California's mountain snowpack has vanished.

Typically, the snowpack provides up to 30% of the state's annual freshwater resources and is at a seasonal peak depth in early April. But in the face of mild Pacific storms that brought rain instead of snow, as well as drier-than-average conditions and a record-warm winter, the snow cover stood at just 5% of average on April 1, by far the lowest level ever recorded.

Conditions are so dire that California Governor Jerry Brown announced on Wednesday the first-ever statewide mandated water restrictions, aiming to achieve a 25% cut in water use across the state. While the governor's executive order would require agricultural interests to report more information to the state about their water use, potentially paving the way for water restrictions in the future, it would not, however, impose new limitations on growers.

See also: What this NASA scientist meant about California having a year of water left

The snow cover is but the latest in a long line of troubling signs showing that the drought the Golden State has been mired in for more than three years now is no ordinary beast. It is the worst on record since 1895, and tree-ring data and other historical evidence shows it may also be the worst to hit California in more than 1,000 years.

There are two crucial differences between the droughts that occurred a millennium ago and modern drought events, however. The first is obvious: There are now about 38 million thirsty Californians living across the state, watering their lawns and golf courses, and irrigating crops in what is the most agriculturally productive state in the country.

The second has been clear to climate scientists for years, but is just now gaining more public recognition. We are now seeing the rise of a new, supercharged type of drought, in which global warming-related temperature extremes combine with dry conditions to transform what would otherwise be an ordinary drought event into a far more severe event.

California's current drought kicked off in early 2012, and now encompasses the entire state as the typical dry season gets underway. Climate scientists have been examining different aspects of the drought, from the role that ocean conditions in far-flung places such as the tropical Pacific Ocean and even the Atlantic Ocean may have played in triggering it to how it falls into a broader historical context.

Here are some of their key findings, interspersed with the drought's milestones so far:

A 5.9-degree Fahrenheit anomaly

California just had its warmest winter on record, obliterating the previous record, which was set just last year, by 1.5 degrees Fahrenheit. In 2014, California had its warmest year since record-keeping began in 1895, with average annual temperatures that were 4.1 degrees Fahrenheit, or 2.3 degrees Celsius, higher than its 20th century average, according to data from the National Oceanic and Atmospheric Administration (NOAA).

The 2014 annual average temperature shattered the state's previous record set in 1934 by 1.8 degrees Fahrenheit. In 2013, the second year of the ongoing drought, California had its sixth-warmest year, and 2012 was its fourth, NOAA's National Climatic Data Center has reported.

It's not the heat, it's the... oh wait, it is the heat.

Studies examining different aspects of this drought point to the unusually warm temperatures that have affected California at the same time as both a cause and consequence of the extreme drought.

The mild temperatures helped intensify the drought, while the lack of sufficient precipitation has also dried out the soil. That in itself can lead to higher temperatures in a positive feedback loop.

One study, published in late March in the journal Proceedings of the National Academy of Sciences, found that as global warming has boosted average temperatures across the West, there is now an increasing risk that a warm year will overlap with a dry one. Such combinations can create significant droughts, the study found, whereas precipitation deficits by themselves can be insufficient in yielding a drought.

A 1,200-year drought history in California, with the 2012-2014 drought at the end of the time series. Image: Griffin and Anchukaitis/GRL 2014

Another study, published in Geophysical Research Letters in late December of last year, found that California's precipitation deficits alone do not make this drought a historic event. Instead, the full impacts only become clear when you combine the precipitation data with temperature information to get an idea of how much moisture is being evaporated from the soil. Higher temperatures mean more evaporation occurs.

From the perspective of soil moisture, the ongoing drought is the worst in at least the last 1,200 years, the study found, adding that the high temperatures in the past few years may have aggravated the precipitation deficits by nearly 40%.

"These observations from the paleoclimate record suggest that high temperatures have combined with the low but not yet exceptional precipitation deficits to create the worst short-term drought of the last millennium for the state of California," the study states.

