The new Congress appears poised to impose tough regulations on greenhouse-gas emissions, but it's unclear how the government will monitor these climate-changing gases to ensure emitters comply with the law.

A number of leading congressional Democrats plan to introduce legislation this year that would establish a cap-and-trade scheme, whereby the government would cap emissions at a specific level and then allot businesses and other entities shares to emit a certain amount. If an organization cut emissions beneath its share, it could then sell the rest of its allowance to another entity.

But such a program can't be effective unless the government knows who's emitting and how much, Rep. Brian Baird, D-Wash., said Tuesday at a hearing on the topic in the House Subcommittee on Energy and the Environment.

"In order to evaluate programs -- either mandatory or voluntary -- for controlling greenhouse-gas emissions, we must be able to track emissions accurately," Baird said.

The federal government has had some experience with tracking emissions in the past. More than a decade ago Congress passed a provision in the Clean Air Act to reduce sulfur dioxide emissions, which contribute to acid rain. Since 1995 the government has operated a cap-and-trade system for the pollutant and monitored emitters to make sure they comply. The program succeeded in lowering emissions, but an equivalent program for greenhouse gases will be much more complicated, Baird said.

"There are many more entities that need to be monitored under a greenhouse-gas control program," he said.

The European Union implemented a cap-and-trade system for carbon dioxide in 2005, but the U.S. Government Accountability Office released a report last November pointing out a number of problems the EU has encountered in regards to tracking emissions. The report found the system's baseline for emission levels was based in large part on projections instead of hard data from specific facilities. As a result, it's been hard to judge whether the system is actually lowering emissions, the report said.

One way to simplify the process is to include only upstream facilities -- those that actually produce fossil fuels -- in the cap-and-trade program, said John Stephenson, director of natural resources and the environment at the GAO.

"The further upstream you go, the easier it is," Stephenson said.

That's because there are fewer entities to monitor. For instance, although Stephenson estimates an upstream cap-and-trade system would include about 3,000 emission sources, a program covering downstream energy users, like power plants, would encompass more than 10,000 entities and would probably only include about half of total U.S. emissions.

However, monitoring emissions at the business level encourages greater efficiency, said Jill Gravender, vice president for policy at the Climate Registry, a non-profit organization that helps its 316 members record and report their greenhouse-gas emissions.

"Having the company carbon footprint is important because it gives companies an opportunity to manage their emissions," Gravender told representatives. "You can't manage what you don't measure."

However, if thousands of companies are suddenly expected to monitor and regulate their emissions under a cap-and-trade program, it's important to also require third-party verification, said Rob Ellis, greenhouse-gas program manager at Advanced Waste Management Systems, which provides professional auditing services to verify organizations' greenhouse-gas emissions records.

"The consequences of and opportunities for fraud are high," Ellis said, urging policymakers to be on the lookout for verification companies that have a vested interest in the organizations they audit.

In addition to deciding what facilities to monitor, policymakers will have to determine which gases to include in a cap-and-trade system. Although witnesses at Tuesday's hearing said it's possible right now to scientifically track and monitor carbon dioxide -- the only gas included in the EU program -- there's less certainty in how to verify levels of other greenhouse-gas emissions like methane and nitrous oxide.

Methane, for instance, comes largely from animal emissions, which are harder to track and quantify than those leaving a power plant smokestack.

That concerned some of the policymakers at Tuesday's hearing, including Rep. Roscoe Bartlett, R-Md., because methane has 20 times the warming potential of carbon dioxide and nitrous oxide is 300 times as potent.

Because of this, some companies are already trying to track all of their greenhouse-gas emissions. Among them is Waste Management, a major trash- and waste-removal and recycling company based in Houston.

In addition to the emissions from company vehicles and the energy used to power its facilities, Waste Management is also tracking its landfill emissions, including methane. This has been difficult to figure out, said Leslie Wong, director of the company's greenhouse-gas programs.

"A broadly accepted protocol does not exist," she told representatives.

As a result, she urged policymakers to carefully consider which gases they include in any legislation aimed at cutting emissions.

Monitoring emissions isn't just a domestic issue.

Some groups are also concerned that, without scientifically based monitoring systems, international attempts to lower greenhouse-gas emissions will be impossible to implement. This is a particularly hot topic right now as the world gears up to replace the Kyoto Protocol, the reigning global agreement on emissions.

"There are wide disparities between countries on the quality, transparency and consistency of emissions reporting," Karen Harbert, president of the Institute for 21st Century Energy at the U.S. Chamber of Commerce, told United Press International. "Serious progress needs to be made on the reporting, monitoring and verifying of emissions."