The Conservative government has consistently downplayed expectations for the Copenhagen conference, where it was hoped the global community would agree on a post-2012 accord to reduce greenhouse-gas emissions.
Earlier this month, Danish Prime Minister Lars Lokke Rasmussen formally invited 191 government leaders and heads of state to Denmark to push along the negotiations for a successor treaty to the Kyoto Protocol.
Other Western leaders, including British Prime Minister Gordon Brown and Australia's Kevin Rudd, quickly got on board.
But the entreaties were rebuffed by Harper at least up until Washington announced that President Barack Obama would be stopping in on the conference next month. Chinese Premier Wen Jiabao has also announced his participation.
Within 24 hours, Harper spokesman Dimitri Soudas announced the prime minister had decided to attend Copenhagen because a "critical mass" of leaders is now going.
The announcement came as the prime minister boarded a plane for the Commonwealth Conference in Trinidad and Tobago, leaving follow-up questions on the decision hanging at 35,000 feet.
Obama will attend Day 3 of the December 7-18 conference, not the leaders' segment slated for the final two days. That has some environmental activists criticizing what they say will simply be an Obama photo-op in Copenhagen.
It was not immediately evident when Harper will attend the conference.
But his participation may signal a change in tone from the deep skepticism he expressed recently at an APEC summit in Singapore.
Harper told reporters at APEC that the assembled leaders shared "a pretty strong consensus... that the countries of the world remain a long way from a binding, legal treaty on climate change."
He also said it may be time to "get our negotiators out of this morass of hundreds of pages and thousands of brackets of (negotiating document) text and into looking at the big picture and coming to some agreement on some big-picture items."
But with UN Secretary-General Ban Ki-moon and Rasmussen both attending the Commonwealth summit to push the climate issue, Harper may have felt pressure to put a more tangible, less atmospheric face on Canada's response.
A number of provincial environment ministers are committed to attending Copenhagen and their participation threatened to embarrass the Harper government internationally.
"We are not out to embarrass the federal government at an international setting," Ontario Environment Minister John Gerretsen told the media.
"But on the other hand, we also want to join with other like-minded subnationals that, in effect, have been more proactive on the whole climate-change agenda over the last couple of years."