evidence: Melting sea ice is acidifying the Arctic Ocean A warmer climate has caused significant amounts of sea ice in the Arctic Ocean to melt over the past few decades. Without an ice barrier between seawater and air, the ocean can rapidly absorb CO2 from the atmosphere. As WEI-JUN CAI and ZHANGXIAN OUYANG explain, this process lowers the pH of the ocean’s surface water—and it is unfolding three to four times more quickly in the Arctic Ocean than in other oceans, threatening all forms of life. Once perennially ice-covered, increasingly large areas of the Arctic Ocean are now ice-free for longer periods. With the loss of its "ice cap," the ocean’s fresh and carbon-deficient surface water is exposed to increasing atmospheric CO2, which it absorbs.