Already the signs are clear. In areas where the sea ice has disappeared, summer air temperatures are five to nine degrees Fahrenheit higher than the average of the previous 20 years. As the differences in temperature between the planet’s north and equator shrink, the changing Arctic is beginning to affect weather patterns across the hemisphere. When there is little ice in the Arctic in summer, there is less winter rain in the United States and Scandinavia, but more rain in the northern Mediterranean and Japan.

That is just a tiny start. As the new Arctic Sea heats up, a pool of warmer air is spreading across the nearby lands. Shrubs and trees are creeping north across the tundra. The resulting dark vegetation soaks up more heat. Again, the warming gains pace. With temperatures rising, odd bubble streams have begun to appear in the tundra lakes. As the frozen lake bottoms melt, micro-organisms begin to break down the rich stores of organic material into methane —a far more powerful greenhouse gas than carbon dioxide, causing more than 20 times as much warming per molecule. Large quantities of methane rising into the atmosphere would boost global temperatures quickly. Out in the shallow Arctic shelf seas, frozen permafrost lies underwater and there, too, bubbling streams of methane are appearing where there were none before.

Away from the lakes, the permafrost is rotting away as it thaws, pouring out carbon dioxide. There is enough carbon stored in the permafrost to more than double the carbon dioxide in the atmosphere—which would produce global temperatures an average of 12 degrees Fahrenheit higher than they are now. Of course, this carbon will not all be released at once. The Arctic will prefer a long, slow settling of scores. The permafrost is many hundreds of yards deep and will melt over hundreds, perhaps thousands of years. As greenhouse gas emissions from the Arctic grow, they will carry with them the threat of faster warming for the rest of the globe. The only way we can combat this change is to reduce our own greenhouse gas emissions further. Year by year, the targets we’ll need to reach to maintain even a modicum of equilibrium will grow tougher. Meanwhile, the Arctic will be tightening the noose.