It is on the edge of the continent, to be sure. But it also sits along a more precarious edge.
With an economy the size of powerful nations, California has put much of its infrastructure and wealth in two regions -- the Bay Area and Los Angeles -- that could be laid to waste at any moment, should a major earthquake fault give way.
It is a state whose people live on the brink of crisis from floods, fires and drought, where water has to be shipped hundreds of miles through a rickety plumbing system.
It is a state that lives, and thrives, on the edge of disaster.
Now, add another to the list of threats: global warming.
California is especially vulnerable to climate change, whether it comes gradually or with catastrophic speed. It also has much to lose.
Consider the sandstone bluffs along the state's 1,100-mile shoreline and the Sierra snow that caps the granite peaks; the sweet cherries in Brentwood and the chardonnay grapes of Napa Valley.
They are each sensitive in varying degrees to climate change.
Even the badly ailing Delta, the heart from which the state's economic lifeblood of water is pumped, is susceptible.
But as much as California is a state of denial and contradiction, it is also a state of inspiration, enlightenment and leadership.
It is a state of dazzling technological invention and bold entrepreneurial innovation.
The edge, it turns out, is also a leading edge.
More than any other state, California has begun to tackle the challenges of global warming.
It is demanding more-efficient cars, lower-polluting fuels and cleaner electricity.
"California has taken a remarkable leadership role in trying to be active in reducing its own fossil fuel emissions," said ecologist Chris Field of the Carnegie Institution at Stanford University. "I think that California has a tremendous history of being an innovator and trendsetter, and I think there's a lot of indication that what California does, the rest of the world does soon after."
The state has good reason to act.
"There's almost no evidence that says climate change is going to be good for any aspect of California," Field said.
In recent years, as the political and social debate raged over whether the global climate is changing in response to man-made greenhouse gases, the scientific debate was already over.
The most prestigious scientific bodies from countries around the world, including Japan, China, India, Russia, France, Germany, the United Kingdom, Italy, Brazil, Canada and the United States, signed a joint statement in 2005 that said global warming is real, that most of it can be attributed to human activities and that it demands prompt action.
On Feb. 2, the Intergovernmental Panel on Climate Change, made up of hundreds of the world's top climate experts, is scheduled to release its latest comprehensive report on global warming, the panel's fourth since 1990 and its first in six years.
The panel's earlier reports have stated with increasing confidence that humans were likely to be a significant contributor to climate change through burning fossil fuels and other activities. The fourth version is expected to be even more definitive.
The evidence for global climate change is now overwhelming. Some highlights:
" The atmospheric concentration of carbon dioxide, the chief greenhouse gas, is about 380 parts per million, up from about 280 parts per million before the 18th century. Tests on ice cores from Antarctica show that today's carbon dioxide concentration is the highest in a record that spans the past 650,000 years.
" The 10 warmest years since record-keeping began in 1880 have all occurred since 1990, and six of the seven warmest years have occurred since 2001. The world is about 1 degree Fahrenheit warmer today than it was 100 years ago.
" Arctic sea ice shrunk to its second lowest level ever last year, only slightly above the record low reached the previous year. Government scientists say the Arctic sea ice has been shrinking about 8 percent per decade since 1979.
" Sea level rose from 4 to 8 inches around the globe during the past 100 years, a jump from the average of about 2 inches per century during the past 18,000 years.
"Since 1979, the troposphere -- the lower layer of the earth's atmosphere, where weather happens -- has warmed up while the layer directly above it, the stratosphere, has cooled. This pattern can be explained only by human activities, including emitting greenhouse gases that trap heat in the troposphere and depletion of stratospheric ozone that absorbs radiation. Increases in natural solar radiation alone would heat both layers.
" A 2003 analysis of 143 studies of wild plant and animal species found that 80 percent of the species that show change are behaving in ways that would be expected with rising temperatures, such as shifting their ranges toward the poles or to higher elevations, flowering earlier in the spring or changing the timing of migration.
Nature or civilization
During the past decade, scientists have tackled the question of whether humans have had a hand in the ongoing climate changes by looking at the expected natural climate variation predicted by models. For example, in places where hurricanes form on the ocean, and where warmer water can mean stronger hurricanes, changes above and beyond the range of natural fluctuations in sea-surface temperatures can be explained only by human activity, said atmospheric scientist Benjamin Santer of Lawrence Livermore Laboratory.
