Rene Clement/Polaris/eyevine

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 (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 CO 2 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 centre’s CO 2 monitoring efforts on Mauna Loa. That gas record, known as the Keeling curve, was started by his father, Charles Keeling.

Source: Scripps Inst. Oceanography/UC San Diego

When monitoring started, the CO 2 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 levelling off (see ‘On the rise’). Emissions of other greenhouse gases are also increasing, pushing the total equivalent concentration of CO 2 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 tonnes 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 CO 2 .

“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 tonnes 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 CO 2 stays in the atmosphere, levels will rise even faster.

Some researchers have suggested that the sinks have already started to clog up, reducing their ability to take up more CO 2 (J. G. Canadell et al. Proc. Natl Acad. Sci. USA 104, 18866–18870; 2007). 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 CO 2 measurements made around the globe. They found no signs of sinks slowing down (A. P. Ballantyne et al. Nature 488, 70–72; 2012). 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.” 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-metre tower to keep tabs on the Amazon (see Nature 467, 386–387; 2010). And Europe’s Integrated Carbon Observation System is setting up stations throughout the continent and at some marine sites to measure CO 2 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 (see page 5). 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 CO 2 concentration at 13 sites around the world.