The new head of environmental group Greenpeace, South African Kumi Naidoo, says he will prioritise the impact of climate change on the world's poor.
A BBC reporter in Johannesburg says it reflects his background as a anti-poverty and anti-apartheid campaigner.
In a BBC interview, he also criticised the US president for failing to make the fate of the earth a priority.
He said Barack Obama no longer reflected the urgency about the planet he had during his election campaign.
Mr Naidoo is the first African to head Greenpeace since it was founded in 1971.
The group has perfected the art of using high-profile media events to exert pressure on politicians and big business.
The BBC's Jonah Fisher in South Africa say for Greenpeace the appointment of Mr Naidoo marks a continuing shift in focus from whaling and nuclear testing to climate change.
Mr Naidoo said human existence on the planet was "fundamentally under threat" by climate change.
"We are seeing every year now, 300,000 more people dying from what can be described as climate-related impacts," he told the BBC.
"We're talking about climate refugees and so on, connecting all of those existing focus on the environment with human concerns, is both the right thing to do."
He said Mr Obama had lost his urgency on the issue.
"During his election campaign... every single speech that he gave, he talked about a planet in peril, referring to climate change," Mr Naidoo said. "We all understood that he 'got' it."
On Sunday, Mr Obama and other world leaders meeting in Singapore said it would not be possible to reach a climate change deal ahead of next month's UN conference in Copenhagen.
Mr Naidoo said he was "quite disappointed" that Mr Obama had not yet announced clearly his intention to attend the summit.
At the two-day Asia-Pacific summit, world leaders dropped a target to halve greenhouse gas emissions by 2050, which was outlined in an earlier draft.
"Anything short of a binding treaty in Copenhagen must be read as a failure of leadership on the part of the political class," AP news agency quoted him as saying on Monday.
"We can't change the science. The science is clear. We have to change the politics. If we can't change the politics, then we have to put our energies into changing the politicians."