A study led by Santer and published in the Proceedings of the National Academy of Sciences in September found that with only natural inputs, 22 of the world's best climate models couldn't generate the 20th century ocean warming that occurred in the Atlantic and Pacific hurricane breeding grounds. But when man-made increases in greenhouse gases were added to the models, the results more closely matched the actual historical sea-surface temperature increases.
As the scientific case was closing on global climate change, a body of science was emerging that attempts to encapsulate what those changes will mean in specific places.
Those findings are not conclusive. But there has been an avalanche of new research that strives to project what global climate changes will mean for regions such as the Arctic, the oceans and low-lying islands.
Hundreds of these studies have focused on California, and many look at possibilities that could have a direct impact on the Bay Area and its residents.
Computer climate models attempt to forecast future climate using equations based on physical and natural laws, along with estimates of how the composition of the atmosphere will change over time. Although the models can only make estimates and are not likely to be exactly right, they are the best tools available. And they are constantly improving as researchers gather more information about the Earth and learn more about the interaction between the atmosphere, oceans and land.
These models now indicate that California's average temperature is likely to rise from 3.6 to 10.8 degrees by the end of the century, depending on whether emissions are curbed or not, and how sensitive the climate turns out to be.
Within that range of temperatures comes a corresponding range of potential changes that could dramatically alter the state.
At risk are some of California's iconic natural landscapes, vital resources, diverse wildlife and world-class tourist attractions.
What might become of water that supplies our showers and irrigates our lawns?
What about ski slopes around Lake Tahoe?
Will winter storms be more destructive? Will bayside property flood? Will coastal erosion ramp up? What about the plants and animals that live around us or in the high mountains we visit?
Scientists are now attempting to answer these questions, and the answers range from unsettling to frightening.
"There are some impacts that we're quite certain about. Then there are (other) impacts where it's not so certain," said Daniel Cayan, a research meteorologist at the Scripps Institution of Oceanography in La Jolla who is among the leading authorities on climate change in California.
"One thing that we're pretty confident about is this pattern of precipitation in western states that is going to be more rain instead of snow," he said.
That shift of rain and snow patterns has large -- and potentially massive -- implications for California.
"If you think about and look at the crazy storage and transport system of water in this state -- the aqueducts and Hetch Hetchy and getting the stuff from the north where the snow is to the south where the people are -- they're pretty antiquated and kind of teetering on the edge," said earth scientist and paleoclimatologist Lisa Sloan of UC Santa Cruz.
"I think water is going to be the single biggest problem that we have, and that's going to be sociological, economical, agricultural and probably tourism."
Opinions vary about how severe climate changes will be and the best way to respond.
But acknowledgment of the effects of climate change has begun to emerge from some unexpected quarters.
Earlier this month, the National Oceanic and Atmospheric Administration announced that 2006 was the hottest year in the continental United States since record-keeping began in 1895. The New York Times reported it was the first time during the Bush administration that a NOAA news release attributed higher temperatures to greenhouse gas buildup.
In December, the U.S. Fish and Wildlife Service proposed listing polar bears as a threatened species because of melting ice, which the bears depend on for hunting, mating, resting and traveling to denning areas.
It was the first time a species has been proposed for listing specifically because of climate change.
The proposal to protect polar bears under the Endangered Species Act cited a study that said a "tipping point" may have been reached in the late 1980s. The agency pointed to studies that predicted the Arctic could be nearly ice-free during summer as early as 2040, and that such conditions apparently have not occurred in 800,000 years.
Rex Tillerson, the CEO of ExxonMobil, the world's largest corporation, slackened his company's skepticism of global warming by calling for a response in a recent speech.
Tillerson maintained the climate science was still somewhat murky but added that it was time to take precautions to protect against the most dire possible results, according to a transcript posted on the Web site of Environmental Defense.
"Consistent with this approach, we should take steps now to reduce emissions in effective and meaningful ways," Tillerson said, according to the transcript. "Improving the fuel economy of our light-duty vehicle fleet is one such way."
No single heat-wave, hurricane, extinction or fire is evidence by itself of global warming, just as no single cold snap disproves it.
But put the hundreds of studies from a half-century of research together and the evidence is overwhelming.
And as the average temperature rises, all kinds of things change.
"The bottom-line message of all this work is that we simply cannot explain all these changes by natural causes alone," Santer said.
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