SAN FRANCISCO, CALIFORNIA—About 1% of the light that strikes plants is re-emitted as a faint, fluorescent glow—a measure of photosynthetic activity. Today, at a meeting of the American Geophysical Union here, scientists released a map of this glow (pictured, with data averaged from August to October of this year) as measured by the Orbiting Carbon Observatory-2. The NASA satellite launched in July with the goal of mapping the net amount of carbon in the atmosphere. But the fluorescence map, an unexpected secondary capability, provides a more direct measure of carbon fluxes: the amount mopped up by plants during photosynthesis or released during respiration. The findings will help scientists disentangle inputs and outputs in places like the Amazon rainforest, where there are both big emissions from deforestation and big sinks from photosynthesis. The map reveals that tropical rainforests near the equator are actively sucking up carbon, while the Corn Belt in the eastern United States, near the end of its growing season, is also a sink. Higher resolution fluorescence mapping could one day be used to help assess crop yields and how they respond to drought and heat in a changing climate.