The much-anticipated United Nations climate summit officially starts in Paris today with a growing sense that it may not deliver any kind of hammer to force countries to curb their emissions, other than a moral one.

Global leaders from 150 countries are there for Day 1 of the conference to provide some momentum to negotiators attempting to craft an agreement to keep global temperatures from rising more than 2 C.

Reporters at a news conference hosted by the Canadian government leading up to the summit got a taste of what the rest of the world might be thinking after the international negotiations wrap up in two weeks.

Canadian Environment and Climate Change Minister Catherine McKenna said the world needs to have a new international deal to legally ensure that countries commit to a target to cut greenhouse gas emissions and agree to improve on their target every five years.

NEW YORK, NY - NOVEMBER 29: People hold a banner as they take part in a global climate march near City Hall on November 29, 2015 in New York City. The protest is part of an international weekend of action that is being mobilized to support climate activists in Paris whose massive march has been prevented by French authorities due to the recent attacks .Around 570,000 people have already marched in 2,300 events in 175 countries around the world. (Photo by Kena Betancur/Getty Images) (Kena Betancur/Getty Images)

But the sticky part is how a UN climate treaty can actually make them do it.

"There will be some parts to reflect the reality that certain countries have concerns with all aspects being binding, that maybe not all aspects will be binding," McKenna said. "But it's really important that we have everyone at the table and come up with an agreement — that's really critical."

That sparked a barrage of questions from reporters about how a treaty can be legally binding and not binding at the same time. And it's a question that is bound to come up as negotiators get down to the business of crafting a new climate treaty that will include 190 countries.

Negotiators want U.S. on board

U.S. President Barack Obama, who is in Paris for the summit, is reluctant to sign any climate treaty that would impose legally binding cuts on the United States, because Congress won't support it.

That happened in 1997 when the U.S. signed the Kyoto climate accord but Congress never ratified it, resulting in an agreement that didn't apply to one of the world's largest carbon emitters.

Canada signed on to Kyoto, but then announced in 2011 that it couldn't meet the targets and pulled out of the treaty.

Negotiators are anxious to avoid a deal that is ultimately unworkable and excludes large polluters.

"This was attempted at the UN, and it didn't work," said Steven Guilbeault, who is with the Quebec-based environmental group Équiterre.

"The UN can't go to Washington or Ottawa or Paris and police what countries are doing. At the end of the day, it is a moral obligation these countries have to do what they say they are going to do.

French President Francois Hollande and Prime Minister Justin Trudeau attend a meeting at the Élysée Palace in Paris, France, November 29, 2015 ahead of the World Climate Change Conference. (Thibault Camus/Reuters)

"It's our obligation as citizens of those countries to put pressure on our governments to make sure they are delivering the goods, that we are reducing emissions."

However Guilbeault, who is a veteran observer of at least 15 UN climate summits, is relatively optimistic about getting a deal in Paris, and he's giving some credit to Canada's new approach.

Prime Minister Justin Trudeau has vowed Canada will play an active role at the summit to "convince and cajole" countries to sign on. Last week, Trudeau pledged to spend $2.65 billion over the next five years to help developing countries adjust to climate change.

"We're emerging from some dark ages," Guilbeault said. "I run into colleagues from Germany or Asia or Latin America and they're thrilled, they are so happy that Canada is back at the table, that we are saying we'll do our fair share."