Euan Nisbet, a University of London earth sciences professor who has traveled the world testing the air for greenhouse-gas pollution, makes his way to a rocky outcropping on Hong Kong Island. He takes out a pump connected to a thin tube and a plastic bag to capture traces of the wind. 'This is a good day for collecting samples,' says Nisbet, looking out to sea. 'There's a good, strong breeze blowing in from the mainland. It's the breath of China.' Nisbet says the world puts too much faith in government estimates of carbon dioxide, methane and other heat-trapping gases blamed for climate change. Companies and countries base emissions calculations on the raw materials that go into a factory or power plant; they don't check the pollution that comes out. 'It's like going on a diet without weighing yourself,' explains Ray Weiss, a geochemistry professor at the Scripps Institution of Oceanography in California. 'We're counting all the calories, believing the bottom-up calculations are right and not making any effort to test by getting on the scale.' As the world turns to climate treaties and emissions- trading markets to tame global warming, scientists and regulators are clashing over a key question: How do we measure the pollution we're trying to reduce? Companies use bottom-up calculations and report their emissions estimates based on inputs: how much coal a plant burns or how much oil a factory consumes. Countries tabulate these estimates and add such nationwide figures as vehicles and even methane-belching sheep within their borders. Nisbet, Weiss and other researchers say this bottom-up approach doesn't reveal what's happening in the air. They're sounding an alarm that greenhouse gases measured in the atmosphere can be double estimates on the ground. In the case of sulfur hexafluoride (SF6), countries report that output has plummeted since 1995 based on counts of equipment and other things that can release the gas. Scientists say measuring the air shows levels have surged since at least 2000. Getting the measurements right is crucial because SF6 traps 23,900 times more heat per molecule than carbon dioxide and lingers for up to 3,200 years, the Environmental Protection Agency says. 'These gases lead to climate change for centuries or millennia, so there's a finite amount you can ever put in the atmosphere,' says Stephen Pacala, director of Princeton University's environmental institute. 'For every release, you're taking something from some subsequent person.' Making sure the numbers add up is important for a more immediate reason: They underpin the only international climate treaty that sets mandatory limits for greenhouse gases. That 192-country pact, known as the Kyoto Protocol, set emissions targets for 38 of the countries that signed on. This year, nations will gather in Cancun, Mexico, beginning Nov. 29 for annual United Nations-led climate talks. Today, $141 billion worth of credits that help countries meet their Kyoto goals are changing hands in global emissions markets. And business is booming in offsets -- the right for countries and companies that still pollute too much to claim credit for green projects elsewhere. These efforts are based on bottom-up calculations being accurate. 'We have to get the emissions numbers right early on,' says Shakeb Afsah, a former World Bank environmental economist who founded Performeks. 'We've seen how the cumulative effect of small errors that accumulated in the financial markets have blown up as little cracks turned into fault lines.' Traders and regulators say Europe's carbon market, called the Emissions Trading System, and the United Nations' offset market, called the Clean Development Mechanism, are making a dent in greenhouse emissions. The global carbon market may surge to as much as $1.4 trillion worth of transactions by decade's end, Bloomberg New Energy Finance says. The 15 countries that were European Union members at the time Kyoto was adopted say they've surpassed their goal of trimming emissions in the five years to 2012 by an average of 8 percent compared with 1990 levels. As of 2008, the now 27-nation EU reported to the United Nations that emissions had dropped 11.3 percent below 1990 amounts. Jean-Pascal van Ypersele, vice chairman of the U.N.'s Intergovernmental Panel on Climate Change, says companies and countries use tried-and-true formulas to calculate emissions. 'Burning 1 kilogram of carbon is going to provide 3.7 kilograms of CO2; that's the law of chemistry,' van Ypersele says. 'As long as you know how much carbon is present in the fossil fuel you burn, you automatically know the amount of CO2.' If there are discrepancies between emissions reduction figures, it may be because countries that calculate them don't all follow one set of rules, van Ypersele says. The Kyoto Protocol requires just 38 developed countries to submit emissions numbers using stringent guidelines. The United States has refused to ratify Kyoto. And fast-growing polluters including China and India can use looser calculations that aren't audited. 'That's not a technical problem,' van Ypersele says. 'It's a political problem.' Nisbet says bottom-up calculations have one big selling point: They're consistent. Measuring the air isn't as precise or as easy. 'You don't always get the same result,' he says. 'But it's what's really there. It's there because we can measure it.' --With assistance from Ewa Krukowska in Brussels and John Duce in Hong Kong. Adapted from Bloomberg Markets magazine. 