(For an analysis of the policy, click [ID:nPEK325868])

BEIJING, Sept 25 (Reuters) - Chinese President Hu Jintao pledged this week to cut the country's "carbon intensity" over the decade to 2020, Beijing's first acceptance that it must control emissions growth.

Following are questions and answers about carbon intensity.

WHAT IS CARBON INTENSITY?

Carbon intensity is the amount of carbon dioxide emitted for each unit of economic output. Often carbon dioxide is measured in tonnes, while gross domestic product (GDP) in a local currency represents economic output, but any units can be used.

Other greenhouse gasses like methane are added to the total by calculating the amount of carbon dioxide that would have the equivalent global warming potential.

Emissions are usually calculated indirectly, through looking at inputs such as the amount of coal burnt in a power plant, rather than attempting to capture and weigh carbon dioxide gas.

WHY HAS CHINA CHOSEN CARBON INTENSITY?

Cutting carbon intensity allows China to meet international demands for it to count and curb its emissions, without giving up its insistence that development must come first while millions of Chinese citizens are still living in poverty.

By agreeing to control its emissions China will also pave the way for a carbon market, as accurate measurements of emissions are a vital cornerstone for any market in permits to emit.

However, if China's economy expands too fast, even massive improvements in carbon intensity may not be enough to contain dangerous increases in emissions.

A carbon intensity figure can be worked out for anything from a single factory to an entire country, and China is likely to give out targets to regions and enterprises as well as setting an overall national goal.

WHY NOT AN EMISSIONS CAP?

China has repeatedly rejected calls to commit to a peak year or level of emissions because of its worries such a target could hinder efforts to lift millions more out of poverty.

A cap could be a logical next step for Beijing if it can meet its initial carbon intensity targets.

Some Chinese experts have said emissions could peak around 2030-2035 with enough spending and the right policies, but officials have been more wary of such ideas. [ID:nPEK276833]

Under the Kyoto Protocol and the U.N. framework which governs efforts to tackle global warming, developing countries do not have any binding obligations to cap emissions.

HOW IS 'CARBON INTENSITY' DIFFERENT FROM 'ENERGY INTENSITY'? China already has a goal to reduce energy intensity 20 percent between 2006 and 2010, but that target measures only the amount of fuel needed to generate each dollar of economic output.

It does not differentiate between different types of power, or encourage controls on non-energy related emissions.

Every gram of fuel conserved brings a corresponding reduction in emissions, but further savings can be notched up by switching from fossil fuels to power sources such as nuclear or renewables.

A factory seeking to meet a carbon intensity target could choose whether to boost efficiency or invest in renewables like wind or solar to provide its power.

Carbon targets could also provide an incentive for emissions cuts which make little sense in energy terms alone, such as an expensive system to capture and burn methane -- which has a much greater warming effect than carbon dioxide -- in a coal mine.

HOW DOES CHINA'S CURRENT CARBON INTENSITY STACK UP?

According to figures published by the United States Department of Energy, China in 2006 emitted 2.85 tonnes of carbon dioxide from fossil fuels for every $1,000 of Gross Domestic Product (GDP), around 15 percent lower than a decade earlier.

In comparison, the United States in 2006 emitted 0.52 tonnes of carbon dioxide for every $1,000 of GDP, while Switzerland produced 0.17 tonnes, and impoverished Chad just 0.07 tonnes.

For further comparisons see: http://www.eia.doe.gov/pub/international/iealf/tableh1gco2.xls (Reporting by Emma Graham-Harrison, Editing by Dean Yates)

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