Nasa has launched its first mission dedicated to measuring carbon dioxide (CO2) from space.
The Orbiting Carbon Observatory (OCO) will help pinpoint the key locations on our planet's surface where the gas is being emitted and absorbed.
CO2's increased concentration in the atmosphere will lead to global climate change, say the major institutions and agencies that study Earth sciences.
The OCO data is intended to help forecast that change more accurately.
Currently, carbon dioxide is regularly sampled at about a hundred sites around the world. The new satellite will be taking roughly 30,000 readings on each orbit.
"We need to make a measurement that is about three times more precise than has ever been made for a trace gas in the Earth's atmosphere," said Dr David Crisp, OCO's principal investigator.
"We regularly measure ozone in the Earth's atmosphere to about 1%. We need to make a measurement of CO2 to about three-tenths of 1% to start answering the questions that face scientists."
The $270m mission was launched by the smallest ground-launched rocket currently in use by the US space agency.
NASA'S OCO MISSION
OCO weighs approximately 440kg
Will fly at a 705km altitude
Passes the equator every pm
30,000 measurements per orbit
Global coverage in 16 days
The Taurus XL vehicle left the Vandenberg Air Force Base in California at 0951 GMT, Tuesday.
It lofted OCO into a near-polar orbit at an altitude of 705km (438 miles). The spacecraft will circle the planet once every 98.8 minutes, passing over the entire globe in the course of 16 days.
Nasa stresses the mission is an experimental one; it first has to establish that the measurement approach it has adopted is a robust one.
OCO carries a spectrometer that analyses the sunlight reflected off the Earth's surface. By splitting that light into its component colours, it will be able to see the part of the spectrum absorbed by carbon dioxide molecules.
By measuring oxygen's presence in the atmosphere also, OCO should be able to arrive at a concentration figure for CO2. The instrument is sensitive to carbon dioxide in the lower reaches of the atmosphere.
"We'll be pumping down about 50 gigabits of data every day," said Dr Crisp.
"We're a very small spacecraft - we'd be a very cosy telephone booth - but we'll pump down data at such a high rate I often joke we'll melt the snow around the base of the down-link station."
CO2 in the atmosphere has seen a steady rise in recent years
This mass of information should help the OCO science team pinpoint the so-called sources (where CO2 comes from) and sinks (where CO2 is pulled out of the atmosphere by land and ocean processes, and stored) of carbon dioxide.
Scientists have calculated that Nature cycles about 330 billion tonnes of carbon every year.
Human activities put about eight billion tonnes into the atmosphere - a tiny sum in comparison but enough, say researchers, to imbalance the system and raise the global mean surface temperature of Earth.
Of that eight billion, studies suggest about half remains in the atmosphere, says Dr Paul Palmer, a collaborator on the mission from the University of Edinburgh, UK.
"The remaining 50% gets taken up by the ocean biosphere and the land biosphere, or so we think. But if you take into account what we know about the oceans and the land, there is still a high percentage - something like 20% - which is poorly understood.
"We don't know where it goes, but we do know that this unaccounted sink changes in magnitude from year to year."
It could be going into land areas where trees, grasslands, crops and soil are absorbing carbon dioxide at a faster rate than previously been acknowledged.
These sinks are likely to include abandoned farmland where forests are re-growing.
They could also take in the northern, high-latitude forests that are experiencing longer, warmer growing seasons, allowing trees and shrubs to "bulk up" and absorb more carbon dioxide.
OCO measures carbon dioxide on the day-side of the Earth
"Even 'Smokey Bear' is a sink," said Professor Scott Denning, an OCO science team associate at Colorado State University, referring to the US Forest Service's anti-fire mascot.
"By putting fires out in our western forests and allowing the wood to accumulate, we are actually sequestering CO2."
To identify those currently poorly understood - or "missing" - sinks, OCO's data will have to be combined with models of how the air is transported through the atmosphere.
The famous US forest mascot may have a part in the story
Ultimately, researchers will use the OCO maps to assess how well the sinks are likely to perform over time.
"[Different climate] models show very different CO2 levels in the atmosphere for the same human emissions. So even though people are producing the same emissions in each of these models, the resulting CO2 in the atmosphere is very different by the end of the century due to the differences in land and ocean behaviour," explained Professor Denning.
"This actually shows up at about 300 parts per million (ppm) of difference in the amount of CO2 in the atmosphere - remember, we're at about 385ppm today. So this is a big difference and is really quite important for understanding future climate."
The OCO team is working closely with the Japanese Gosat ("Ibuki") mission which launched its carbon observatory last month.
The orbits of the two spacecraft will cross six times each day.
The groups use different measurement approaches, which will provide a cross-check on each other's data. Both will take their calibration from ground stations which, although limited in number, can measure CO2 with much higher precision at their locality.
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