Each year during the Northern Hemisphere spring, the greening of the planet begins. Trees sprout their leaves, plants grow and vegetation takes hold north of the equator, where nearly 70 percent of Earth’s total land mass lies.

As photosynthesis ramps up, plants breathe in carbon dioxide (CO 2 ), and atmospheric levels of CO 2 begin to drop. Then in fall and winter, when trees lose their leaves and foliage declines, CO 2 levels begin to rise again. This up-and-down sequence creates an annual cycle of minimum and maximum levels of atmospheric CO 2 .

“The exchange between vegetation and the atmosphere makes this sort of wavy pattern, and what seems to be happening now is that we’re reaching an annual minimum that is above 400 ppm [parts per million],” says Joao Teixeira, Atmospheric Infrared Sounder (AIRS) Science Team leader at NASA's Jet Propulsion Laboratory (JPL). “Unless something dramatic happens with humans and the planet, it will never be 400 again in the next several decades."

Seeing global concentrations above the 400 ppm threshold at a time of year when atmospheric CO 2 is typically at its lowest level is a critical turning point.

Climate milestone

Ice core records show that until the Industrial Revolution, atmospheric CO 2 levels remained fairly steady at around 280 ppm. By 1961, CO 2 data collected at a monitoring station at the summit of Hawaii’s Mauna Loa volcano showed that atmospheric CO 2 levels were rising steadily by about 2 ppm per year. Nearly half a century later, in 2005, CO 2 concentrations had increased to 380 ppm.

The station at Mauna Loa is considered the “gold standard” for monitoring atmospheric CO 2 from the surface. But the 2002 launch of the AIRS instrument made it possible for researchers to map CO 2 levels in the troposphere on a global scale. “Suddenly, we have measurements over the ocean, over the land and over the poles, and now we can track these levels over time,” says Edward Olsen, a scientist and AIRS team member at NASA JPL.

In May 2013, the Mauna Loa station recorded CO 2 levels above the benchmark 400 ppm for the first time. Although that measurement marked a climate milestone, AIRS data have now confirmed a more significant landmark: the annual minimum CO 2 level has now exceeded 400 ppm—not just in one location, but over the entire globe. “We take these measurements all over the world in different places, and we average everything out,” says Teixeira. “So AIRS gives us the global mean value of CO 2 in the troposphere.”

New territory

Seeing global concentrations above the 400 ppm threshold at a time of year when atmospheric CO 2 is typically at its lowest level is a critical turning point. “The significance of the minimum exceeding 400 ppm is that the natural processes that draw down atmospheric CO 2 are not sufficiently strong to bring the level back down again,” says Olsen.