The California heat of the past 12 months is like nothing ever seen in records going back to 1895. The 12 months before that were similarly without precedent. And the 12 months before that? A freakishly hot year, too.
What's happening in California right now is shattering modern temperature measurements—as well as tree-ring records that stretch back more than 1,000 years. It's no longer just a record-hot month or a record-hot year that California faces. It's a stack of broken records leading to the worst drought that's ever beset the Golden State.
The chart below shows average temperatures for the 12 months through March 31, for each year going back to 1895. The orange line shows the trend rising roughly 0.2 degrees Fahrenheit per decade, just a bit faster than the warming trend observed worldwide.
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 statewide water restrictions in U.S. history, with $10,000-a-day penalties against water agencies that fail to reduce water use by 25 percent.
Evolution of an Epic Drought
These maps show the worsening conditions over the last four years. More than 44 percent of the state is now in “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
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, with 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 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 long-term 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.