In the Sierra Nevada, the snowpack is shrinking.
These developments, attributable to a warming climate, threaten one of California's most indispensable resources: water.
Without water and the ability to move it efficiently over hundreds of miles -- to cities, suburbs, farms and factories -- California would be unrecognizable from the highly developed, fertile and industrial powerhouse it is today.
The threats to California's water supply, in many ways the state's lifeblood, are not mere possibilities.
They are here. And now.
"What we're beginning to see clearly in California -- and these are not projections -- is we've clearly seen sea-level rise of about a half a foot at the Golden Gate. That's real data," said John Andrew, chief of special planning for the state Department of Water Resources. "The snow coming down from the Sierra -- earlier snowmelt -- that's real data."
Many scientists say those trends will hasten in the coming decades and reach troublesome levels as the climate continues to absorb increased greenhouse gases from motor vehicles, power plants and other sources.
Water, already the subject of perpetual and fierce battles among farmers, urban water agencies and environmentalists, will become even more scarce.
The effects of a warming climate are already being measured all over the world, from the Arctic to Antarctica.
In California, tide gauges have recorded a sea-level rise of about 7 inches at the Golden Gate during the past 100 years.
That is bad news.
In the western United States, where crops generally require irrigation and states are dependent on mountain snowpack to water their farms and cities, a warming climate means less snow.
And that means less water for the long, hot summer.
A rising sea means the fragile Delta levees that protect farms, houses and the state's primary water delivery system will come under increased pressure when the high tides roll in. More sea salt will be pushed toward the state's main Delta drinking and irrigation water intakes.
Sierra snows melt away
For a hint of what a warmer California might look like, recall May 2005. A spring storm moved into California that was warmer than usual. At relatively high elevations around Yosemite National Park, where the storm clouds would normally drop snow, it rained.
The rain quickly washed into tributaries and streams that rapidly filled and broke riverbanks in the low-lying areas, causing widespread flooding in Yosemite Valley. Campgrounds were drowned, water pooled on thoroughfares and park officials closed the Yosemite Valley for a day.
It was a relatively small storm that caused an unusual amount of havoc.
"This is an analogue of what might happen," said Daniel Cayan, a researcher at the Scripps Institution of Oceanography at UC San Diego. "As time goes on, we're expecting those warmer storms to be more frequent."
Today, the Sierra snowpack is effectively California's biggest reservoir, quenching about 40 percent of the state's thirst for water.
But it is already diminishing. Computer climate models suggest it will shrink further.
Statewide, the loss could amount to 3 million to 4 million acre-feet per year within 50 years. That is more water than is delivered each year by the California Aqueduct, the 444-mile canal that supplies Southern California with Delta water from Byron. By comparison, the state's share of the Colorado River is 4.4 million acre-feet.
"It happens slowly, so we're kind of like that frog in the water that's boiling -- you might not notice it until there's some real consequences," said Steve Hall, executive director of the Association of California Water Agencies.
Climate models predict a temperature increase in the next 100 years between 3.6 to 10.8 degrees Fahrenheit.
For every 1.8 degrees Fahrenheit of added warmth, the snow level in the Sierra is expected to move 500 feet higher on the mountain.
A temperature increase of 3.6 degrees Fahrenheit in California would raise the snow level about 1,000 feet and cut the state's snowpack in half, according to one simulation by climate scientists.
In the worst case, the Sierra snowpack could shrink by as much as 90 percent, models suggest.
"That rain/snow line is going to rise to the summit" in much of the northern Sierra Nevada, said Jeff Mount, director of the Center for Watershed Sciences at UC Davis. "That changes everything."
Although Mount Shasta, the tallest peak in Northern California, would continue to boast a snowpack, much of the Sierra Nevada from Lake Tahoe north could have dramatically less snow.
Without slow melting snow, runoff in rivers such as the Feather, Yuba and American could be very high while the rain is falling, then very low as the watershed drains.
Natural variability ruled out
More than 150 peer-reviewed scientific papers have been published on climate change and California's water, studies that range from technical reports meant to improve how scientists can more accurately regionalize climate change projections, to how Californians might adapt to them, to documenting changes already taking place.
"There's an increasingly clear message that we're already here," said Peter Gleick, president of the Oakland-based Pacific Institute, who helped write a summary of those studies for water officials in 2003.
In the mid-1980s, Gleick published the first detailed assessment of how climate change would affect California's water supply. He concluded then that the state would see more rain, less snow and a greater risk of floods and drought -- conclusions that have only strengthened with time.
A respected scientist and environmental advocate, Gleick is firmly in the scientific mainstream when he says that humans are at least partly responsible for global warming.
