastructure – such as water systems and wastewater management projects – is increasingly vulnerable to flooding as a result of sea level rise. Flooding can cut communities off from essential services, such as hospitals and markets, with low-income and other marginalised groups disproportionately affected. Inland encroachment of seawater leads to the saltwater intrusion and threatens agriculture in low-lying coastal areas. Vertical land motion can also amplify the risk of rising seas.