For more than two decades, satellites have tracked the total amounts of water held in glaciers, ice sheets, lakes, rivers, soil and the world’s vast natural reservoirs underground — aquifers. An extensive global analysis of that data now reveals fresh water is rapidly disappearing beneath much of humanity’s feet, and large swaths of the Earth are drying out.

Scientists are seeing “mega-drying” regions that are immense and expanding — one stretching from the western United States through Mexico to Central America, and another from Morocco to France, across the entire Middle East to northern China.

There are two primary causes of the desiccation: rising temperatures unleashed by using oil and gas, and widespread overpumping of water that took millennia to accumulate underground.

“These findings send perhaps the most alarming message yet about the impact of climate change on our water resources,” said Jay Famiglietti, a hydrologist and professor at Arizona State University who co-authored the study. “The rapid water cycle change that the planet has experienced over the last decade has unleashed a wave of rapid drying.”

Since 2002, satellites have measured changes in the Earth’s gravity field to track shifts in water, both frozen and liquid. What they sent back shows that nearly 6 billion people — three-fourths of humanity — live in the 101 countries that have been losing water.

Each year, these drying areas have been expanding by an area roughly twice the size of California.

Canada and Russia, where large amounts of ice and permafrost are melting, are losing the most fresh water. The United States, Iran and India also rank near the top, with rising temperatures and chronic overuse of groundwater.

Farms and cities are pulling up so much water using high-capacity pumps that much of the water evaporates and eventually ends up as rain falling over the ocean, measurably increasing sea level rise.

Water flows from a well to irrigate an orchard in Visalia.

(Irfan Khan / Los Angeles Times)

The study, published in the journal Science Advances, found that these water losses now contribute more to sea level rise than the more widely understood melting of mountain glaciers or the Antarctic or Greenland ice sheets.

The staggeringly rapid expansion of the drying regions was surprising even for the scientists. Famiglietti said it is set to worsen in many areas, leading to “widespread aridification and desertification.”

“We found tremendous growth in the world’s land areas that are experiencing extreme drought,” Famiglietti said. “Only the tropics are getting wetter. The rest of the world’s land areas are drying.”

The wave of drying has prompted many people across the world’s food-growing regions to drill more wells and rely more heavily on pumping groundwater.

The researchers estimate that 68% of the water the continents are losing, not including melting glaciers, is from groundwater depletion. And much of that water is to irrigate crops.

Where aquifer levels decline, wells and faucets increasingly sputter and run dry, people drill deeper and the land can sink as underground spaces collapse.

The loss may be irreversible, leaving current and future generations with less water.

Famiglietti said the potential long-term consequences are dire: Farmers will struggle to grow as much food, economic growth will be threatened, increasing numbers of people will flee drying regions, conflicts over water are already increasing, and more governments will be destabilized in countries that aren’t prepared.

The researchers estimated that the world’s drying regions have been losing 368 billion metric tons of water per year. That’s more than double the volume of Lake Tahoe, or 10 times Lake Mead, the largest reservoir in the United States.

All that water, year after year, has become a major contributor to sea level rise, which is projected to cause worsening damages in the coming decades.

Previous studies have shown dropping groundwater levels, dry regions getting drier and these water losses contributing to sea level rise. But the new study shows these changes are happening faster and on a larger scale than previously known.

“It is quite alarming,” said Hrishikesh Chandanpurkar, an Arizona State research scientist who co-authored the study. “Water touches everything in life. The effects of its irreversible decline are bound to trickle into everything.”

He likened the global situation to a family overspending and drawing down their savings accounts.

“Our bank balance is consistently decreasing. This is inherently unsustainable,” Chandanpurkar said.

The draining of groundwater, often invisible, hides how much arid regions are drawing down their reserve accounts, he said. “Once these trust funds dry out, water bankruptcy is imminent.”

The researchers examined data from two U.S.-German satellite missions, called Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow On.

The scientists ranked California’s Central Valley as the region where the fastest groundwater depletion is occurring, followed by parts of Russia, India and Pakistan.

In other research, scientists have found that the last 25 years have probably been the driest in at least 1,200 years in western North America.

Over the last decade, groundwater losses have accelerated across the Colorado River Basin.

And farming areas that a decade ago appeared in the satellite data as hot spots of drought and groundwater depletion, such as California’s Central Valley and the Ogallala Aquifer beneath the High Plains, have expanded across the Southwest, through Mexico and into Central America.

The satellite data show that these and other regions are not only shifting to drier conditions on average, but are also failing to “live within the means” of the water they have available, Chandanpurkar said.

“The truth is, water is not being valued and the long-term reserves are exploited for short-term profits,” he said.

He said he hopes the findings will prompt action to address the chronic overuse of water.

In the study, the researchers wrote that “while efforts to slow climate change may be sputtering,” people urgently need to take steps to preserve groundwater. They called for national and global efforts to manage groundwater and “help preserve this precious resource for generations to come.”

In many areas where groundwater levels are dropping, there are no limits on well-drilling or how much a landowner can pump, and there is no charge for the water. Often, well owners don’t even need to have a meter installed or report how much water they’re using.

In California, farms producing vast quantities of nuts, fruits and other crops have drawn down aquifers so heavily that several thousand rural households have had their wells run dry over the last decade, and the ground has been sinking as much as 1 foot per year, damaging canals, bridges and levees.

The state in 2014 adopted a landmark groundwater law that requires local agencies to curb widespread overpumping. But it gives many areas until 2040 to address their depletion problems, and in the meantime water levels have continued to fall.

State officials and local agencies have begun investing in projects to capture more stormwater and replenish aquifers.

Arizona has sought to preserve groundwater in urban areas through a 1980 law, but in much of the state, there are still no limits on how many wells can be drilled or how much water can be pumped. Over the last decade, out-of-state companies and investors have drilled deep wells and expanded large-scale farming operations in the desert to grow hay and other crops.

Famiglietti, who was previously a senior water scientist at NASA’s Jet Propulsion Laboratory, has extensively studied groundwater depletion around the world. He said he doesn’t think the leaders of most countries are aware of, or preparing for, the worsening crisis.

“Of all the troubling findings we revealed in the study, the one thing where humanity can really make a difference quickly is the decision to better manage groundwater and protect it for future generations,” Famiglietti said. “Groundwater will become the most important natural resource in the world’s drying regions. We need to carefully protect it.”