By Jonathan Amos
Science reporter, BBC News, San Francisco
The US space agency is set to launch a satellite that can map in detail where carbon dioxide is in the atmosphere.
Nasa's Orbiting Carbon Observatory (OCO) will pinpoint the key locations on the Earth's surface where CO2 is being emitted and absorbed.
CO2 from human activities is thought to be driving climate changes, but important facts about its movement through the atmosphere remain elusive.
The agency believes the technology on OCO can end some of the mysteries.
"This is Nasa's first spacecraft specifically dedicated to mapping carbon dioxide," principal investigator David Crisp told BBC News.
"The objective of the OCO mission is to make measurements that are so precise that they can be used to look for surface 'sources' and 'sinks' of CO2."
Dr Crisp has been presenting details of the mission here at the American Geophysical Union's (AGU) Fall Meeting.
As he did so, OCO's launch on a Taurus XL rocket from Vandenberg Air Force Base in California was booked for 23 February.
CO2 in the atmosphere has seen a steady rise in recent years
Nasa already has a CO2 detection instrument on its Aqua satellite but this looks at the greenhouse gas some five to 10km above the surface.
OCO, on the other hand, will detail the concentration of carbon dioxide closer to the ground where its warming effect is most keenly felt.
The observatory will be engaged in what amounts to carbon accountancy. Its fortnightly global maps of CO2 concentration will help the mission team work out where the gas is entering the atmosphere and where it is being absorbed by land plants and the oceans.
OCO weighs just under half a tonne
Scientists have calculated that nature cycles about 330 billion tonnes of carbon every year.
Human activities put about 7.5 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.
"We know where most of the fossil fuel emissions are coming from; we also know where things like cement manufacturing are producing large CO2 emissions," explained Dr Crisp, who works at Nasa's Jet Propulsion Laboratory.
"But there are other things such as biomass (forest) burning and clearing; and we don't have a good quantification of the CO2 released by those processes.
"If you take out the fossil fuels - for which we understand the CO2 source to within 10% - and look at the rest of the carbon dioxide that's introduced into the atmosphere by our activities, it's uncertain by 100%.
"The idea is that OCO will help us to constrain that a whole lot better."
The sinks for CO2 - the places where it is absorbed - also have many mysteries associated with them.
The Earth is thought to be absorbing about 50% of the carbon dioxide we put out - the majority of it going into the oceans. But science's description of the other major absorbers is poor, commented UK Earth-observation scientist Shaun Quegan.
"There's a bunch of atmospheric collection flasks dotted around the planet and when we apply the models to their data, the models all show there is a carbon sink in northern mid-latitudes," he said.
"But whether that's in North America, in Siberia, or wherever and what's causing it is a big debate."
Since science does not have a good handle on where the CO2 is being absorbed, researchers can have only limited understanding of how CO2 sinks are likely to evolve as the climate changes.
"Let's say we found that the boreal forests in Canada and Siberia were the primary sinks of CO2 because of their incredibly rapid growth during summer months when the Sun is up," speculated Dr Crisp.
"Well those environments are changing dramatically right now.
"Will they still be the primary absorbers of CO2 as time goes on? We don't really know how big an impact they're having right now.
"This is why OCO is so essential."
The observatory carries a single instrument - a spectrometer that breaks the sunlight reflected off the Earth's surface into its constituent colours, and then analyses the spectrum to determine how much carbon dioxide and molecular oxygen is present.
The data can be used to work out atmospheric concentrations.
OCO will map carbon dioxide over 1,600-sq-km (620 sq miles) regions of the Earth's surface to an accuracy of just fractions of 1%.
However, to locate the sources and sinks, scientists will need to combine the information with models that estimate how CO2 is being moved and mixed through the air.
NASA'S A-TRAIN SATELLITE CONSTELLATION
1. OCO will head the 'train' of satellites when it gets into orbit. It will measure the concentration of carbon dioxide in the lower atmosphere
2. Aqua will lag OCO by 15 minutes. It is collecting information about the Earth's water cycle - water in the oceans, the air and on the land
3. Cloudsat will allow for the most detailed study of clouds to date. It should better characterise their role in regulating the climate
4. Calipso views clouds just moments after Cloudsat has looked at them. Its primary interest is the way aerosols interact with clouds
5. Parasol is a French satellite that can distinguish natural from human-produced aerosols. It makes polarised light measurements
6. Glory will join the train in June. One task will be to measure the 'energy budget' of Earth, to determine accurately the global temperature
7. Aura also has a big European investment. It looks at atmospheric chemistry, and is producing remarkable global pollution maps
Once in orbit, OCO will join a fleet of other satellites - known as the A-Train - which carry a range of instrumentation to give a rounded picture of Earth's atmospheric and water systems.
The spacecraft cross the equator in the early afternoon on a path that takes them over broadly the same observation point in quick succession.
OCO will be followed into orbit next year by a Japanese carbon mission known as the Greenhouse gases Observing SATellite (GOSAT).
Europe is considering two carbon observatories - A-SCOPE (Advanced Space Carbon and Climate Observation of Planet Earth) and a mission called BIOMASS - which could fly in 2016.
Professor Quegan, from the UK's University of Sheffield, is working on the BIOMASS proposal.
"The spacecraft would measure global forest biomass at scales of about one hectare," he said.
"It's a crucial natural resource and ecosystem service - for materials, for energy, for biodiversity - there's a good correlation between how much biomass you've got and how much biodiversity you've got - and for climate and water protection."
"So from a carbon cycle science aspect, forests have some critical parameters that need to be pinned down."
In the carbon cycle, natural fluxes are the biggest, accounting for about 330 gigatonnes per year, and are in near equilibrium
The roughly 7.5 gigatonnes coming from all human sources may be sufficient to tip this system out of balance, warming the Earth
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