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.
"The Arctic is warming so much faster than mid-latitudes, and it's that difference in temperature that drives the jet stream," a river of air that triggers and steers storms, says Jennifer Francis, an atmospheric scientist at Rutgers University who focuses much of her research on the Arctic and is the lead author on the study.
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.