A couple of new papers exploring California’s drought triggered what I thought this morning was some overly simplistic back and forth on the twitters about whether climate change is to blame. I think that’s the wrong question.

The first paper, which I wrote about last week, was the Griffin/Anchukaitis paleo look at the thing. They argued that while as measured by precipitation alone, the last three years were not unprecedented, when you add in the impacts of rising temperatures, it was the worst drought in 1,200 years (which is as far back as their tree ring data go).

Then today, Richard Seager and Marty Hoerling released an analysis suggesting that natural variability was sufficient to explain the drought:

The current drought is not part of a long-term change in California precipitation, which exhibits no appreciable trend since 1895. Key oceanic features that caused precipitation inhibiting atmospheric ridging off the West Coast during 2011-14 were symptomatic of natural internal atmosphere-ocean variability.

Model simulations indicate that human-induced climate change increases California precipitation in mid-winter, with a low-pressure circulation anomaly over the North Pacific, opposite to conditions of the last 3 winters. The same model simulations indicate a decrease in spring precipitation over California. However, precipitation deficits observed during the past three years are an order of magnitude greater than the model simulated changes related to human-induced forcing. Nonetheless, record setting high temperature that accompanied this recent drought was likely made more extreme due to human-induced global warming.

My take:

2/ This drought has taught us important things about the envelope of climate variability California faces, whatever the cause.

— jfleck (@jfleck) December 8, 2014

4/ Scientific back-and-forth helps shine some light, but all science is telling us droughts like this are more likely than we thought.

— jfleck (@jfleck) December 8, 2014

5/ "More likely than we thought" is the key thing to understand, whether natural variability, climate change, or some of both, is behind it.

— jfleck (@jfleck) December 8, 2014