How long will scientists have to measure annual economic damages from hurricanes before they can confidently say that global warming is making storms stronger? In An Inconvenient Truth, Al Gore claimed the evidence is already clear in the damage trends of the last several decades. But a new study finds that any warming-related increase in hurricane damages won’t be detectable for a century a more.

Last week I blogged about a study (Bouwer, L.M. 2011) debunking a misconception — popularized in Al Gore’s film, An Inconvenient Truth — that we know global warming intensifies extreme weather events because economic damages from extreme weather keep going up, decade after decade.

Gore did not realize that the economic loss data he was looking at had not been adjusted (“normalized”) to take into account changes in socio-economic factors — notably population, wealth, and the consumer price index — that massively affect how much damage a particular weather event inflicts.

As discussed in last week’s post, Laurens M. Bouwer of the Institute for Environmental Research in the Netherlands analyzed 22 previous studies attempting to find an anthropogenic warming “signal” in normalized weather-related loss data. Bouwer’s key conclusion:

The studies show no trends in losses, corrected for changes (increases) in population and capital at risk, that could be attributed to anthropogenic climate change. Therefore, it can be concluded that anthropogenic climate change so far has not had a significant impact on losses from natural disasters.

But what about the future? Most IPCC climate models project an increase in the strength of tropical storms and hurricanes as the oceans warm. When will the climate-change contribution to hurricane-related economic losses (assuming there is one) be detectable in normalized loss data?

That is the question Ryan Crompton, Roger Pielke, Jr., and K. John McAneney explore in a recent study. The short answer is that nobody reading this post today will likely be around when (if) the warming signal emerges!

The researchers set out to determine “the time it would take for anthropogenic signals to emerge in a time series of normalized US tropical cyclone losses.” That is, they seek to determine the anthropogenic signal’s “emergence timescale.” By “cyclone,” the authors include all Atlantic tropical storms (up through category 5 hurricanes) with maximum sustained wind speeds of at least 63 kph.

To project changes in hurricane behavior over time, the authors used the IPCC’s 18-model ensemble plus other projections from four of the ensemble’s leading models (Geophysical Fluid Dynamics Laboratory, Japanese Meteorological Research Institute, Max Planck Institute, and Hadley Centre UK Meteorological Office).

Here’s what they found:

The emergence timescale of these anthropogenic climate change signals in normalized losses was found to be between 120 and 550 years. The 18-model-based ensemble signal emerges in 260 years.

The researches thus ”urge extreme caution in attributing short-term trends (i.e. over many decades and longer) in US tropical cyclone losses to anthropogenic climate change,” stating that “anthropogenic climate change signals are unlikely to emerge in US tropical cyclone losses on timescales of less than a century under the projections examined here.”

Note, the study does not mean scientists will not know for 120-550 years whether global warming intensifies hurricanes. As the authors write: “Our result confirms the general agreement that it is far more efficient to seek to detect anthropogenic signals in geophysical data rather than in loss data.” Nonetheless, if the study means what I think it does, it will be a long time before any “short-term” (multi-decadal) trend in hurricane losses can be attributed to global warming rather than to socio-economic factors and/or natural variability.

What is the policy implication? ”Our results argue strongly against using abnormally large losses from individual Atlantic hurricanes or seasons as either evidence of anthropogenic climate change or to justify actions on greenhouse gas emissions. There are far better justifications for action on greenhouse gases.” The authors don’t specify those “better justifications,” which presumably are outside the scope of their paper.

Although not mentioned by the authors, the study should pour cold water on some CO2 tort cases. In Comer v. Murphy Oil, for example, plaintiffs sued a wide range of energy, fossil fuel, and chemical companies for economic damages from Hurricane Katrina, alleging that the companies’ emissions contributed to global warming, which in turn increased the power of the storm.

Armed with the timescale emergence study, defendents in such a case could argue that their contribution to a hurricane’s power is not only undetectable today but will likely remain so for a century or more.

For a more technical review of the timescale emergence study, see “Detecting Footprint of Man in Tropical Cyclone Damage Data” at CO2Science.Org.

Al Gore’s film An Inconvenient Truth bombarded audiences with image after image of hurricanes, tornadoes, floods, forest fires, and drought, creating the impression of a world in climate chaos. Gore blamed the alleged upsurge in extreme weather on global warming, that is, mankind’s sins of emission. One of Gore’s mighty pieces of evidence was a dramatic increase in insurance payments for weather-related damages. As he writes in his best-selling book of the same title:

Over the last three decades, insurance companies have seen a 15-fold increase in the amount of money paid to victims of extreme weather. Hurricanes, floods, drought, tornadoes, wildfires and other natural disasters have caused these losses [An Inconvenient Truth, p. 101].

Gore presented a chart similar to this one:

Seeing is believing, right? The problem, of course, is not merely that correlation (warmer weather/bigger losses) does not prove causation. More importantly, the economic data depicted in the chart have not been adjusted (“normalized”) to offset increases in population, wealth, and the consumer price index.

Consider this fact: More people today live in just two Florida counties, Dade and Broward, than lived in all 109 coastal counties from Texas to Virginia in 1930. Florida’s population grew by more than 17.5% in the past decade alone and today is 48% larger than in 1980. There’s tons more stuff in harm’s way than there used to be. No wonder damages are bigger than in the good old days!

Most studies that “normalize” economic loss data find no evidence of a trend towards more violent or destructive weather. Here, for example, is a chart from a study on normalized hurricane damages by Pielke, Jr et al. 2008:

Figure description: U.S. hurricane damages, 1900-2005, if all hurricane strikes had hit the same locations but with year 2005 population, wealth, and consumer price index.

A study published earlier this year in the Bulletin of the American Meteorological Society (Bouwer, L.M. 2011. Have disaster losses increased due to anthropogenic climate change?) examines 22 previous studies on the oft-asserted link between climate change and weather-related damages.

Here’s what the researcher, Laurens M. Bouwer of the Institute for Environmental Studies in the Netherlands, found:

All 22 studies show that increases in exposure and wealth are by far the most important drivers for growing disaster losses. Most studies show that disaster losses have remained constant after normalization, including losses from earthquakes (see Vranes and Pielke 2009). Studies that did find increases after normalization did not fully correct for wealth and population increases, or they identified other sources of exposure increases or vulnerability changes or changing environmental conditions. No study identified changes in extreme weather due to anthropogenic climate change as the main driver for any remaining trend.

Bouwer concludes:

The studies show no trends in losses, corrected for changes (increases) in population and capital at risk, that could be attributed to anthropogenic climate change. Therefore, it can be concluded that anthropogenic climate change so far has not had a significant impact on losses from natural disasters.

CO2Science.Org has an excellent review of the Bouwer study. On a related issue, World Climate Report reviews a recent study finding no long-term increase in the number of Atlantic tropical storms and hurricanes over the past 130 years. The apparent increase in storm frequency turns out to be an artifact of the data, that is, a product of the increase in spatial coverage and accuracy of hurricane monitoring systems.