The Copenhagen summit is set to do "real business" because of President Obama's decision to attend its crucial final stage, says Climate Change Minister Nick Smith.
Dr Smith said the summit was rapidly gaining momentum, with 105 world leaders now signed up for the final day.
The United States President had originally planned to stop on his way to a Nobel Peace Prize gathering in Norway this week, but announced at the weekend he would now go for the world leaders' meeting on December 18.
This is the stage Prime Minister John Key will attend, and is when most of the final top-level negotiations on greenhouse gas reductions are expected to be be hammered out.
Indian Prime Minister Manmohan Singh yesterday said he would also be attending the world leaders' meeting in what was deemed another encouraging sign.
Dr Smith said Mr Obama's rethink was the leading factor in the increasing optimism for the summit, which begins today.
A spokesman for Mr Key said he welcomed Mr Obama's u-turn, adding that if progress was to be made, it required the attendance of the major emitters.
The United States is the second biggest emitter behind China, while India is the fourth biggest.
Mr Key decided to attend Copenhagen only last week, after earlier saying he would go only if a deal was close.
Dr Smith said the mantra of his negotiating team was "for New Zealand to do its fair share" to stop global warming.
The first week of the fortnight-long summit will mainly involve officials, before environment ministers arrive next week.
Dr Smith said the growing numbers of world leaders signing up for the final stage meant there would be "enormous pressure" to deliver a deal.
He said New Zealand's ambition was for the summit to end with a high-level political statement with a strong commitment to working through 2010 on the details of an international treaty.
He said the impact on New Zealand depended on how much developed countries like New Zealand agreed to reduce emissions, how much money was committed to a global fund to help developing nations reduce emissions, and the extent to which developing countries agreed to constrain their emissions.
