The loss of sea ice in the Arctic at the end of the 20th Century is "unprecedented" in the past 1,450 years in its duration and magnitude, an indication of human-influenced climate change, a study said.
The ice, which melts every summer before cold weather makes it expand again, shrank this year to its second-smallest size since 1979, covering 4.33 million square kilometers (1.67 million square miles), according to the US National Snow and Ice Data Center. Although previous sea ice declines have occurred at a similar pace, they don't match the extent of the melt, the study authors said.
"This drastic and continuous decrease we've been seeing from the satellites does seem to be anomalous," Christophe Kinnard, a study author and a geographer at the Centro de Estudios Avanzados en Zonas Aridas in La Serena, Chile, said in a telephone interview. "It does point to a continuation of this trend in the future."
The researchers used ice core records, tree ring data, lake sediment and historical evidence to reconstruct the amount of Arctic cover. The thickness and extent of sea ice have declined dramatically over the last 30 years, the researchers said.
Arctic sea ice influences the global climate, since 80 percent of the sunlight that strikes it is reflected back to space. When the ice melts in the summer, it exposes the ocean surface, which absorbs about 90 percent of the light, heating the water, according to the National Snow and Ice Data Center. That influences climate patterns.
"You increase the radiation that's absorbed by the oceans, that's one of the strongest climate feedback mechanisms," Kinnard said. "The more sea ice you lose, the more energy you get in the ocean, which warms the atmosphere."