Tony Blair has said time is running out for tackling climate change.
Mr Blair wants to use Britain's G8 presidency to push for change
The prime minister, speaking ahead of a major speech on the issue, said he had been shocked by scientists' warnings about the growth of the problem.
He told BBC Radio 1: "We will start to notice within reasonably short periods of time real difficulties."
Environmental campaigners fear Mr Blair's speech on Tuesday evening will contain no firm policies to combat global warming.
But media reports say he may call for measures to control aviation pollution.
The prime minister will set out his vision to the Prince of Wales' Business and the Environment charity on Tuesday evening in London.
On Monday, Tory leader Michael Howard accused Mr Blair of squandering the chance to lead efforts against climate change.
The prime minister is expected to respond by saying he wants to use Britain's presidency of the G8 next year to push the major industrialised nations towards environmentally sound policies.
He will warn of forecasts that sea levels could rise by another 88cm by the end of the 21st Century, threatening 100 million people around the planet.
He will echo the government chief scientist's warnings that "unchecked climate change has the potential to be catastrophic in both human and economic terms".
On Tuesday morning, Mr Blair told a private meeting with environmental professionals: "It's a huge issue but time is running out.
"When I had my last presentation from government scientists on this I was shocked about how the speed of this is gathering."
Chief scientist Sir David King said earlier this year that climate change was a bigger problem than the threat of terrorism.
The prime minister told the meeting he did not think the US Senate would ratify the Kyoto agreement on climate change.
But at the G8 summits, which Britain chairs next year, governments had to accept the scientific evidence explicitly and say how to take the process forward.
Parts of the UK have been affected by extreme weather this year
Mr Blair said he wanted to develop technologies such as solar energy.
He added: "The problem for a lot of people as individuals, is that it's not that they
don't care, they just don't know how to implement environmentally sustainable
"They get confused about whether they are really making a difference or not,
and is it just a waste of time."
After the meeting, he told Radio 1's Newsbeat: "This is a serious issue and it is going to get worse... because every year we are piling more green house gases in to the atmosphere...
"There are whole communities that are going to be affected. The time to act is now."
BBC environment correspondent Sarah Mukherjee said Mr Blair's speech risked disappointing environmental campaigners if no specific policies were outlined.
She said one campaigner who worked closely with the government had said he believed the prime minister wanted to stop climate change - but was not convinced Mr Blair knew what that meant in practice.
Some reports say the prime minister will push for the aviation industry to be brought within the EU's emissions trading scheme.
The prime minister may also face pressure to take action to push US President George Bush to take climate change seriously.
The US has yet to sign up to the Kyoto Protocol under which industrialised nations agreed to limit their greenhouse gas emissions.
Liberal Democrat environment spokesman Norman Baker said if Mr Blair was sincere he would scrap government plans for more roads and airport terminals.
Mr Baker asked: "How many hurricanes and tornadoes will it take for the prime minister to realise that paying lip service to the environment is just no use?"