The Arctic Ocean’s ice cover has thinned by about 40 percent in the last 20 to 40 years, according to a new study published in the journal Geophysical Research Letters. The study’s researchers, from the University of Washington, were surprised by the dramatic shift, but couldn’t say whether it was the result of human-caused climate change or natural cycles. The study fits with other research that has shown Arctic ice is retreating, causing problems for wildlife.