The fate of U.S. climate control legislation is in the hands of the Senate, where it faces an uphill climb. Democratic leaders hope to put it to a vote in October.

The House of Representatives narrowly passed its version of a bill to mandate reductions in industrial emissions of carbon dioxide and other greenhouse gases blamed for global warming.

Here are some scenarios on how the battle in Congress could play out in coming months:

* BARBARA BOXER'S 'TWEAKS'

The California Democrat, who chairs the Senate Environment and Public Works Committee, says she's taking the House-passed bill and making some tweaks. Boxer says she will formally introduce her bill in early September.

Environmentalists and others speculate Boxer might opt, as a starting point, for a slightly higher goal for reducing carbon emissions -- say 20 percent below 2005 levels by 2020, instead of the 17 percent in the House bill.

A big question is whether and how Boxer will tweak the initial sale or giveaway of pollution permits utilities and manufacturers will be required to obtain. President Barack Obama wanted all permits to be sold; the House ended up doing nearly the opposite, with 85 percent being given away. Boxer is under pressure from all sides.

Senator John Kerry says the Senate bill will have tougher controls than the House-passed bill to discourage abusive financial market speculation on trading of pollution permits.

Boxer and other committee heads have been given a late September deadline for producing a bill.

* 99 OTHER SENATORS' TWEAKS

The magic number is 60 in the Senate. That's how many votes are needed in the 100-member Senate to rescue bills from opponents' delaying tactics. Reaching 60 votes on the Senate floor will require plenty of deal-making and just about every senator could get involved. Among the possibilities:

-- A lower target for reducing emissions. Some moderates want a 14 percent reduction in carbon output by 2020.

-- Prominent senators like John McCain demand that nuclear energy be included in the cap and trade program lowering carbon emissions and letting companies sell pollution permits to each other.

-- Coal-state senators want more breaks and some of them are in a strong position to influence the legislation.

-- If the Senate manages to pass a bill, differences would still need to be worked out with the House, probably early next year.

* THE RENEWABLES ROUTE

Some senators who are lukewarm about a sweeping climate change bill argue there's not enough time to pass one this year. Their solution: Just pass legislation already approved by the Senate Energy and Natural Resources Committee requiring utilities to generate 15 percent of their electricity by 2021 from renewable sources like solar and wind power. The bill also encourages other clean energy investments.

Senate Democratic leaders would like to couple this with the bigger cap and trade climate bill. But if that's not possible, they could opt for the renewables piece, which also would expand some offshore oil drilling.

* LEAVE IT TO EPA

If Congress fails to pass a climate change bill by the end of the year, when countries from around the world meet in Copenhagen to mull coordinated steps to slow global warming, Obama has a fall-back plan: The U.S. Environmental Protection Agency is poised to go ahead, maybe next March, with its own rules on limiting carbon emissions. Environmentalists think federal legislation would be more effective, but if that's not politically possible, regulation by the executive branch might be the next-best option.

(Reporting by Richard Cowan in Washington, editing by Philip Barbara)

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