The snow pack under the towering giant sequoias in the Sierra Nevada Mountains is melting fast now under the hot spring sun.
It does so every year, of course, but virtually all climate scientists say it will melt faster and faster on average over the next several decades as a direct result of the invisible carbon dioxide that human machinery keeps pouring into the air.
There is far more carbon dioxide in Earth's atmosphere now than there has been at any time during the entire 2,000-year lifetimes of the more mature of these powerful, massive trees.
Far more carbon is now in the atmosphere than at any time in at least the past 650,000 years.
Human civilization is only about 10,000 years old -- born in the retreat of the last ice age -- and made possible in part by the relatively stable global climate since that ice age ended.
The oldest of these trees were already taking youthful gulps of air at the same time Jesus and Caesar drew breath. One tree is so big you can walk inside the folds of its huge twisted trunk -- into a little room -- and step out again into the dappled sunlight to feel the soft dry bark with your palm.
On snowshoes, we walked out into Round Meadow with Sequoia National Park rangers Chris Waldschmidt and Kyle Nelson on one of their regular treks to measure the snow pack for the California State Water Board.
The rangers were in short sleeves under the blue spring sky in this football-field-size forest meadow ringed by great trees, and sunglasses guarded against the lingering glare of the snow pack where marsh flowers will soon bloom amid fresh rivulets.
In the San Joaquin Valley below -- some 20 miles to the west -- an agricultural miracle unfolds that would simply not be possible without the irrigation water that melts down from the mountains during spring and summer.
That icy water gets distributed among orchards and fields by one of the most complex and sophisticated water management systems on the planet.
It is estimated to produce nearly half of America's fruits and vegetables, and -- every spring and fall -- 95 percent of America's lettuce, and most of America's raisins, walnuts, almonds and some 200 other food crops.
Waldschmidt tramped out in his snowshoes across the snow, pulling a yellow measuring tape.
"We measure the snow pack every 100 feet," he said. "You can see there's a marker on that tree back there ... and another on that tree on the edge of the meadow up ahead. We keep a straight line between them."
His partner Nelson held the back end of the tape and checked the line of sight between the two trees. They take these snow pack readings around the Sierras four times each winter.
Some locations require a full day's trek to wild hidden sites, or a winter overnight in subzero tents, but farmers down in the San Joaquin eagerly await their news. Calculations about water allotment, crop rotation and the harvesting schedule rely on their data.
Waldschmidt drove a long hollow aluminum tube down through the soft snow -- some two feet in this spot -- until he hit mud, then carefully pulled it back out, turned it horizontal and looked through a thin slit to measure the depth of the snow core inside.
The ancient sequoias towered around us, ringing the quiet scene as a couple of preseason hikers appeared at the edge of the meadow and stopped to take in the scene.
Nelson then held out a simple scale -- two hooks dangling from a measuring spring hooked on a ski pole he'd brought along. They calculated the weight of the snow inside and then dumped out.
"Multiply the weight of the snow by the depth of the snow core, and you get the water content," Waldschmidt explained.
This spring, there's good news for the farmers below. The snow pack is nearly double the normal amount for this time of year. Since the system of reservoirs and dams built to serve just the San Joaquin Valley can hold a full year's supply of water, the farmers know there will be plenty in their irrigation canals and ditches ... in 2006, at least.
But weather is not climate.
Weather is what happens in one locale today or this week or this season. Climate is long-range patterns over large areas, and the news about the climate is serious and a growing topic of conversation among America's providers in the San Joaquin.
Surveys across the American West have found that as average global temperature creeps up, the mountain snow pack is, on average, melting sooner every year.
This is bad news, say state and federal Water Board officials, because about three-quarters of the water in the American West comes from snow packs.
Slow-melting snow packs have always provided free water storage for summer and early fall -- storage so vast that no feasible amount of dam and reservoir building could ever begin to replace it -- ever hope to capture all that water lost too early.
So when early snowmelt sends the water roaring down the gullies and canyons too soon, summer drought can follow as it has more and more in recent years.
Global warming was always predicted to bring heavier precipitation events -- heavier snowfalls and heavier rains -- at least in the early years, and in more erratic patterns.
Most of the above-normal snowfall in the Sierras this year came all at once -- in March -- and if the pattern of recent years holds, the snow may melt early, giving dam keepers major flood control worries as they try to keep the San Joaquin's limited dam and reservoir capacity from overflowing into towns, roadways and fields.
Climate modelers on the far western side of the San Joaquin Valley, at Stanford University, have told farmers that despite this year's high snow pack, rising average global temperatures mean that 90 percent of Northern California's annual snow pack -- which provides most of the valley's irrigation water -- could disappear well before the end of the century.
Before coming up to this pristine mountain meadow, we visited with a number of San Joaquin farmers in their orchards and fields.
They told us that before the miracle of modern irrigation came along, most of the San Joaquin was pretty dry by midsummer, almost desertlike in places.
The valley's natural rainfall is only eight inches a year, and most of that has usually fallen before the spring growing season has really begun.
For the future of this great American food supply, they say they always have to lift their eyes to the mountains, lost in the far haze across these vast abundant fields.