A UN summit on climate change will see the world "change course" and begin to seriously tackle global warming, UN climate envoy Mary Robinson said Wednesday.

More than 120 leaders are to attend the summit on Tuesday called to inject new momentum in efforts to address climate change ahead of a crucial conference in Paris next year.

"The message from the climate summit and the message going forward to Paris is that it's not business as usual with a little bit of green attached," Robinson told AFP in an interview.

"It's changing course. It's taking decisions that will bring us back to a trajectory that will give us a safe world that stays below 2 degrees Celsius. We are not on course for that. We need to change course."

The United Nations is seeking to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, but scientists say current emission trends could hike temperatures to more than twice that level by century's end.

US President Barack Obama is to outline his vision for reining in greenhouse gas emissions, but key polluters China and India are sending lower-level representatives.

Robinson dismissed suggestion that the absence of leaders from China and India, the world's number one and number three polluters, had dealt a blow to the summit's ambitions.

"What we see is that China is being represented at a very serious level. The vice premier is third in line and the highest authority on climate change and development. So for the Chinese he is not a lesser person."

China is sending Vice Premier Zhang Gaoli while India's new much-awaited Prime Minister Narendra Modi is dispatching his environment minister.

"We will have the largest gathering of heads of state ever on the climate issue at a time when the world knows that we need to move urgently," said Robinson.

Momentum and surprises

The former Irish president was appointed by UN Secretary General Ban Ki-moon in July to shepherd preparations for a summit seen as key to avoiding a repeat in Paris of the failure of the 2009 Copenhagen gathering.

The summit is being held on the eve of the high-level UN General Assembly meeting where world leaders will try to agree on a common approach to address the Islamist threat in Iraq and Syria, the Ebola crisis, the Israeli-Palestinian conflict among other issues.

"The climate summit hopefully will generate quite a bit of momentum and some surprises and a sense that we can actually get on top of this big issue," said Robinson.

"It is the most urgent pressing problem facing the world. Despite all the other problems, this is the biggest."

While the summit is not a formal negotiating session -- those are held under the UN Framework Convention on Climate Change (UNFCCC) -- Ban has urged leaders to outline their action plan and to commit to a deal on reducing greenhouse gas emissions in Paris in December 2015.

Robinson said the summit will also yield concrete announcements, including a declaration on carbon-pricing and partnerships on forestry, green bonds and climate-smart agriculture.

"There will be a declaration on carbon-pricing that a significant number of states and major companies will sign up to, saying we have got to put a price on carbon," said Robinson.

There will not be a single carbon price, she added, but the declaration will help spur the global movement toward putting a price on carbon dioxide emissions.

Robinson will take part in the "People's Climate March" on Sunday that organizers hope will draw 150,000 demonstrators on the streets of New York to demand action on climate change.