On Tuesday, a converted Gulfstream V jet loaded with scientific instruments took off from an airport in Colorado on a 24-day mission to measure carbon dioxide concentrations at thousands at different locations and altitudes over the Pacific Ocean between the North and South Poles.
It is the fourth time the jet has made such a journey and the data it gathers will be used to provide hard information about the dynamic state of the Earth's atmosphere so that computer models of the global climate can be made more reliable as a tool for predicting the impact of rising carbon dioxide concentrations.
The $40m (£24.8m) jet, a former executive toy, had its bar removed to make room for a further $40m worth of highly sophisticated equipment for measuring between 50 and 100 different compounds, including carbon dioxide.
It will fly in regular up-and-down swoops, rising from an altitude of 1,000ft to a maximum of 45,000ft, and back down to 1,000ft. Between Anchorage and Hawaii, for instance, it will complete 28 dips, taking 150 air samples with each swoop. It will fly over the entire Pacific Ocean as far as the polar regions 85 degrees north and 55 degrees south, covering an area as far west as Australia and as far east as the Gulf of Mexico.
Strange as it may seem, until the "Hippo" pole-to-pole observations began two years ago no one had actually measured carbon dioxide concentrations in such detail from one end of the Earth to the other at different altitudes, from sea-level to 45,000ft. The project is run by the US National Centre for Atmospheric Research and it has already revealed that the many dozens of gases and other compounds that make up the atmosphere vary significantly from one place to another and from one altitude to another.
It is this heterogeneity that will be fed into the computer models of the climate which have in the past assumed a rather homogeneous and hence inaccurate picture of the different greenhouse gases present in the atmosphere.
"It's not so much to prove the models wrong, it's to help the modellers do the right thing by giving them the data to show them whether they are right or not," said David Fahey of the US National Oceanic and Atmospheric Administration, one of the hundreds of scientists working on the Hippo project.
"Once you can show the models have skill then you can ask important questions like 'What is going to happen if...?'. That's what the world really needs to know," Dr Fahey added.
The fact that average carbon dioxide concentrations in the atmosphere have risen from a pre-industrial level of about 280 parts per million (ppm) to a present concentration of about 380ppm is incontrovertible.
But such measurements have relied on solitary recording stations, such as those on the mountain of Mauna Loa on Hawaii, and have given little insight into the dynamic variability of the concentrations at different locations , heights and seasons.
"Hippo is a fantastic mission, it's really outside the box. It is giving us a curtain of data that goes from 85 degrees north to 55 degrees south and the only way you can attempt that kind of coverage is by satellite, but there is no satellite measuring CO2," Dr Fahey said.