Reuters: Arctic sea ice, a key component of Earth’s natural thermostat, has thinned sharply in recent years with the northern polar ice cap shrinking steadily in surface area, government scientists said on Monday. Thinner seasonal sea ice, which melts in summer and freezes again every year, now accounts for about 70 percent of the Arctic total, up from 40 to 50 percent in the 1980s and ’90s, the researchers said, citing new satellite data. At the same time thicker ice, which lasts two …