How high will the sea rise?

Tide gauges and, since the early 1990s, satellite altimetry record a rise that has accelerated over the period of measurement. The two methods agree where they overlap, which matters because they measure from different places: a tide gauge reads the sea against the land it stands on, a satellite reads it from space.

The contributions can be added up independently: the ocean expands as it warms, glaciers lose mass, the Greenland and Antarctic ice sheets lose mass, and the water stored on land changes. The sum matches the measured total within its uncertainties, which is a strong check on all of them.

The deep ocean takes centuries to warm through, and ice sheets respond to a change in climate over centuries to millennia. Sea level in 2300 depends on what happens this century, and continues rising afterwards under every scenario.

This is a place where the usual framing of a target year is misleading. The commitment is long even where the rate this century is moderate.

Much of the West Antarctic ice sheet rests on bedrock below sea level, sloping downward inland. Warm water reaching the grounding line thins the ice, which lets the grounding line retreat into deeper water, which thins it further.

Whether that runs away, and how fast, is not settled. A 2016 model that added the collapse of tall ice cliffs and the fracturing of ice shelves by meltwater produced far larger contributions than earlier work; later studies have questioned whether the cliff mechanism operates as modelled.

The assessed likely range for 2100 is 0.28 to 0.55 metres under the lowest emissions scenario and 0.63 to 1.01 metres under the highest, measured from the 1995 to 2014 average. Those ranges leave out the ice-sheet processes that cannot be projected with confidence. Include them and the 95th percentile under the highest scenario is 2.3 metres by 2100. That figure carries low confidence, and a rise approaching two metres cannot be ruled out.

Land rises and sinks, ocean currents pile water up in some places, and the gravitational pull of an ice sheet holds water near it, so losing ice in Greenland lowers the sea locally and raises it more than average far away.

The consequence is that some coasts will see substantially more than the global mean and some less, and planning from the global number is an error in both directions.

Parts of West Antarctica may already be committed to eventual collapse under warming that has already happened. The observational record is too short and the process too slow to tell from measurements.

A commitment that takes centuries to express itself is invisible to a satellite record that began in 1992, which is a limit on the question and not a reassurance.