Editors note: This is the first in a year-long Associated Press series exploring the implications of climate change for California and the challenges in addressing them.The January wind whips down from the peaks that ring the northwest shore of Lake Tahoe, the snapping cold and snow-covered grandeur of the mountains providing a somewhat misleading backdrop to Brant Allens task.While all around him are the certain signs of winter, the marine biologist is reading other signs that tell him winters in the Sierra Nevada arent what they used to be.At the end of a wooden pier, he checks a metal box filled with wires and computer chips that gauge the temperature in the nations deepest glacial lake. The data recorded at this pier and at four buoys tell an alarming story of a lake that has been warming gradually over the past three decades, due at least in part to global climate change that scientists say has led to shorter winters in the Sierra.The potential consequences of those changing winters are profound, for Lake Tahoe and all of California, especially as temperatures are predicted to rise markedly over the next century.What those changes might be and how the nations most populous and geographically diverse state will prepare for them is the subject of increasingly intense debate. Californias economy, geography and search for solutions also has made it a model for the nation and the world.Throughout California, rising temperatures threaten to transform a landscape that ranges from the Joshua trees of the Mojave Desert to the redwoods of the North Coast. In doing so, drastic challenges are expected for a population expected to reach 55 million by 2050, from the millions who live in coastal cities to the farmers who have made California the nations top agricultural state.The forecast is dim: diminished snowpacks that melt too early, causing floods and water shortages; submerged coastal homes and eroded beaches as sea levels rise; crops unable to survive in longer, hotter summers; charred forests that fall victim to more intense wildfires.Its not a place where we would be comfortable, said Connie Millar, an historic scientist at the U.S. Forest Service. If we dont lasso this thing, it could ramp up into catastrophic conditions.In wine country, for example, high temperatures could ripen grapes up to two months early, affecting the quality of the grapes behind a $3.2 billion industry.What types of grapes would be able to be grown would perhaps change the types of wine that would be able to be produced, said Dave Whitmer, agricultural commissioner for Napa County.

The Earths temperature has risen about 1 degree Fahrenheit in the last 100 years, with the rate of warming tripling in the last three decades. Last year was the warmest on record in the U.S., a year that saw California suffer through a withering summer heat wave.Nowhere else in California are the signs of climate change more evident than in the Sierra, the 400-mile long range that provides the snowpack essential for the states water supply.The landscape already has begun to change. Native conifer trees, squirrels, mice and other cold-weather mammals have migrated to higher elevations. Yosemite National Parks largest glacier, the Lyell Glacier discovered by John Muir, has retreated to the highest peaks.The iconic lake that straddles the state border with Nevada has seen its average temperature rise by nearly 1 degree Fahrenheit since 1969. While that change might seem slight, the continuing increase is enough to put at risk the native fish and the ongoing efforts to restore the lakes signature clarity. Higher temperatures promote algae growth; algae growth diminishes water clarity.Global warming is something that doesnt happen overnight. It builds up over time, and changes occur slowly, Allen said. Having nearly 40 years of data on water clarity and water temperature allow us to document these changes that are occurring.The peaks that hold the states summer water supply tell another story of climate change. The snowpack melts up to a month earlier than it did in the early 1900s.Californias massive network of aqueducts and reservoirs, which channel the runoff to roughly two-thirds of Californians and the prime farming land in the Central Valley, arent designed to handle the earlier snowmelt.Those early spring rains that warmer weather will bring compound the problem, melting even more snow sooner. That threatens to overload Californias vast network of levees, many of which already are in fragile condition.As he unloaded his snowmobile in woods alongside Lake Tahoe, Don Enos said there have been fewer weekends to enjoy the winter snow with his buddies.It seems like theres no pattern anymore to the winter, said Enos, who has been coming to Lake Tahoe from the San Francisco Bay area town of Capitola for about 20 years. It seems unpredictable.

The threats posed by climate change have not gone unnoticed by Californias lawmakers and government agencies.State water officials say the state needs to plan now to capture more of the Sierra runoff in future decades, perhaps by building more reservoirs. They also say upgrades to the aqueduct system are crucial.Gov. Arnold Schwarzenegger and legislative Democrats made headlines around the world last year by agreeing on a landmark bill to reduce emissions of greenhouse gases. The states strategy requires vehicles to run cleaner, oil companies to produce cleaner-burning fuels and manufacturers to cut their factory emissions.Schwarzenegger and Democratic leaders say they want to do more, even though there already are questions about how companies will meet the initial state targets.Officials with California Steel Industries Inc., about 50 miles east of Los Angeles, say theyre not sure how the company can become more efficient to meet the states new emission requirements. The factory already has reduced by half the amount of energy used to produce a ton of steel.Weve probably gathered in the low-hanging fruit. Its going to be tough for us to conserve much more energy, said Brett Guge, a company vice president.Auto manufacturers, meanwhile, have sued California over its attempt to regulate vehicle emissions.Ultimately, state mandates are likely to be only a fraction of whats needed to minimize the consequences that many scientists say already are in motion.Critics go further, saying the kind of regulations California seeks to impose could drive industries elsewhere, either in the U.S. or abroad. Some environmental scientists argue that Californias efforts are a waste of time and money, saying the expected warming over the next century will not be enough to cause catastrophe.The gloom and doom scenarios are just not warranted, said Pat Michaels, a senior fellow of environmental studies at the libertarian Cato Institute in Washington, D.C. It might not be very wise to undertake extensive programs right now that will in fact do very little about planetary warming.

California, the worlds 12th largest emitter of greenhouse gas emissions, has set a target to reduce its emissions by about 25 percent by 2020.It is the most sweeping greenhouse gas mandate in the world, exceeding the limitations of programs in Europe and in the northeastern United States. For that reason, the United Kingdom, Japan, Sweden and other countries are carefully watching whether the mandate will hurt the worlds eighth-largest economy or position its businesses in the forefront of a new era.One problem is that greenhouse gases know no boundaries. Researchers have detected particulate matter from China and other Asian countries in the mountains around the San Francisco Bay area and elsewhere in the western U.S.Without strict measures in other countries, Californias efforts may produce only modest effects within the state.Nevertheless, state leaders say their go-it-alone approach is worth the gamble because California has too much at stake. Schwarzenegger has said as much, saying governments must act to prevent the effects from getting worse.Its a potentially costly experiment, and the results are uncertain. To do nothing, however, is an option many experts say is irresponsible.If you look at California, we have our own resources at stake, said Daniel Cayan, a climate researcher at the Scripps Institution of Oceanography at the University of California, San Diego Scripps. We have a responsibility and an opportunity to be a leader, to work ourselves out of this problem, he said.