Environmentalist and former US vice president, Al Gore, said the United Nations report on climate change released on Friday makes it a near certainty that global warming is real and needs to be addressed immediately. He took this message to Silicon Valley on Friday, urging businesses to push clean technology.

"The degree of certainty, which was already very high, is now as close to certain as scientists are ever willing to say something is certain," Gore — also a member of the board at Apple — said in an address to Silicon Valley business and community leaders in San Jose, California.

The International Panel on Climate Change's fourth report on the subject since 1990 says human activity, including the burning of fossil fuels, is the primary cause of global warming.

Gore implored his audience of 1,500, many of them executives and employees of technology companies in Silicon Valley, to use their collective knowledge and resources to promote green technology that causes less pollution and can reverse the effects of climate change.

"The world faces an unprecedented challenge, and Silicon Valley can make an unprecedented contribution to meeting that challenge," Gore told them. "You can chart a course and change the future of civilisation."

Technology companies recently have been focusing more on green technology, such as designing computer processors that generate less heat, building systems that better manage electricity use in data centres, improving manufacturing processes and recycling old computers. Gore urged them to do more.

He criticised the George Bush administration's reaction to the UN report, which was to restate President George Bush's opposition to mandatory reductions in greenhouse emissions.

"It's not as if the oil lobby has too much influence in the White House. The problem is the oil lobby is the White House," he said.

Gore also describe as "unethical" a lobbying tactic by oil companies and other critics of climate change to pay $10,000 for each scientific paper produced that disputes the potential harm of global warming.

"(Gore's speech) was a stirring challenge," said Russell Hancock, chief executive officer of Joint Venture Silicon Valley Network, which hosted the event. "We need to organise and mobilise so that Silicon Valley can keep doing the kind of innovation that made us famous."

Chip maker Cypress Semiconductor incubated a startup that makes solar power systems, said Eric Benhamou, chairman of Cypress. SunPower is now a standalone company.

"I think we are a unique centre of technology innovation in Silicon Valley, and I think we have the best research universities relevant to the climate protection challenge," said Benhamou.