By Richard Black
Environment correspondent, BBC News website, Poznan
Al Gore was optimistic about some developing countries' plans
Leaders will have to embrace tougher targets on reducing emissions if they want to prevent dangerous climate change, according to Al Gore.
Speaking here at the UN climate conference, the former US presidential candidate said the "sclerotic" politics of today had to change.
His speech was met with rapturous applause by thousands of delegates.
But environmental groups here criticised the EU's climate and energy package, agreed earlier in Brussels.
EU leadership is widely seen as vital in clinching a global deal at the next UN conference in Copenhagen in a year's time; and campaigners say European governments have concluded a weak agreement.
But Mr Gore, who won last year's Nobel Peace Prize for his work on climate change, said he saw more reasons for optimism than pessimism.
"To those who say it's too difficult to conclude a deal by Copenhagen, I say it can be done, it will be done, let's finish this process."
One of the reasons Mr Gore gave for his optimism was that a number of developing countries have come forward with firm pledges on restraining the rise in greenhouse gas emissions.
China plans to improve energy efficiency
He cited China's plan to improve energy efficiency, Brazil's intention of reducing deforestation and Mexico's adoption of emissions targets.
"Today, no-one is saying that China is standing in the way," he said.
But he said the science mandated moving from a target of keeping atmospheric greenhouse gas levels below 450 parts per million (ppm) - a level that is regarded by many countries as a threshold above which climate impacts are likely to become severe - to 350ppm, which would be much harder to achieve.
Capacity for reducing emissions and for adapting to climate impacts needed to be improved in developing countries - but political capacity also needed to be increased, he said, in the west.
"Political systems in the developed world have become sclerotic. We have to overcome the paralysis that has taken over politics in these countries, rather than spending so much time on OJ Simpson and Paris Hilton and Anna Nicole Smith."
Mr Gore highlighted the Himalayas as an area of the Earth that is feeling climate impacts.
The Himalayas are threatened by climate change
The mountain range acts as a natural reservoir, feeding water into major rivers such as the Brahmaputra, Ganges and Salween.
The potential of climate change to disturb the freshwater supply to more than a billion people makes it, he said, a moral and spiritual issue.
"It is wrong for this generation to destroy the habitability of our planet and ruin the prospects of every future generation."
Some of his other examples of climate impacts, such as the shrinking of Lake Chad, were more contentious.
While climatic factors are believed to play a role in the diminishing amounts of water flowing into the lake, its shrinkage is believed to be more down to local factors such as over-extraction and over-grazing.
But his speech won praise in the corridors afterwards.
Nepal's environment minister Ganesh Shah told BBC News: "What he said was very encouraging in terms of describing climate change in humanitarian terms and as something of great urgency."
"As a minister of Nepal, it is very heart-touching, and I think global attention will now be on the Himalayas."
The UN conference is supposed to conclude on Friday evening here in Poznan, but there are signs that the final sessions may continue through the night - something of a tradition in recent years.
Delegates have been keeping one eye on the EU talks in Brussels, where heads of state from member countries reached agreement on their energy and climate "package", although it has yet to be ratified by the European Parliament.
UN officials describe the EU decision to retain a target of cutting emissions by 20% from 1990 levels by 2020 as a success.
"This is a sign of developed countries' resolve and courage that the world has been waiting for in Poznan," said Yvo de Boer, executive secretary of the UN climate convention.
"This will contribute to propelling the world towards a strong, ambitious and ratifiable outcome in Copenhagen in 2009."
Environment groups, though, were dismayed about a number of concessions offered to industry.
"At present, the offer they have made on cutting their own emissions is woefully inadequate," said Ruth Davis, head of the climate team with the UK's RSPB (Royal Society for the Protection of Birds).
Mr Gore's words, though - and those of US Senator John Kerry on Thursday - may have begun to convince delegates that if EU leadership on climate is faltering, the US under Barack Obama is poised to take over.