U.S. President George W. Bush stood his ground on greenhouse gas emissions as he went into a meeting Wednesday with Chancellor Angela Merkel — but predicted consensus on a future framework for climate change at this week's Group of Eight summit.

As leaders headed to Heiligendamm, and Bush and Merkel worked to hash out their differences on climate change, riot police struggled to keep thousands of protesters from reaching the summit site in picturesque, coastal northern Germany.

Officers fired a water cannon to force demonstrators, some hurling stones at police, back from the 7.5-mile fence built to protect the eight world leaders gathering Wednesday for three days of talks.

Other protesters blocked roads from the airport and the small-gauge railway used to transport journalists to Heiligendamm, forcing organizers to ferry them to the site by boat, police said.

Merkel — who has made climate change the centerpiece of Germany's G-8 leadership — was using the hours before the start of the summit to champion agreement at the summit.

She is pushing specific targets for reduction of the carbon emissions believed to cause global warming, including a "two-degree" target under which global temperatures would be allowed to increase by no more than 3.6 degrees Fahrenheit before being brought back down.

Practically, experts have said that means a global reduction in emissions of 50 percent below 1990 levels by 2050. Merkel supports a global carbon-trading market as one tool.

A May report from a U.N. network of more than 2,000 scientists estimated that the world must stabilize the amount of greenhouse gases in the atmosphere within eight years to keep global temperatures from spiking to disastrous levels.

The U.S. has now acknowledged that global warming is a serious problem that must be addressed, and that doing so requires a global goal. Europe and others have come around to Washington's view that no solution is viable without the participation of developing energy guzzlers such as China, India and Brazil, and that economic growth can't be sacrificed for progress on climate.

Bush told reporters before his meeting with Merkel that he would not give ground on global warming proposals that would require mandatory caps on greenhouse gas emissions. Instead, he backed his own proposal to have the United States and other nations that spew the most greenhouse gases meet and — by the end of next year — set a long-term strategy for reducing emissions.

Bush wants to bring India, China and other fast-growing countries to the negotiation table. He envisions that each country will set goals on how they want to improve energy security, reduce air pollution and cut greenhouse gases in the next 10 to 20 years.

"The United States can serve as a bridge to help find a solution," Bush said.

He predicted that leaders would produce a consensus for a post-Kyoto framework after the landmark treaty expires in 2012.

British Prime Minister Tony Blair said he believed leaders would reach agreement on cutting greenhouse gases. In an interview with the Guardian newspaper, Blair said the United States was "on the move" over global warming.

"The key elements of this are an acceptance that the climate is changing in a dangerous way as a result of human activity. Secondly, we need a global agreement that includes all the main players, including China and America, and at the heart of that there has to be a global target for a substantial cut in emissions," Blair was quoted as saying. "I believe it is possible to get all that way."

Leaders also were expected to confront Russian President Vladimir Putin about his country's increasingly frosty relations with the United States and Europe. The days leading up to the summit were dominated by jarring rhetoric from Putin over U.S. plans to base a new missile defense system in nearby Czech Republic and Poland.

Bush discounted Putin's warning.

"Russia is not an enemy," he told reporters. "There needs to be no military response because we're not at war with Russia. Russia is not a threat."

He cited Iranian President Mahmoud Ahmadinejad's declaration that it was "too late" to stop Iran's nuclear program as justification for basing a shield in Europe.

"Therefore, let's build a missile defense system," Bush said, adding that it was time to return to the U.N. Security Council to tighten pressure on Iran to give up its suspected weapons program.

Blair said he expects to have a frank conversation with Putin at the summit. The missile defense plan "has always been about the danger of rogue states" — not Russia, he told the British Broadcasting Corp. in an interview aired Wednesday.

"The truth of the matter is that, for all sorts of reasons, it is not something that is really about Russia at all and yet suddenly it is put up by Russia in this way, in quite a confrontational way," he said.

"Now I think the sensible thing, and this is what I'll do certainly when I meet President Putin, is just to have a frank conversation about the state of the relationship between not simply Britain but Europe and Russia," he said before departing for Heiligendamm for his farewell G-8 summit.

Merkel has fretted about the dispute's effect on U.S.-Russia relations and beyond but has had her own frosty meeting with him, criticizing his crackdown on political opponents. They are to meet again Wednesday.

The other G-8 leaders attending the three-day summit are Italian Prime Minister Romano Prodi, Japanese Prime Minister Shinzo Abe, French President Nicolas Sarkozy and Canadian Prime Minister Stephen Harper. European Commission President Jose Manuel Barroso is representing the European Union.

Summit talks also are expected to include discussion about aid to Africa, tensions in the Middle East and G-8 cooperation with emerging countries.