04 January 2011

Signals of Anthropogenic Climate Change in Disaster Data

[UPDATE: Joe Romm blows a gasket on this paper.  Have a look at his post and you'll get a sense why it is that he dodged an opportunity to debate me last year (an offer that remains open) even after a donor promised $20,000 to Doctors Without Borders if he'd participate.] 

At ClimateWire, Evan Lehmann has a lengthy overview (also here at the NYT) of a new paper by Ryan Crompton, John McAneney (both of Macquarie University) and me on detecting signals of human-caused climate change in disaster losses. Ryan, who is wicked smart, did the heavy lifting and heavy thinking on it and deserves credit for what should be a widely influential article.

Lehmann writes in the article:
Economic losses are seen as a potent storyteller about climate change. If greenhouse gases could be shown to increase financial damages, that might accelerate efforts to develop stronger buildings codes, influence insurance prices for coastal homes, and discourage development in risk-prone areas.

But the research tells a different story, at least for hurricanes. As a backdrop, it uses a landmark study published in Science last January finding that the number of strongest hurricanes, categories 4 and 5, could double in 100 years because of climate change.

The researchers begin by assuming that's true. Then they apply hurricane damage data from the past century to those future hazards, adjusting for growth in population, inflation and wealth.

The results indicate that future hurricane damages won't produce a tangible "climate signal" for at least 120 years, and perhaps not for 550 years. The average time before a signal might be seen is 260 years, according to the combined findings of an 18-model ensemble used by the researchers. In that year, 2271, climate change is expected to increase damage by 106 percent, more than double.

The researchers know this is a touchy topic. It could be perceived as an effort to downplay the impacts of climate change, or be seen with alarm by environmentalists advocating for action now to cut carbon pollution.

"It's not to dispute that [global warming] is happening or what influence it will have on hurricanes," said Ryan Crompton, a co-author and a catastrophe risk expert with Risk Frontiers, a research organization at Macquarie University near Sydney, Australia, that is funded in part by the insurance industry.
The study that we build off of is Bender et al. 2010 which argues that under their projected changes in the behavior of hurricanes a signal will not be detectable in the geophysical data until later this century.  If it takes that long to detect a signal in the geophysical data, it is just common sense that it will take longer to see that signal emerge in loss data.  Tom Knutson, a co-author of Bender et al. agrees:
. . . the economic loss research confirms what Tom Knutson, a research meteorologist with the National Oceanic and Atmospheric Administration and an expert on climate change and hurricanes, has considered common sense: Climate change will reveal itself first in the hazard, then in the damage.

"Nothing too earth-shattering here," Knutson said. "This study actually doesn't tell me much that I already sort of have a notion of anyway. Namely, that if one is looking for a signal of climate change in the hurricane record, damage is not the first place to go to look for it."
Our 2006 Hohenkammer workshop with Munich Re reached this same conclusion. Our paper makes a pretty convincing and straightforward argument (in my view) and should make it clear why it is just plain wrong to attribute recent disasters (and even recent trends in disasters) to human-caused climate change.

While our paper focused on Atlantic hurricanes, the same sort of dynamics are likely to be present for other phenomena in other locations, as well as in aggregated global loss data.  This paper provides a convincing reason why the literature review of normalization studies conducted by Bouwer last year did not find any peer reviewed papers detecting and attributing trends in disasters to greenhouse gas emissions.  Such attribution is not likely in the near term, at least based on the magnitude of changes projected by Bender et al.

The paper is forthcoming in Environmental Research Letters -- here is the citation, abstract and concluding section:
Crompton, R. P.,  R. A. Pielke Jr. and K. J. McAneney, 2011 (forthcoming). Emergence time scales for detection of anthropogenic climate change in US tropical cyclone loss data, Environmental Research Letters V. 6, No. 1.


Recent reviews have concluded that efforts to date have yet to detect or attribute an anthropogenic climate change influence on Atlantic tropical cyclone (of at least tropical storm strength) behaviour and concomitant damage. However, identification of such influence cannot be ruled out in the future. Using projections of future tropical cyclone activity from a recent prominent study we estimate the time it would take for anthropogenic signals to emerge in a time series of normalized US tropical cyclone losses. Depending on the global climate model(s) underpinning the projection, emergence time scales range between 120 and 550 years, reflecting a large uncertainty. It takes 260 years for an 18-model ensemble-based signal to emerge. Consequently, under the projections examined here, the detection or attribution of an anthropogenic signal in tropical cyclone loss data is extremely unlikely to occur over periods of several decades (and even longer). This caution extends more generally to global weather-related natural disaster losses.


This study has investigated the impact of the Bender et al [11] Atlantic storm projections on US tropical cyclone economic losses. The emergence time scale of these anthropogenic climate change signals in normalized losses was found to be between 120 and 550 years. The 18-model ensemble-based signal emerges in 260 years.

This result confirms the general agreement that it is far more efficient to seek to detect anthropogenic signals in geophysical data directly rather than in loss data [14]. It also has implications for the emergence time scale of anthropogenic signals in global weather-related natural disaster losses given these losses are highly correlated with US tropical cyclone losses (supplementary discussion and supplementary table 1). Our results suggest that the emergence time scales are likely to be even longer than those determined for US tropical cyclone losses given that different perils will have different sensitivities to future anthropogenic climate change and may even change in different directions. We note that US tropical cyclone losses may become increasingly less correlated with global weather-related records as the loss potentials of developing countries in particular continue to rise rapidly, irrespective of future changes in climate [15]. This means that the relationship between the signal emergence time in US tropical cyclone losses and global losses may weaken over time.

Based on the results from our emergence time scale analysis we urge extreme caution in attributing short term trends (i.e., over many decades and longer) in normalized US tropical cyclone losses to anthropogenic climate change. The same conclusion applies to global weather-related natural disaster losses at least in the near future. Not only is short term variability not ‘climate change’ (which the IPCC defines on time scales of 30 to 50 years or longer), but anthropogenic climate change signals are very unlikely to emerge in US tropical cyclone losses at time scales of less than a century under the projections examined here.

Our results argue very 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. Policy making related to climate necessarily must occur under uncertainty and ignorance. Our analysis indicates that such conditions will persist on timescales longer than those of decision making, strengthening the case for expanding disaster risk reduction in climate adaptation policy [15].
If you'd like a pre-publication copy of the paper please send me an email at pielke@colorado.edu.