The next few days could be a "defining moment for the EU" as its leaders consider tough new emissions targets, the EU Commission president says.
Some countries are reluctant to switch from fossil fuel power plants
Jose Manuel Barroso said the EU's credibility hung on its matching words with action to fight climate change.
In Brussels, EU leaders are expected to commit to cutting carbon emissions by 20% by 2020 compared to 1990 levels.
But how this burden will be shared is still subject to argument, as are plans to increase energy market competition.
In the space of little over a year, climate change and what to do about it has shot to the top of the EU's agenda, says the BBC's Jonny Dymond in Brussels.
Across Europe leaders have been stressing the urgency of action - but now they have to make good on all the talk, he says.
Before the two-day summit opened on Thursday, Mr Barroso said: "Are we really credible or not?
"We have been speaking about climate change; now we have an opportunity to take decisions that are really important, not only for Europe but for the rest of the world.
"Only if we take the initiative are we able afterwards to engage the rest of the world," he added.
"It's not just about Europe, it's also that we need the United States, we need China, we need India, we need others to come with us."
It is thought EU leaders may agree to a deeper cut of 30% in emissions by 2020 if other developed and emerging nations, notably the US, India and China, join in.
How a 20% or 30% cut will be achieved is likely to be the focus of lively debate in Brussels.
Poorer Eastern European countries, which are more dependent on heavy industry and carbon-heavy coal, say they will struggle to make the investment in wind farms and solar power necessary to meet the targets.
A European Commission proposal - that 20% of EU energy consumption should be met by renewable sources by 2020 - is therefore thought less likely to be adopted at the two-day summit.
France, which depends heavily on nuclear power, is opposed, saying that, too, should be considered a clean source of energy.
EU leaders hope to limit the damage to the environment
German Chancellor Angela Merkel, who is chairing the talks, said she expected "very difficult negotiations".
There is also likely to be tough talking over Mr Barroso's attempts to sharpen competition in the European energy market, by taking distribution out of the hands of big energy producers.
"I believe that only with that separation we can create more choice for consumers [and] more attractive conditions for investment," Mr Barroso said.
He said he knew that "unbundling" would be unpopular with "those who have some kind of dominant positions".
This will principally affect France and Germany, where big energy companies dominate both production and distribution networks.