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Is California's drought man-made?

California is experiencing its worst drought in recorded history.

99.86 percent of the state is in drought. Half is suffering from "exceptional drought"—a term used by researchers to describe only the worst drought conditions. The Sierra Nevada mountains' snowpack levels measure just . That's bad news for the Golden State, which relies on melted snow from these mountains for much of its water supply.

Thousands of wells—another main source of water—have already run dry. Many still functioning are so over-pumped that the ground is actually sinking, more than a foot per year in some places, as people suck the water out from underneath the soil.

Scientists aren't sure whether climate change is causing the drought

Although nobody disputes the drought's severity, people do disagree about its cause.

The two main organizations that investigated the drought's root causes reached opposite conclusions. Stanford University researchers say helped set the stage for California's drought. The National Oceanic and Atmospheric Administration (), one of the most respected authorities on climate change, thinks California's drought is just bad luck.

Specifically, they disagree about the cause of a "high pressure ridge" in the atmosphere off the California coast. This pressure zone acts like a boulder in a stream, diverting rain and snowstorms away from California. Last year, the ridge spanned 2,000 miles across and four miles tall.

Stanford research shows that elevated levels of greenhouse gases, the driver of climate change, cause this pressure zone to appear much more frequently. NOAA scientists believe the pressure zone is the result of natural weather cycles.

Scientists do agree that climate change makes the drought worse

Stanford and NOAA both agree that man-made climate change is in California and across the globe. Hotter weather leads to more severe droughts.

That's because and makes it more likely that precipitation will fall as rain rather than snow. Although snow and rain are both important sources of drinking water, snow is more useful. Soil can gradually absorb melting snow. Heavy downpours can't be absorbed all at once, so much of the rain runs off into our gutters and rivers. Rain tends to drain into the ocean rather than into the soil where it can be reused.

Snow melt keeps soil fertile and provides drinking water for 10 million Californians. A stable water supply is also important for its dams and hydroelectric power plants, which generate 15 percent of California's electricity. By decreasing the likelihood of snow, warmer weather jeopardizes California's economy.

In short, climate change may or may not be causing California's drought. But it certainly reduces snowfall. That has turned what could have been an ordinary drought into something much more severe.

A drier future?

Using tree rings and other natural clues, scientists discovered long droughts were common over California’s past thousand years. Droughts spanned decades and sometimes even hundreds of years, long before people started to change the climate.

Although climate change was not always a factor, studies show it could influence future droughts. Similar to how speeding increases the likelihood of a severe accident, more greenhouse gases increase the probability of a severe drought.

The current drought could be a preview of what's to come. NASA warns that unmanaged climate change will likely cause 30-year "megadroughts" across much of the United States by the end of this century.

If Californians and Americans want to protect their water supplies, they must start fighting climate change.