Uncertainty is growing over the future of the Kyoto Protocol, the first legally binding treaty to cut greenhouse gases blamed for heating up the planet.

Nearly 40 industrialized nations -- all except the United States -- are supposed to meet agreed emissions targets during the pact's 2008-12 first phase but some countries are way behind their targets and will likely miss them.

The disappointing outcome from December's climate talks in Copenhagen has cast a cloud over whether Kyoto will be extended for a second period from 2013, raising questions over the shape of a future legally binding climate pact that all nations can agree on.

Here are some questions and concerns 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 by an average of 5.2 percent below 1990 levels during 2008-12.

A total of 189 nations have ratified the pact. Developing nations do not have binding emissions targets under Kyoto but are encouraged to take voluntary steps to curb the growth of carbon dioxide pollution from power stations, cars and industry.

* IS IT LEGALLY BINDING?

Kyoto has legal force from February 16, 2005. It represents more than 60 percent of developed nations' total emissions.

The United States, the world's second largest greenhouse gas emitter after China, decided against implementing Kyoto 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.

* WILL ALL COUNTRIES MEET THEIR TARGETS?

Unlikely. Kyoto targets for 37 rich nations that are committed to cut emissions range from an 8 percent cut for the European Union from 1990 levels to a 10 percent rise for Iceland. But some countries, including Canada and Spain, say they will miss their targets by a wide margin, despite the threat of penalties.

This has led to criticism that Kyoto is a failure, raising doubts nations will meet much tougher targets for the second compliance period from 2013.

* SO WILL KYOTO FADE AWAY?

There's a real risk of this happening. The current U.N. climate talks remain far apart on deciding the shape of a future, broader, climate pact that extends the fight against climate change.

One idea is to extend Kyoto into a second period and create another climate treaty, perhaps based on the Copenhagen Accord, which wasn't formally adopted by the U.N. meeting in Denmark in December, and is therefore not legally binding.

Big developing nations India and China, while broadly supporting the Accord, firmly back Kyoto and the current U.N. talks to try to extend it and do not want to turn the Accord into a legal text. The United States refusal to join Kyoto and the failure of the U.S. Congress to pass emissions targets is another big uncertainty.

WHAT ABOUT KYOTO'S CARBON MARKET MECHANISMS?

A key part of Kyoto is a scheme that allows rich nations to meet a part of their emissions targets by buying carbon offsets from clean-energy projects in developing nations.

Called the Clean Development Mechanism (CDM) it has generated billions of dollars in investment in wind farms, hydro power plans, solar water heaters and biomass power generators in poorer nations, creating valuable offsets that can be traded for profit or used to meet greenhouse gas targets.

If U.N. climate talks drag on and Kyoto isn't extended, a major issue is what happens to the CDM after 2012, since the whole notion of binding emissions caps drives the market for the offsets.

(Writing by Alister Doyle and David Fogarty; Editing by Ed Lane)

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