Aug 26 (Reuters) - More than 100 nations will meet in Vienna on Aug 27-31 to discuss ways to widen and extend the U.N.'s Kyoto Protocol for fighting climate change after 2012.

Here are some frequently asked questions about Kyoto:

* WHAT IS THE KYOTO PROTOCOL?

-- It is a pact agreed by governments at a 1997 U.N. conference in Kyoto, Japan, to reduce greenhouse gases emitted by developed countries to at least 5 percent below 1990 levels by 2008-12. A total of 175 nations have ratified the pact.

* IS IT THE FIRST AGREEMENT OF ITS KIND?

-- Governments agreed to tackle climate change at an "Earth Summit" in Rio de Janeiro in 1992 with non-binding targets. Kyoto is the follow-up and is the first binding global agreement to cut greenhouse gases.

* IS IT LEGALLY BINDING?

-- Kyoto has legal force from Feb. 16, 2005. It represents 61.6 percent of developed nations' total emissions. The United States, the world's biggest source of emissions, came out against the pact in 2001, reckoning it would be too expensive and wrongly omitted developing nations from a first round of targets to 2012.

* HOW WILL IT BE ENFORCED?

-- Countries overshooting their targets in 2012 will have to make both the promised cuts and 30 percent more in a second period from 2013.

* DO ALL COUNTRIES HAVE TO CUT EMISSIONS BY FIVE PERCENT?

-- No, only 35 relatively developed countries have agreed to targets for 2008-12 under the principle that richer nations are most to blame. They range from an 8 percent cut for the European Union from 1990 levels to a 10 percent rise for Iceland.

* WHAT ARE THESE 'GREENHOUSE GASES?'

-- Greenhouse gases trap heat in the earth's atmosphere. The main culprit is carbon dioxide, produced largely from burning fossil fuel. The protocol also covers methane, much of which comes from agriculture, and nitrous oxide, mostly from fertiliser use. Three industrial gases are also included.

* HOW WILL COUNTRIES COMPLY?

-- The European Union set up a market in January 2005 under which about 12,000 factories and power stations are given carbon dioxide quotas. If they overshoot they can buy extra allowances in the market or pay a financial penalty; if they undershoot they can sell them.

* WHAT OTHER MECHANISMS ARE THERE?

-- Developed countries can earn credits to offset against their targets by funding clean technologies, such as solar power, in poorer countries. They can also have joint investments in former Soviet bloc nations.

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