With critical problems from climate change to overpopulation threatening the world's environment, scientists at UC Berkeley and Stanford led a call by more than 500 of their international colleagues Thursday for more urgent action to combat the global threats.

Spurred by Gov. Jerry Brown, the Bay Area scientists said their declaration was a renewed challenge to governments and policymakers in every nation to resolve these five major issues:

-- Climate disruption: The world now faces "more, faster climate change than (at any time) since humans became a species," the scientists said in their declaration.

-- Extinction: "Not since the dinosaurs went extinct have so many species died out so fast, both on land and in the oceans."

-- Ecosystems: "We have plowed, paved, or otherwise transformed more than 40 percent of Earth's ice-free land."

-- Pollution: "Environmental contaminants in the air, water and land are at record levels and increasing, seriously harming people and wildlife in unforeseen ways."

-- Population growth: "Seven billion people alive today will likely grow to 9.5 billion by 2050," posing consumption pressures that may well prove dangerous, the scientists warned.

"By the time today's children reach middle age, it is extremely likely that Earth's life-support systems, critical for human prosperity and existence, will be irretrievably damaged by the magnitude, global extent, and combination of these human-caused environmental stressors, unless we take concrete, immediate actions to ensure a sustainable, high-quality future," the scientists said.

Given to the governor

Brown received the 20-page initiative Thursday during a Silicon Valley meeting at the Ames Research Center in Mountain View. It was signed by 520 researchers on environmental issues, led by UC Berkeley paleontologist Anthony Barnosky and Stanford biologist Elizabeth Hadly.

Many international scientists have warned for years that stresses on the Earth's environment - particularly climate change - are increasingly dangerous. A report by Barnosky published last year in the journal Nature provided some of the most persuasive evidence yet that the world is fast nearing an environmental "tipping point" when global disaster could become unavoidable.

Brown was so struck by the tipping point evidence, Barnosky recalled, that he asked in phone conversations why if "global change was such a big deal," that the scientists so often talked only to themselves about it, and only published their evidence in obscure scientific journals. His questions were well taken, Barnosky said.

"Now our statement is designed to educate policymakers and the public everywhere about the real urgency of the evidence," Barnosky said.

Brown urges action

The governor has been criticized by many environmentalists recently for his controversial proposals on delta water issues and carbon cap and trade, but in response to this new appeal he urged the scientists to take more direct action themselves.

"This is not just about science, this is about activism, this is about convincing people," Brown said, adding that their effort needs to include reaching out to oil executives, manufacturers, politicians and the public.

The serious issues are being drowned out in the public, he said, by what he called trivial news and events.

"We're really in a war here, a contest for ideas, and this crowd is representing the losing end right now because it's not happening," he told the scientists.

If the conversation doesn't change and action doesn't happen on climate change in the next five years, Brown said, "it's over."

The session with Brown came during the annual meeting of a nonprofit called Sustainable Silicon Valley, which is supported by many of the region's major high-tech industries and is aimed at tapping into them for new solutions to the world's environment problems.

San Francisco Chronicle staff writer Wyatt Buchanan contributed to this report.

David Perlman is The San Francisco Chronicle science editor. E-mail: dperlman@sfchronicle.com