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Global Warming and Severe Weather: Is There a Link?

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Crossposted with TheGreenGrok.com.

Are 2011's weather-related disasters part of a long-term trend?

Looking back, many will recall 2011 as a momentous year. It sure seemed to be a doozie when it came to disasters. Hurricane Irene. The Joplin tornado. Floods in Mississippi. Fukushima. And so on. But how unusual was it really? And if it was, why? Did global warming have anything to do with it?

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Knowing when to act has been a subject of philosophy throughout the ages. Ben Franklin, a Founding Father, was keen on prevention. Shakespeare's Falstaff, not so much. When it comes to climate, whose book do you take a page out of?

For Bean Counters the Answers Look Straightforward

The National Oceanic and Atmospheric Administration (NOAA) reports that:

  • the number of severe weather events in the United States costing a minimum of $1 billion increased from about 1.2 per year in the early 1980s to about 5 in the 2000s, and
  • 2011 saw a record high of 14 such events.

A summary appearing in last week's edition of Nature (based on data collated by the reinsurance company Munich Re) showed that over the past 30 years the number of weather-related severe events has been on the rise. (See graph here.)

But Counting Begs the Question: What to Count

While tallying up the number of disasters in a given year may seem pretty straightforward, in practice it's not. Things can get complicated. For example: How to decide where to draw the line between a disaster and just a really bad event? With storms, for instance, a single person getting hit by lightning would be tragic but would hardly be categorized as a disaster. But what if 10 people were killed in a storm, or 100, or 500? At what point does something become a disaster?

To avoid the counting-threshold problem, analysts often turn to metrics that integrate across all events -- for example, the total damage costs (or alternatively the total insured losses) that natural disasters impose on the insurance industry in a given year.

And through that lens, 2011 was an unmitigated disaster for disasters, the most expensive year on record with damages soaring past 2005's previous record of $220 billion to a total of about $380 billion. It was also the insurance industry's most expensive year on record for natural disasters, with total insured losses of $105 billion.

However, if global warming is your concern, it's important to note that severe weather was not the only or even the main culprit for 2011 -- about two-thirds of the total damages and about half of the insured losses can be attributed to two non-weather-related disasters: New Zealand's February earthquake and Japan's earthquake and tsunami in April.

But counting dollars has its drawbacks too.

Socio-Economics Confounds Things

As we all know, things change -- populations grow, people move, new buildings are constructed in different places. And these changes tend to increase total damages as well as make insurance companies more vulnerable to losses from disasters. For example, Americans' flocking to coastal communities has undoubtedly significantly increased our susceptibility to losses from hurricanes.

This is a point that Roger Pielke has often made (see here and here), and it's one that a recent paper in Climatic Change by Fabian Barthel and Eric Neumayer of the London School of Economics appears to confirm. Analyzing worldwide losses from severe events over the past two decades, Barthel and Neumayer concluded [pdf] that in particular the socio-economic factor of the "accumulation of wealth in disaster-prone areas is ... by far the most important driver of future economic disaster damage." This suggests that even the billion-dollar threshold used by NOAA to track severe weather over time should be taken with a grain of salt.

What Factoring Out the Socio-Economics Teaches

But Barthel and Neumayer did not allow the confounding influence of socio-economic changes to stop them from trying to answer the question of whether or not disasters were on the rise.

To adjust "for the fact that a hazard event of equal strength will typically cause more damage nowadays than in past years," they normalized the losses from events over time by factoring out the effects of inflation, population growth, rising wealth per capita, and insurance penetration. After doing so, they found no evidence of a significant upward trend in losses from non-geophysical disasters globally over the 18-year period from 1990 to 2008. However, using data over a longer time period, they did find an upward trend for West Germany (from 1980 to 2008) and for the United States (from 1973 to 2008).

Specific weather-related events that contributed to this upward trend included convective storms (like flash floods and hail storms) and winter storms. The authors speculate that the absence of a trend in the global data may be an artifact of the short-term data set used -- because of the difficulties of teasing out a significant signal from inherently noisy data.

What Role Does Global Warming Play?

So it's by no means a slam dunk, but an argument can be made that extreme weather events are up at least in some parts of the world. Let's suppose that's true. Then why? Is global warming the culprit? Answering those questions is a whole lot harder than answering the question of whether severe weather events are on the rise.

On the one hand, our basic understanding of the way the climate system works suggests global warming plays a role in making weather more extreme. We expect more temperature extremes, more heavy storms and the floods they bring, and more severe droughts and the wildfires they bring.

But it's one thing to expect something and another to observe it. And, given the vagaries and randomness of weather, rigorously identifying the role of a long-term climate trend in fostering any single given event is extremely difficult. There's some hope for a statistical approach, similar to the sort used for predictions like "there's a 50 percent chance of rain," but that's very much a work in progress.

The Path Forward: Dueling Philosophies

It seems like the recent past has delivered some strange weather. Is it part of a long-term trend? A trend due to global warming? If you require rigorous scientific evidence, the answer, at least today, is that we just don't know.

On that basis, some people seem content to sit back on the whole global-warming debate until we scientists can say we know for sure and answer those questions with rigorous nods yes. It's an approach that is perhaps encapsulated by the philosophy of Shakespeare's Falstaff who advises that "the better part of valor is discretion."

But there's also a philosophy that urges people to hedge their bets and act, even with incomplete knowledge. Think Ben Franklin's adage, "an ounce of prevention is better than a pound of cure."

Falstaff or Franklin? Choose your philosophy carefully -- in the case of global warming the world will have to live with it for decades to centuries, and in the words of songwriter Jerry Livingston, "that's a long, long time."

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