Did Global Warming Cause California’s Drought?

  • Date: 21/05/16
  • David R. Henderson, Charles L. Hooper, Defining Ideas

Was California’s drought part of a long-term decline in precipitation? No. According to the National Oceanic and Atmospheric Administration (NOAA),“The current drought is not part of a long-term change in California precipitation, which exhibits no appreciable trend since 1895.”

Image credit: Barbara Kelley

When it comes to global warming and climate change, how many times have you heard the phrase, “the science is settled”? The “settled” refrain is often accompanied by “the fact” that 97 percent of climate scientists believe that humans are the main contributor to global warming.

Some global warming science is certainly settled, but most of it is not. A case in point is droughts. Alarmists claim that global warming causes droughts—and, on the surface, this claim makes sense. Just think of Death Valley, a place that’s notoriously hot and dry. If the world moves closer, even slightly, toward Death Valley’s heat, wouldn’t it also get dryer? It’s even easier to link global warming to droughts when prominent people make that connection for us and scold those who might disagree.

President Obama and California’s Governor Jerry Brown, for example, have concluded that climate change caused California’s 2011-2015 drought. In justifying his imposition of water rationing to deal with the drought, Brown said in 2015, “I can tell you, from California, climate change is not a hoax. We’re dealing with it, and it’s damn serious.” In a February 14, 2014 press release, Obama said, “Droughts have obviously been a part of life out here in the West since before any of us were around and water politics in California have always been complicated, but scientific evidence shows that a changing climate is going to make them more intense.” He added, “Unless and until we do more to combat carbon pollution that causes climate change, this trend is going to get worse.” President Obama has even reserved a special place on his website for climate change deniers who “are blocking progress in the fight against climate change. Find the deniers near you—and call them out today.”

President Obama and Governor Brown believe the science is settled and carbon emissions lead to droughts. Before we test the veracity of their beliefs, consider that many of the warmest places on Earth, such as rainforests, are both warm and wet. Further, some of the driest places on Earth, such as Antarctica and Siberia, are also the coldest. The coldest city in the world, Oymyakon, Russia, has a mean annual temperature of -15.5 °C and gets only 8.3 inches of precipitation per year. Antarctica’s McMurdo Dry Valleys, obviously cold, are the world’s driest locations.  So we can’t just assume that warmer equals drier.

Droughts are defined by reduced precipitation and increased evaporation, and California’s 2011-2015 drought had both. What happened over the last three winters was a major reduction in precipitation (a reduction of 0.9 mm per day) with only a minor increase in evaporation (less than 0.1 mm per day). In short, the reduction in precipitation was an order of magnitude more than the increase in evaporation. “California lost essentially one full year of precipitation,” according to Richard Seager, a climate model specialist at Columbia University’s Lamont-Doherty Earth Observatory. “The [reduced] precipitation was the essence of this drought,” added Marty Hoerling, a meteorologist at the NOAA Earth System Research Lab: “Farmers were praying for rain, not cooler temperatures.”

Next, consider long-term trends. Have droughts become more frequent? No. Justin Sheffield concluded in a 2012 Nature paper that “there has been little change in drought over the past 60 years,” the period that includes half the warming of the last 100 years. Moreover, if global warming were an important cause of drought, the world should have had more droughts. It hasn’t. Sheffield identified a weakness in the widely used Palmer Drought Severity Index (PDSI), a calculation used by climatologists to standardize drought assessments. He said, “More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years.”

Was California’s drought part of a long-term decline in precipitation? No. According to the National Oceanic and Atmospheric Administration (NOAA),“The current drought is not part of a long-term change in California precipitation, which exhibits no appreciable trend since 1895.”.

So droughts haven’t become more common and California isn’t getting less precipitation. Is the world getting more wet/dry extremes as the atmosphere warms? Again, the answer is no.

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