Beware of Sound Bite Harvey Attribution

Imagine the western U.S. equivalent of Hurricane/Tropical Storm Harvey.

Rain nearly nonstop in California for 6 weeks.  Runoff turning the Central Valley into an inland sea.  Sacramento buried under 10 feet of water.  Damages of more than a half-trillion dollars.

Now that would be a climate catastrophe right?

Except, it already happened.

During December.

In 1861.

Granted, the damages were not more than a half-trillion dollars.  That's merely the lower estimate of storm damage if it happened today.  But in December 1861, a megaflood in California killed thousands of people and 800,000 cattle.  A catastrophe by any standards, and it happened before significant global warming.

Source: USGS, Ingram (2013)

I write this to emphasize caution in interpreting the causes and contributors to the severity of floods produced by Harvey during and following the immediate aftermath of the storm.  The meteorology and hydrology of extreme events is mutlifaceted and highly complex and deserving of a more careful and cautious analysis.

Let's think back to the California Megaflood of 1861.  It it happened today, how would the press and public view it?  It would be unprecedented in "modern" (20th century or later) times and viewed as a disaster at least on par with Harvey. Paleoclimatic evidence suggests that such events have a "return interval" of 300 years, so we know such an exceptional event is possible when natural variability brings all the key ingredients together.  On the other hand, we expect global warming to serve as an amplifier of events of this type (see The Coming Megafloods).  In addition, we've changed the landscape of both the mountains and the Central Valley in ways that greatly affect hydrologic response.  So, will the next California megaflood be natural, caused by global warming, or caused by our land-surface modifications?  One can imagine all sorts of great clickbait headlines, but clearly, the causes are mutifaceted.

And, it is especially difficult to quantify the effects of global warming on a single event.  Global warming is having an effect on our weather and there are good reasons why we expect heavy precipitation events to increase in frequency and become more severe.  We should be concerned about this.

However, careful analysis is needed to truly understand the ingredients leading to the flooding produced by Harvey and the degree to which global warming has contributed to an increase in the likelihood of an event of such magnitude.  

Further, I am concerned about giving people a false impression that climate change is the only issue at play when it comes to our vulnerability of natural disasters.  Most scientists recognize this is the case, but the headlines command the attention of the public.

Andrew, Katrina, Harvey, and other tropical cyclones serve as reminders of our incredible vulnerability to natural disasters.  Paraphrasing Will Durant, "Civilization exists by meteorological consent, subject to change without notice."

In addition, we aren't prepared for the climate of the 20th century, let alone the one coming in the 21st century.  As a society, we are not only exposed to extreme weather events associated with heavy precipitation, but we're essentially doubling down on that exposure through ongoing, concentrated development and poor land-use practices and planning in vulnerable areas.

If we are to build a more weather and climate resilient nation, we need to recognize the multifaceted nature of the challenge.  Harvey serves as a wakeup call (unfortunately, we've had them before) and is not the last jaw dropping weather disaster we will face.