UPDATE 3-U.S. Senate panel approves Democratic climate bill

* Senator Boxer hails "great signal" to Copenhagen

* Republicans boycott committee vote

* Senator Baucus says opposes 20 pct carbon cut target (Adds reaction in paragraphs 6-9)

By Richard Cowan

WASHINGTON, Nov 5 (Reuters) - A controversial climate change bill cleared its first hurdle in the U.S. Senate on Thursday, allowing President Barack Obama to tout progress in the run-up to next month's global warming talks in Copenhagen.

Democrats on the Senate Environment and Public Works Committee ignored a Republican boycott and used their majority to approve the legislation that would require U.S. industry to reduce emissions of carbon dioxide and other greenhouse gases 20 percent by 2020, from 2005 levels.

"I think this is a great signal for Copenhagen that there's a will to do what it takes to advance this issue," committee Chairman Barbara Boxer told reporters after her panel voted.

The committee vote also came as international negotiators held a contentious climate change meeting in Barcelona, their final session before the Copenhagen summit starts Dec. 7.

But Democrats are likely to fall far short of their goal of passing legislation in the full Senate before Copenhagen as Boxer's bill lacks enough support for full approval.