Three decades of data, graphed at the Arctic Sea-Ice Monitor.
The Christian Science Monitor explains why it matters to more than polar bears:
Earth's icy skull cap, floating atop the Arctic Ocean, has reached its lowest summer extent since satellites first began keep in track in 1979, and by some estimates its lowest reach in nearly 1,500 years...More at the links, which I found at Paul Douglas' incomparable On Weather blog.
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 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. 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...
As the temperature difference shrinks, the jet stream's speed slows and the north-south meanders it makes as it snakes from west to east grow longer. Both changes slow the jet stream's pace, contributing to the blocking patterns that lead to persistent bouts of heat, cold, or precipitation...
The jet streams' elongated meanders can bring one storm after another to parts of the continent while keeping other parts relatively storm-free. And the slowdown in the jet stream's migration across the hemisphere sets up the blocking patterns that can hold those conditions in place for weeks.
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