Near the moonscape summit of the Mauna Loa volcano in Hawaii, an infrared analyzer will soon make history. Sometime in the next month, it is expected to record a daily concentration of carbon dioxide in the atmosphere of more than 400 parts per million (p.p.m.), a value not reached at this key surveillance point for a few million years.
There will be no balloons or noisemakers to celebrate the event. Researchers who monitor greenhouse gases will regard it more as a disturbing marker of humanity’s power to alter the chemistry of the atmosphere and by extension, the climate of the planet. At 400 p.p.m., nations will have a difficult time keeping global warming in check, says Corinne Le Quéré, a climate researcher at the University of East Anglia in Norwich, UK, who says that the impact “is getting very dangerously close to reaching the 2 °C target that governments around the world have pledged not to exceed”.
It will be a while, perhaps a few years, before the global CO2 concentration averaged over an entire year, passes 400 p.p.m.. But topping that value at Mauna Loa is significant because researchers have been monitoring the gas there since 1958, longer than any other spot. “It’s a time to take stock of where we are and where we’re going,” says Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, California, who oversees that center’s CO2 monitoring efforts on Mauna Loa. That gas record, known as the Keeling curve, was started by his father, Charles Keeling.
When monitoring started, the CO2 level stood at 316 p.p.m., not much higher than the 280 p.p.m. that characterized conditions before the industrial revolution. But since the Hawaiian measurements began, the values have followed an upward slope that shows no sign of leveling off (see ‘On the rise’). Emissions of other greenhouse gases are also increasing, pushing the total equivalent concentration of CO2 in the atmosphere to around 478 p.p.m. in April, according to Ronald Prinn, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge.
Data compiled by Le Quéré and other members of the Global Carbon Project suggest that humans contributed around 10.4 billion tons of carbon into the atmosphere in 2011. About half of that is taken up each year by carbon ‘sinks’ such as the ocean and vegetation on land; the rest remains in the atmosphere and raises the global concentration of CO2.
“The real question now is: how will the sinks behave in the future?” says Gregg Marland, an environmental scientist at Appalachian State University in Boone, North Carolina, who helps to compile the emissions data.
The sinks have grown substantially since Keeling began his measurements, when carbon emissions totalled about 2.5 billion tons a year. But climate models suggest that the land and ocean will not keep pace for long.
“At some point the planet can’t keep doing us a favor, particularly the terrestrial biosphere,” says Jim White, a biogeochemist at the University of Colorado Boulder. As the sinks slow down and more emitted CO2 stays in the atmosphere, levels will rise even faster.