With U.N. climate talks under way in Copenhagen, much of the attention has focused on countries' competing proposals to cut greenhouse gas emissions, and just how steep cuts included in a final agreement might be.

But an equally important and harder to answer question, according to many scientists, is whether it will be possible to tell whether any one country is holding up its end of the deal. Right now, researchers are confident they know how much carbon is produced worldwide by human activities and what fraction of that remains in the atmosphere or is absorbed by "sinks" on land or in oceans.

What they can't do, and may not be able to do for at least another decade, is independently monitor an individual nation's contribution to the overall total.

"I think this is one of the most important sleeper issues at this meeting," said Tony Haymet, director of the Scripps Institution of Oceanography, at a Copenhagen press conference last week. "If we succeed in two weeks of having a global regulation of carbon dioxide and other greenhouse gases by a cap-and-trade mechanism, somebody has to certify that a ton of carbon has been buried, or a ton of carbon has been avoided, or a ton of carbon has been emitted. And exactly the same verification has to be done for the other greenhouse gases."

It's a sensitive issue. While the U.N. Framework Convention on Climate Change requires countries to submit national emissions inventories, the data is self-reported, not required regularly from all countries, and there is no independent data to verify it.

At the Copenhagen talks, U.S. negotiators have pushed China and India to allow outside verification of their greenhouse gas output, something those countries have resisted. "It's a matter of principle," Chinese Vice Foreign Minister He Yafei said last week.

How scientists measure greenhouse gas emissions

Observatories: The National Oceanic and Atmospheric Administration maintains observatories in Barrow, Alaska; Mauna Loa, Hawaii; and American Samoa, and in Antarctica at the geographic South Pole. The Mauna Loa facility shown at left spawned the famous "Keeling Curve," a continuous record of carbon dioxide measurements collected since 1958 that shows the greenhouse gas steadily building up in the atmosphere. Tall towers: NOAA began placing instruments on tall towers in the early 1990s, according to agency scientist Pieter Tans, in an effort to begin collecting measurements that can help scientists track regional differences in CO2 levels. The agency's network now includes roughly 10 sites, many of them existing radio, cell phone or television transmission towers. Shown here is the San Francisco's Sutro Tower, which now holds instruments to track greenhouse gases along with television broadcast antennas. Aircraft: Scientists use specially equipped aircraft to measure CO2 concentrations at various altitudes, giving them information about how the greenhouse gas mixes into the atmosphere. Commercial airplanes can also be part of the mix. An E.U. pilot project installed CO2-measuring instruments on Lufthansa jets, while Japan maintains a similar program with Japan Airlines. Satellites: Japan's GOSAT, launched in January, is the first satellite designed to monitor greenhouse gases worldwide. Its NASA counterpart, the Orbiting Carbon Observatory, crashed on launch weeks later. Satellites can measure CO2 and other heat-trapping gases in places that aren't covered by existing ground, tower or aircraft measurements. Ships: Scientists have also placed instruments on commercial vessels, or "ships of opportunity," to collect measurements of greenhouse gas levels as they ply their regular trade routes. -- Lauren Morello Photo credits: Mauna Loa observatory courtesy of NOAA/Retired Cmdr. John Bortniak, NOAA Corps. Research aircraft courtesy of Carlye Calvin, University Corporation for Atmospheric Research. Sutro Tower courtesy of NOAA.

For scientists, developing the capacity to verify compliance with a global climate deal requires a shift in focus.

"Traditionally, for decades, we wanted to be in the middle of nowhere," said Pieter Tans, a scientist at the National Oceanic and Atmospheric Administration's laboratory in Boulder, Colo., which maintains an international network of sites that monitors concentrations of carbon dioxide and other greenhouse gases.

Still wrestling with the basics of verification

That's because scientists were trying to answer basic questions like how much the amount of carbon dioxide in the atmosphere grew from one year to the next, for which taking measurements away from greenhouse gas emissions hot spots ensured the most accurate results.

But increasingly, the questions scientists and policymakers are asking require zeroing in on sources and sinks of CO2 emissions. And that's not something the current network of monitoring sites does well.

"Our coverage is still really thin," said Tans. "There are places where we have practically no instruments."

Even in the United States, where Tans said coverage is considered to be relatively good, NOAA maintains only about a dozen permanent measuring sites, supplementing the data they collect with observations from specially equipped aircraft. In practical terms, that means the agency uses data collected by CO2 monitoring instruments installed on a tall tower at an Energy Department site in South Carolina to represent conditions in the entire southeastern United States.

