Credit: NASA

Carbon dioxide sources and sinks can now be measured from space at high resolution since the Orbital Carbon Observatory 2 (OCO-2) successfully joined a constellation of six Earth-observing satellites, on 2 July.

Termed the Afternoon-Train (or A-Train) because it passes over the equator at 1.30 pm each day, the satellite constellation already provides detailed data on clouds, aerosols, water, climate and air quality. The carbon dioxide measurements collected by the OCO-2 are expected to complement these observations with unprecedented details. The satellite will deliver regional-scale information on both carbon emissions, such as those from large metropolitan areas like Los Angeles, and carbon uptake by the terrestrial biosphere.

Most of the A-Train spacecraft were launched between 2002 and 2006. The designated lead craft, the original Orbiting Carbon Observatory, was set to join the constellation in February 2009. However, it was lost during launch and only now has it been possible to send its second incarnation, OCO-2, into orbit.

Global observations of carbon sources and sinks are now as important as ever. Over the past few years, the ranking of nations in terms of their total carbon emissions from fossil fuel burning and cement production has shifted from the developed world towards emerging economies, such as China, India and Brazil (Earth Syst. Sci. Data 6, 235–263; 2014). These countries typically do not have networks of sophisticated ground observations. Plans to improve the density of observational networks of local carbon budgets are largely limited to highly developed countries in Europe and North America (Nature Geosci. 2, 3–4; 2009). Worldwide coverage with satellite observations, at a resolution sufficient to identify emission or uptake hot spots, will help to close these observational gaps, which is key to understanding the human perturbations of the Earth's carbon cycle.

An important limitation of OCO-2 is its relatively short lifetime, currently set at only two years. To document how carbon emissions shift around the globe as the worldwide economy evolves, long-term monitoring would be better. Follow-up missions are already being planned, including a duplicate instrument that could be installed on the International Space Station — if the funds become available to follow through with the mission, after NASA's most recent budget omitted the project (Nature 510, 451–452; 2014).

The task of keeping the global carbon budget and its impact on the Earth's climate in check cannot be evaded. To that end, we need to know where emissions come from, where carbon is being taken up by ocean or land biosphere, and how sources and sinks evolve over time around the globe. Given the stakes, the funds to make these measurements on a long-term basis should be made available.