Atmospheric CO2 concentrations rose during the last deglaciation, but the carbon sources are unclear. Climate and carbon cycle simulations suggest that permafrost melting was the main source of carbon between 17,500 and 15,000 years ago. The image shows small streams of summertime meltwater in Kobbefjord, southwest Greenland.
Letter p683; News & Views p648
IMAGE: JAKOB SIEVERS, AARHUS UNIVERSITY, DENMARK
COVER DESIGN: TULSI VORALIA