Tans said he believes that the number of sites included in the global monitoring network would have to increase by "one to two orders of magnitude" before scientists could monitor individual countries' emissions with the precision needed to enforce an international agreement.

"Can we globally have a sense of what we're doing correctly in terms of emissions trajectories?" said Pep Canadell, executive director of the Global Carbon Project. "Yes."

Scientists know, for example, that humans emit 8 gigatons of carbon into the atmosphere every year, and about 4 of those remain in the atmosphere. The other half is absorbed by oceans or vegetation on land. And they are also able to break down those emissions by continent, in some cases. But Canadell believes it will be at least a decade before scientists are able to provide independent estimates of individual countries' CO2 emissions.

He and other experts said building an effective system to check treaty compliance will also require collecting measurements of the atmosphere's greenhouse gas content using satellites, which can provide coverage in places ground measurements haven't reached.

Giant questions linger

Such precise measurements would also help scientists keep tabs on natural sources and sinks of carbon, and alert them to any changes in their behavior. Researchers with the Global Carbon Project, for example, released a report last month that suggests Earth's carbon sinks aren't keeping up with humans' rising greenhouse gas output. Their study, published in Nature Geoscience, concluded that the fraction of CO2 that remains in the atmosphere has grown from 40 percent in 1959 to 45 percent in 2008.

"All we know is that Earth is providing this free service that is sucking up half of the carbon we're emitting," said Anna Michalak, a professor of atmospheric science and civil engineering at the University of Michigan. "So one of the giant questions we're trying to answer is, what's going to happen in the future? As we emit more and more carbon, are the oceans and the land going to start taking up even more -- or stop taking up carbon altogether and start emitting carbon back out?"

But the first efforts to launch satellites designed specifically to measure greenhouse gases have brought mixed results. Japan successfully launched its greenhouse gas monitoring satellite, known as GOSAT, in January. But weeks later, a similar U.S. satellite, the Orbiting Carbon Observatory, failed at launch. A faulty nose cone kept the $278 million satellite from separating from its launch rocket, sending it crashing into the ocean near Antarctica (ClimateWire, Feb. 25).

That's a blow for policymakers who would like a way to independently verify whether countries are meeting their emissions pledges, a National Academy of Sciences committee said this summer.

"Current methods for estimating greenhouse gas emissions have limitations for monitoring a climate treaty," the panel warned in a report urging the Obama administration to build a copy of the original Orbiting Carbon Observatory, deeming it a crucial tool to monitor the effectiveness of a new climate agreement.

An existing method that combines atmospheric circulation models with CO2 measurements from ground stations, aircraft and satellites can only estimate land-based emissions "for some large continents," the report said, while GOSAT is not capable of fine enough measurements to monitor treaty compliance. The Japanese satellite cannot discern emissions from a single power plant, for example, something a rebuilt OCO might able to do.

Do we rebuild a fallen satellite?

"The hope was that we would have OCO in space and the observations would have been of high enough precision ... that we could see CO2 pollution plumes from cities," said Ross Salawitch, a University of Maryland professor who served on the OCO's science team.

But even resurrecting the failed U.S. satellite wouldn't be a permanent solution, the NAS panel said, though it could help establish an emissions baseline during a new treaty's crucial early years, and buy time for scientists to design and launch a more sophisticated replacement.

Anna Michalak, a University of Michigan scientist who served on OCO's science team, agreed. "In general, if we don't expand the monitoring network to have continuing measurements of CO2 from space, it's going to be difficult to verify these treaties."

But just how the Obama administration will proceed isn't clear.

NASA hasn't said whether it will build an OCO copy -- an expensive proposition at at a time when the space agency's budget is tight, and a choice to construct a second satellite could force the agency to delay other programs.

Still, Congress appears interested in the prospect. Last weekend, the House and Senate approved a spending bill that would provide up to $50 million to cover the initial costs of rebuilding the OCO. The legislation would set aside $25 million in the fiscal 2010 budget and directs NASA to contribute another $25 million from money its science directorate received in earlier years, which could include stimulus funds.

David Crisp, the NASA scientist who served as project manager for OCO, said that $50 million could help keep scientists and engineers who worked on the original satellite on staff. It might also allow the space agency to purchase key parts that require a long lead time to manufacture.

"This is not what we asked for," he said, "but it is enough to keep us going and make some progress this year."