60- to 100-year "mega-droughts"

A representation of the summer moisture in the U.S. Central Plains and Southwest. The brown line represents the variation in dryness since the year 1000. Colored lines on the right represent a trend toward dryness not seen in the previous millennium. Image: Cook et al./Science Advances, 2015

That same study also warns that the climate system is "capable" of precipitation deficits that are more severe and longer-lasting than what we are seeing now. With climate projections showing a trend to higher average and higher extreme temperatures across the West, it's likely that any drought will be more severe than it would have been without manmade warming, the study — along with others — warns.

Some researchers are already calling the broader drought that has been affecting the West for more than a decade now a "mega-drought." This term sounds apocalyptic, but refers to long-lasting and widespread drought conditions.

Historical data gleaned from tree rings, coral reef records and other sources shows that droughts have struck the Southwest with a ferocity and durability. They could paralyze modern society, which has become so dependent on abundant water supplies for agriculture, energy generation, human consumption and other uses.

Soil moisture 12 inches below ground projected through 2100 for a high greenhouse gas emissions scenario. Image: NASA

For example, the Sierra Nevada Mountains have seen droughts that lasted for 100 to 200 years. Droughts that spanned the continental U.S. would have made the Dust Bowl look like a minor sandstorm hit during the 12th and 13th centuries.

Of course, this occurred long before humans began pumping climate-warming greenhouse gases into the atmosphere. But average temperatures then were similar to what they are now in the Southwest. Because greenhouse gases are increasing rapidly — to levels not seen in all of human history, any such "mega-drought" today would be even worse, scientists say.

A study published in February in the journal Science Advances, for example, found that droughts in the Southwest and Central Plains during the latter half of this century could be drier and longer-lasting than any similar event those regions have seen in the last millennium.

"Natural droughts like the 1930s Dust Bowl and the current drought in the Southwest have historically lasted maybe a decade or a little less," said Ben Cook, climate scientist at NASA's Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory at Columbia University in New York City, and lead author of the paper, in a statement.

"What these results are saying is we're going to get a drought similar to those events, but it is probably going to last at least 30 to 35 years."

The researchers pegged the current likelihood of a mega-drought at 12%, but said the risk could skyrocket to 80% in these parts of the country between 2050 and 2099, depending on the amount of greenhouse gas emissions.

24 trillion gallons of water lost

A worker uses a pressure washer to clean headstones at the San Francisco National Cemetery on April 1, 2015 in San Francisco, California. Image: Justin Sullivan/Getty Images)

The drought has caused the state's farmers to tap into groundwater supplies for irrigation water, leading to a massive drawdown in aquifers that take decades to centuries to be recharged by natural rainfall.

"In each of the last three years, California has lost more than 8 trillion gallons of water, which is more than all 39 million Californians use each year in their homes, and for industrial and municipal use," NASA water scientist and UC-Irvine professor Jay Famiglietti told Mashable.

500 millibar mega-ridges

While this is still an ongoing area of research, the current California drought has its roots in variations in ocean conditions and atmospheric circulation across the Pacific and North America. Studies have shown that California droughts are more frequent when certain parts of the Pacific are unusually mild, while parts of the Atlantic are unusually cool, which was the setup for part of this event.

The ridge of high pressure is seen here on a 500 millibar pressure anomalies map from Feb. 15, 2015. The red areas correspond to milder-than-average conditions associated with the ridge. Image: WeatherBell Analytics

For more than a year now, a giant blob of unusually mild waters has set up shop in the northeast Pacific and Gulf of Alaska, with an area of high pressure — or a "high pressure ridge" — forming above it. This ridge has periodically intensified to levels that have made many meteorologists scratch their heads in amazement. It has also been a persistent feature of the airflow across the Northern Hemisphere during the past year or more.

Weather forecasters have nicknamed it the "ridiculously resilient ridge" for its tenacity. Every time computer models project it will break down, it seems to persevere. This ridge has acted like an atmospheric detour, redirecting storms far to the north of California and keeping mild air flowing into the state all year long.

Scientists are divided over whether manmade global warming is influencing this weather pattern, or if it is mainly a natural occurrence.

Regardless, the proximate cause of the drought is less of a concern to policymakers and climate scientists alike, who all see the writing on the wall. The future California is likely to be a much drier, milder and in many ways, harsher, place.