Polar sea ice is essential for maintaining a livable global climate, with global risks from its decline ranging from disruption of weather and ocean currents; to accelerated Greenland and Antarctic melt and associated sea-level rise; to extinction of ice-dependent species at the base of the food chain for humans and many polar and marine mammals. Sea ice coverage at both poles has declined by 40–60% since satellite measurements began in 1979, with nearly all Antarctic sea ice decline occurring precipitously since 2016. While most attention is given to the September sea ice minimum in the Arctic, this loss has occurred year-round, in all months of the year including sea ice maximums, when the ice reaches its largest extent. A record-low maximum occurred in the Arctic in March 2025, and Antarctica’s record-low maximum was set in September 2023. Global sea-ice coverage, combining both poles, reached a record all-time low in February 2025.

Sea ice has declined not only in extent, but in thickness. Much of the Arctic Ocean used to be covered in thick, multi-year ice that was 4–7 years old. Such “old” ice has virtually disappeared, with even two or three-year-old ice comprising under 10% of today’s sea-ice coverage. Antarctic sea ice plays an essential role in several ways, including formation of Antarctic Bottom Water: the densest water mass on the planet, driving the entire global ocean “conveyor belt.” A 40% decline in sea ice in the Weddell Sea has reduced the production of Antarctic Bottom Water in this region by almost a third.

Future sea ice survival is extremely sensitive to current and future human emissions of greenhouse gases. If governments course-correct to 2025 NDCs consistent with 1.5°C of warming or below at 2100, sea ice may slowly begin to recover in the 2070s and beyond. At least one ice-free Arctic summer event seems increasingly likely however before 2050, and the summer ice-free period would increase with additional warming. NDCs that result in global mean temperatures of 2°C or above would lead to ice-free conditions in the Arctic every summer, with high-risk and unpredictable global impacts. Loss of Antarctic sea ice and associated ice shelves is less certain, but holds even greater long-term and non-reversible risks because the sea ice and ice shelves are essential to protecting Antarctica’s ice sheet, and holding sea-level rise to adaptable levels in coming decades and centuries.