12-Month Average Temperature (°F), April-March. (NOAA/Bloomberg)

The last 12 months were a full 4.5 degrees Fahrenheit (2.5 Celsius) above the 20th-century average. Doesn't sound like much? When measuring average temperatures, day and night, over extended periods of time, it's extraordinary. On a planetary scale, just 2.2 degrees Fahrenheit is what separates the hottest year ever recorded (2014) from the coldest (1911).

California's drought has already withered pastures and forced farmers to uproot orchards and fallow farmland. It's costing the state billions each year that it goes on. Governor Jerry Brown issued an executive order this month for the first mandatory state-wide water restrictions in U.S. history, with $10,000-a-day penalties against water agencies that fail to reduce water use by 25%.

Evolution of an Epic Drought

(U.S. Drought Monitor/Bloomberg)

These maps show the worsening conditions over the last four years. More than 44% of the state is in now “exceptional drought” (crimson). It’s a distinction marked by crop and pasture losses and water shortages that fall within the top two percentiles.

California has seen droughts before with less rainfall, but it's the heat that sets this one apart. Higher temperatures increase evaporation from the soil and help deplete reservoirs and groundwater. The reservoirs are already almost half empty this year, and gone is the snowpack that would normally replenish lakes and farmlands well into June.

The chart below shows a measure of drought known as SPEI. It takes into account both rainfall and heat, and again shows the state in uncharted territory.

Hot + Dry = California's Worst Drought

Standardized Precipitation-Evapotranspiration Index, 48 Months Ended in February. (WRCC/UI)

That's part of what makes this drought so troubling. The 4.5 degree above-normal temps that California has seen are unprecedented, but not entirely unexpected. The International Panel on Climate Change, which includes more than 1,300 scientists, forecasts global temperatures to rise anywhere from 2.5 to 10 degrees Fahrenheit over the next century, depending largely on how quickly humans reduce our dependence on fossil fuels.

That means the conditions that are wreaking so much havoc in California today fall well within the range of what may be considered "normal" in the not-too-distant future. The impact of such warming is expected to vary dramatically by region. The scientists' forecast for the U.S. Southwest: increased heat and drought—and decreased water supplies and agricultural yields.

We aren't nearing the end of California's climate troubles. We're nearing the beginning.

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