russia North Sea Arctic Ocean greenland Pacific Ocean Atlantic Ocean Arctic Circle 1984 Sea Ice EXTENT in EARLY march Hudson Bay Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Atlantic Ocean Pacific Ocean Arctic Circle 1984 Hudson Bay Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Atlantic Ocean Arctic Circle 1984 Hudson Bay Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland 1984 Arctic Circle Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years 1984 russia Arctic Ocean greenland Arctic Circle Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Pacific Ocean Atlantic Ocean Arctic Circle 2018 Sea Ice EXTENT in EARLY march Hudson Bay Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Atlantic Ocean Pacific Ocean Arctic Circle 2018 Hudson Bay Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Atlantic Ocean Arctic Circle 2018 Hudson Bay Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland 2018 Arctic Circle Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years 2018 russia Arctic Ocean greenland Arctic Circle Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Pacific Ocean Atlantic Ocean Arctic Circle 1984 Sea Ice EXTENT in EARLY march Hudson Bay Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Atlantic Ocean Pacific Ocean Arctic Circle 1984 Hudson Bay Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland Atlantic Ocean Arctic Circle 1984 Hudson Bay Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years russia North Sea Arctic Ocean greenland 1984 Arctic Circle Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years 1984 russia Arctic Ocean greenland Arctic Circle Sea Ice EXTENT in EARLY march Age of Arctic sea ice Canada 2 3 4 5+ years The New York Times

In the Arctic Ocean, some ice stays frozen year-round, lasting for many years before melting. But this winter, the region hit a record low for ice older than five years.

This, along with a near-record low for sea ice over all, supports predictions that by midcentury there will be no more ice in the Arctic Ocean in summer.

As darker, heat-absorbing water replaces reflective ice, it hastens warming in the region. Older ice is generally thicker than newer ice and thus more resilient to heat. But as the old ice disappears, the newer ice left behind is more vulnerable to rising temperatures.

“First-year ice grows through winter and then to up to a maximum, which is usually around in March,” said Mark A. Tschudi, a research associate at the Colorado Center for Astrodynamics Research at the University of Colorado, Boulder. “As summer onsets, the ice starts to melt back.”

Some of the new ice melts each summer, but some of it lingers to grow thicker over the following winter, forming second-year ice. The next summer, some of that second-year ice survives, then grows even thicker and more resilient the next winter, creating what is known as multiyear ice. Some ice used to last more than a decade.

100% 5+ year old ice 80 4 yr. 3 yr. 60 2 yr. 40 1 year old ice 20 0 1984 1988 1992 1996 2000 2004 2008 2012 2018 Percentage of Arctic Ice in Early March 100% 5+ year old ice 80 4 yr. 3 yr. 60 2 yr. 40 1 year old ice 20 0 1984 2000 2018 Percentage of Arctic Ice in Early March 100% 5+ year old ice 80 4 yr. 3 yr. 60 2 yr. 40 1 year old ice 20 0 1984 1992 2000 2008 2018 Percentage of Arctic Ice in Early March The New York Times

Today, Arctic sea ice is mostly first-year ice. While the oldest ice has always melted when currents pushed it south into warmer waters, now more of the multiyear ice is melting within the Arctic Ocean, leaving more open water in its wake.

This is especially bad for animals like narwhals, the so-called unicorns of the sea, that use sea ice to avoid predators like killer whales. As the sea ice disappears, killer whales spend more time in narwhal waters, eating the narwhals and driving them from the richest feeding grounds.