When Charles Keeling first began measuring the atmosphere’s carbon dioxide levels in the late 1950’s, he noted first that they stood at about 315 parts per million (ppm), or 315 molecules of carbon dioxide for every million molecules of air.
Soon after, though, he found that the concentrations were rising, thanks to the burning of fossil fuels (today, they stand at around 395 ppm and they’re still rising). But he also noticed that the upward curve of CO2 concentrations had a sawtooth pattern. That pattern saw CO2 rise sharply in the fall in the Northern Hemisphere -- when leaves died and fell off the trees to rot -- then drop slightly in spring as new leaves emerged to start drawing in CO2 for photosynthesis. (Leaves fall and sprout in the Southern Hemisphere, too, in an exactly opposite pattern, but there’s so much more ocean and so much less land south of the Equator that the Northern effect is a lot stronger).
Now NASA has put together an animation that shows this process in a much more vivid way. Based on observations from two instruments on the Aqua spacecraft, the animation shows how the disappearance of leaves (green) leads to an increase in atmospheric CO2 (yellow-orange), first in one hemisphere, then in the other — and just as Keeling showed a half-century ago, the effect in the Northern Hemisphere is a lot stronger.