"I can't tell you if it's 80-20, 70-30, 60-40 or 50-50," Gleick said. "But we can't explain the changes that we're seeing without invoking human influences. They cannot be just the results of natural variability."
The human fingerprint on climate change is becoming clearer, both globally and in California.
Six years ago, a team of scientists using data from stream gauges and records gathered by gardeners and other volunteers of when lilacs and honeysuckles first bloom found that spring has been arriving earlier in western states since the 1970s.
Scientists hypothesized that the shift was caused by a natural climate fluctuation.
The phenomenon, known as the Pacific Decadal Oscillation, affects weather patterns and is associated with water temperatures in the northern Pacific that tend to switch back and forth between warm and cool phases every couple of decades or so.
If the spring warming was largely caused by the oscillation, that would mean that the earlier snowmelt and blooms were simply an expression of the natural rhythm.
But in subsequent studies, natural causes were mostly rejected as an adequate explanation.
A more detailed 2005 paper in the Journal of Climate, which considered four years of additional data, confirmed the early blooming trend.
And it added that stream gauges from mountainous watersheds throughout the western states continued to show spring runoff coming one to four weeks earlier -- even though the weather had shifted back to its cooler phase after summer 1998.
Despite the end of 21 years in the warm phase, the onset of spring was continuing to creep back earlier in the year.
"Our analysis suggests that (the natural weather pattern) contributes to some of the observed ... trends but is not sufficient to explain them all," the paper's authors wrote.
The paper did not dismiss the possibility that other forces might be at work, but it suggested the oscillation and greenhouse gases were the likely contributors and that the natural climatic fluctuation could not explain "a large part" of the trends in stream flow.
Cayan, the Scripps researcher and a co-author on both papers, said he is "quite certain" that warming since the 1980s is a result of human-caused greenhouse gas buildup in the atmosphere, even if there is no definitive link between all of the changes and greenhouse gases.
"I think we would be foolish to dismiss this as an early symptom of global warming," he said.
For California, the 2005 paper carried especially dire news.
"These trends are large throughout the western United States but largest in the Pacific Northwest and the Sierra Nevada," the paper reported.
As it turns out, more snow in the Rockies falls in colder, higher elevations, where snow would not necessarily melt with a few degrees of warming.
But closer to the West Coast, the mountain snowpack is mostly in warmer environments, especially the lower and middle elevations, where a small increase in temperature has a larger effect.
The northern Sierra Nevada, California's most important snow country, is at a lower elevation than the southern portion of the mountain range and is especially sensitive to warming temperatures.
In September, some of the same scientists published a report in the Journal of Climate that said warmer winters and early springs were also causing more precipitation to fall as rain instead of snow.
Again, the trend was strongest in the Sierra Nevada and the Pacific Northwest.
"The largest shifts from snowfall to rainfall actually occurred at sites that warmed less, because these sites had mean temperatures warm enough that moderate warming was sufficient to impact the precipitation form," the study concluded.
'A world-class hog wrassle'
So what do less snow, more rain, rising seas and earlier snowmelt mean in a state that already has huge difficulties supplying water to cities and farms while maintaining ecosystems?
It likely means that cities, farms and the environment will be competing more for less water and that the state's water managers will have to restrict flows more often or find new ways to store water in dams and reservoirs.
Water officials and Gov. Arnold Schwarzenegger argue that a warming climate is a good reason to build more reservoirs. Building those storage facilities is politically controversial and expensive.
Others, including Gleick, the Pacific Institute president, argue that conservation, efficiency and underground water storage are better options.
Unlike other western states, California has less reservoir space compared with the amount of water in the snowpack. Dams on the Colorado River upstream of California, for example, can store several years' worth of runoff. But reservoirs in Northern California can capture only about one year's worth, water officials said.
Dam operators will have to carefully balance the need to capture as much water as possible with the need to preserve enough reservoir space to protect downstream cities, including Sacramento, from flooding.
If they overestimate the flood threat and draw reservoirs down too far, they are more likely to end up with water shortages in the summer.
If they fill the reservoirs too early, they risk flooding downstream communities.
Mount said the interests of cities and farms will be in direct conflict, and it will be impossible for water managers to satisfy both.
"You've got a world-class hog wrassle right there," he said.
And it will not be just farms against cities.
The environment will have a new claim on water, too.
If mountains drain earlier in the year, less cold water will be available for salmon and other fish. Policymakers could turn back to building more reservoirs as a solution.
"I see a giant tug of war," Mount said.
Mike Taugher covers natural resources. Reach him at 925-943-8257 or firstname.lastname@example.org.