<p>A Dartmouth study provides the first evidence that the Arctic’s frozen soil is the dominant force shaping Earth’s northernmost rivers, confining them to smaller areas and shallower valleys than rivers to the south. But as climate change weakens Arctic permafrost, the researchers calculate that every 1 degree Celsius of global warming could release as much carbon as 35 million cars emit in a year as polar waterways expand and churn up the thawing soil.</p>