Indeed, the ice hit hardest by the long-term decline is the thick ice that once survived several years of thaw and freeze. With more of the Arctic Ocean starting the freeze season as open water, an increasing proportion of winter ice heading into the melt season is relatively thin – more vulnerable to wind-driven break-up when the melt season returns, which can speed melting.
The summer sea-ice cover at the end of the melt season has been declining since the early 1970s, although since 1979 satellites have provided the most consistent measurements of the decline.
The decline coincides with warming at the top of the world that has been occurring twice as fast there as it has for the northern hemisphere as a whole as the global climate warms. Climate scientists attribute the general warming to rising concentrations of atmospheric greenhouse gases – mainly carbon dioxide – from burning fossil fuels since the start of the Industrial Revolution as well as from land-use changes.
The higher pace of Arctic warming, linked in part to rising greenhouse gases as well as to the interplay between ice, snow, and ocean that is reinforcing the warming trend, has implications for more than caribou and polar bears.
This so-called Arctic amplification increases the likelihood of severe weather at mid-latitudes in the northern hemisphere, where most people live, according to a study published earlier this year in the journal Geophysical Research Letters.