The Arctic melt season lengthens by about 7.5 days as autumn freeze-up arrives later
A review of roughly 50 years of satellite data led by Melinda Webster at the University of Washington finds the Arctic melt season has lengthened by about 7.5 days since 1979, mostly from a later autumn freeze-up. Webster tells Earth.com that sea ice reflects 60–90 per cent of sunlight while open water reflects about 7 per cent, so the delay lets the ocean absorb more heat and form thinner ice. The Antarctic trend remains harder to pin down, the review notes.
Science··Morning
Sea ice's high albedo is being eroded by a later autumn freeze
A review gathering roughly 50 years of satellite measurements — 15 to 20 more years than earlier syntheses — finds the Arctic melt season has grown about 7.5 days longer since 1979, mostly from ice now freezing up later in autumn. Melinda Webster of the University of Washington's Applied Physics Laboratory, who led the review, tells Earth.com that sea ice reflects 60–90 per cent of incoming sunlight back into space while open ocean water reflects only about 7 per cent, so every extra open-water day lets the ocean absorb more heat that then delays the next freeze.[1], [2]
Thinner ice ripples into algae, fish and denning wildlife
Webster says the added heat produces a cycle: warmer autumn water forms thinner ice, which melts more easily the following year and lengthens the season further. The changes also reach the food web — Arctic algae adapted to low light beneath thick ice and snow face altered conditions as snow cover thins, which Webster says could mean less food for fish and wider effects on fisheries. Thinner snow has also made sea ice smoother, affecting the dens polar bears and seals use to raise their young.[2]
Warming four times faster than average, while Antarctica stays a puzzle
The review sets the change against Arctic warming running about four times faster than the global average, and describes the state of ice cover as both an indicator and an amplifier of climate change. Webster's team notes the Antarctic side is harder to pin down: that sea-ice system behaves differently, since the Antarctic is land surrounded by water rather than an ocean ringed by land, and the logistics of deploying instruments there are harder, so Antarctic sea ice held steady for years before recently falling to unusually low levels.[1]
