Near the moonscape summit of the Mauna Loa volcano in Hawaii, an infrared analyser 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 (ppm), 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 ppm, nations will have a difficult time keeping global warming in check, says Corinne Le Quere, 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 degrees celsius 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 ppm. 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 centre’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 ppm, not much higher than the 280 ppm that characterised conditions before the industrial revolution.

Rapid increase recorded

But, since the Hawaiian measurements began, the values have followed an upward slope that shows no sign of levelling off. Emissions of other greenhouse gases are also increasing, pushing the total equivalent concentration of CO2 in the atmosphere to around 478 ppm in April, according to Ronald Prinn, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge. Data compiled by Le Quere 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 at 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 favour, 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. While some researchers have suggested that the sinks have already started to clog up, reducing their ability to take up more CO2, others disagree. Ashley Ballantyne, a biogeochemist at the University of Montana in Missoula, worked with White and others to examine records of emissions as well as CO2 measurements made around the globe. They found no signs of sinks slowing down. But it is difficult to be sure, says Inez Fung, a climate modeller at the University of California, Berkeley. “We don’t have adequate observing networks.”

Monitoring carbon sinks

The largest global network, operated by the US, National Oceanic and Atmospheric Administration, had to trim 12 stations in 2012 because of budget cuts. Some of the most crucial areas, such as the tropics, are also the least monitored, although researchers are seeking to fill in the gaps. Scientists from Germany and Brazil are building a 300-meter tower to keep tabs on the Amazon. And Europe’s Integrated Carbon Observation System is setting up stations throughout the continent and at some marine sites to measure CO2 and other greenhouse gases.

Satellites, too, could monitor carbon sources and sinks. Two orbiters are already providing some data, and NASA plans to launch the much anticipated ‘Orbiting Carbon Observatory-2’ next year. An earlier version of that satellite failed during its 2009 launch. Even as new resources come online, however, researchers are struggling to keep the Mauna Loa station going. “The amount of money that I’m able to obtain for the programme has diminished over time,” says Keeling, whose group monitors CO2 concentration at 13 sites around the world. “It’s kind of silly that we chose to go all ostrich-like,” says White of the funding difficulties. “We don’t want to know how much CO2 is in the atmosphere, when we ought to be monitoring even more.”