A new assessment from NASA’s Sea Level Change Team and the United Nations Environment Programme examines what would happen if global temperatures temporarily rose above 1.5 degrees Celsius and later declined. The central finding is that cooling the atmosphere would not quickly reverse changes already underway in the ocean and ice sheets.

Global average sea level has risen by more than 10 centimeters since satellite measurements began in 1993, and the rate of increase has more than doubled over that period. The report projects another 10 to 15 centimeters of rise by 2050 across the emissions pathways it considered.

After mid-century, the amount of additional rise depends more strongly on how high temperatures peak and how long the overshoot lasts. Warmer ocean water expands, while glaciers and ice sheets can continue losing mass long after atmospheric temperatures stop increasing.

The assessment is intended to help coastal governments plan for a long-lived risk rather than treat a later temperature decline as a rapid reset. NASA’s sea-level work with Pacific island countries, including Tuvalu and Kiribati, illustrates why local elevation data and adaptation planning remain important even under successful future cooling.