by Roger Pielke
In 2012, I was part of a research project that looked at trend in the number of tropical cyclones which made landfall around the world.[83] We found that there have been no significant trends (up or down) in global tropical cyclone landfalls since 1970 (when data allow for a comprehensive perspective; we have data going back further for various parts of the world), or in the overall number of tropical cyclones. The graph below shows those data, updated through 2013.
Figure 5.4: Global Tropical Cyclone Landfalls (1970-2013)
Source: After J. Weinkle, R. Maue, and R. Pielke, Jr., “Historical Global Tropical Cyclone Landfalls, Journal of Climate 25, No. 13 (2012): pp. 4729-4735. Thanks to R. Maue for the updated data.
Our collaborator Ryan Maue has analyzed data on total tropical cyclone activity worldwide since 1970 (not just the ones which make landfall).[84] Those data can be seen in the graph below.
Figure 5.5: Total Count of Tropical Cyclones of Tropical Storm (Top Curve) and Hurricane Strength, 12-Month Running Sums (1970 - 30 Nov. 2013)
Source: R. Maue.
There is simply little evidence to support claims that tropical cyclones, or hurricanes, have become more common or intense on climate timescales, a conclusion that is strongest for landfalling storms. It is thus no surprise that normalized loss studies have also failed to find increasing trends.[85]
A Final Note on U.S. Hurricanes
As of this writing, over halfway through the 2014 hurricane season, the United States is currently in a remarkable stretch with no major hurricane (Category 3+) landfalls, as shown in the figure below. The last major hurricane to strike the U.S. was Wilma back in 2005. That streak may have ended before you read this, or it might be still ongoing. Either way, it has been a remarkable period with no major U.S. hurricane landfalls.
The five-year period ending 2013 has seen 2 total hurricane (Cat 1+) landfalls. That was a record low for any five-year period since 1900. Two other five-year periods have seen 3 landfalls (years ending in 1984 and 1994). Prior to 1970 the fewest landfalls over a five-year period was 6. From 1940 to 1957, every 5-year period had more than 10 hurricane landfalls (1904-1920 was almost as active). These data suggest that the U.S., even with Superstorm Sandy, has been in a relatively benign period of hurricane activity, at least as compared to past eras.
Figure 5.6: Days Between Major Hurricane Landfalls in the U.S. Since 1900
There were 78 major hurricane landfalls in the U.S. from 1900 to 2013. Source: NOAA.
The Bottom Line on Tropical Cyclones
With respect to the central question that is the focus of this short volume, the data are clear. Those who assert that disasters are getting more costly because of climate change (human caused or not) are going to have to look at phenomena other than U.S. hurricanes or tropical cyclones around the world. There is no evidence to suggest that hurricanes have become more common, intense or costly for any reason other than more people and their property exist in locations vulnerable to the impacts of these powerful storms.
Floods
As with tropical cyclones, there is little evidence in support of claims that floods have become more common or more intense. The IPCC AR5 concludes:
In summary, there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale.[86]
The IPCC SREX went into a bit more detail:
· There is limited to medium evidence available to assess climate-driven observed changes in the magnitude and frequency of floods at regional scales.
· There is low agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of these changes.[87]
In early 2014, more than a dozen contributors to the IPCC SREX published a peer-reviewed paper specifically on floods, to expand their discussion of the state of the science.[88] The authors are clear that they are speaking for themselves, and not the IPCC, but they explain that their conclusions are “congruent” with those found in the SREX. Here are a few of their conclusions, which they note were focused on riverine floods, and not storm surges caused by coastal storms.
The authors explain: “a direct statistical link between anthropogenic climate change and trends in the magnitude/frequency of floods has not been established.” More specifically,
[N]o gauge-based evidence has been identified for a clear climate-driven, globally widespread, observed change in the magnitude/frequency of river floods during the last decades. There is thus low confidence regarding the magnitude/frequency and even the sign of these changes.
One important reason for this conclusion is that there are limited time-series data available in many regions of the world.
The authors are explicit in noting that trends in “extreme precipitation” have not translated to increased riverine flooding:
Despite the diagnosed extreme-precipitation-based signal, and its possible link to changes in flood patterns, no gauge-based evidence had been found for a climate-driven, globally widespread change in the magnitude/frequency of floods during the last decades.
The authors conclude their analysis with a plea to focus attention on more important issues than establishing a linkage between greenhouse gases and flood trends:
There is such a furor of concern about the linkage between greenhouse forcing and floods that it causes society to lose focus on the things we already know for certain about floods and how to mitigate and adapt to them. Blaming climate change for flood losses makes flood losses a global issue that appears to be out of the control of regional or national institutions. The scientific community needs to emphasize that the problem of flood losses is mostly about what we do on or to the landscape and that will be the case for decades to come.
Amen.
Tornadoes
Tornadoes occur in many regions around the world, but are most commonly found in North America. I recently was part of a research team that sought to apply the normalization methods that we first developed for U.S. hurricanes to tornadoes.[89]
Figure 5.7: Normalized U.S. Tornado Damage (1950-2013)
Updated courtesy K. Simmons.
The figure above shows an estimate of how much tornado damage would occur in the United States, if each year’s tornadoes occurred with today’s levels of population and development. The worst year for damage was 1953, but 1965 and 2011 aren’t far behind. In terms of loss of life, 2011, with 560 deaths, saw the most casualties since 1925, when 794 people died. Overall, however, the United States has seen a long-term decrease in both property damage and loss of life related to tornadoes. Yet even with this decline, 2011 reminds us that large impacts are always possible.
The graph reflects data from almost 58,000 tornadoes from 1950 through 2013. Using damage estimates of the U.S. government’s National Oceanic and Atmospheric Administration, we used several approaches to normalize the losses to 2014 values in order to estimate how much damage would occur if historical tornadoes occurred with today’s levels of population and development. The figure above shows our central estimate.
As with our work on hurricanes, a first question to ask is: how do we know if our estimates are any good?
And as with hurricanes we perform several independent checks. One is that we know which years had exceptionally large losses: 1953, 1965, 1974 and 2011. These four years show up clearly in our dataset as outliers.
A more sophisticated check is to compare trends in the incidence of tornadoes with trends in damage. Because counts of tornadoes are independent of damage estimates, they can serve as a basis for evaluating the appropriateness of our adjustments. Logically, we would expect that trends in damage and trends in tornado incidence would go in the same direction. This check is a bit tricky because meteorologists have changed how they track tornadoes over time, requiring us to break the overall dataset into a series of shorter periods. When we do this, we find excellent agreement between the damage trends and the trends in tornado incidence, giving us some conf
idence in our approach.
Average annual losses for the entire 63-year period across the U.S. are $5.9 billion (in 2014 dollars). However, for the first 32 years of the dataset (1950-1981) the annual average was $7.6 billion and since 1982, a period of 31 years, the annual average has been $4.1 billion, a drop of more than 50%.
Does the drop in average annual damage mean that there have actually been fewer tornadoes? Not necessarily.
The IPCC SREX explains that the quality of the data makes any conclusions about long-term trends problematic: “There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail.”[90] In our analysis we concluded that the data are “suggestive” of an actual decline in tornado incidence, but do not say anything stronger, and recommend further research.
Drought
As with tropical cyclones, floods, and tornadoes, there is little evidence to support claims that drought has increased globally on climate time scales.
The IPCC SREX concluded:
There is medium confidence that since the 1950s some regions of the world have experienced a trend to more intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughts have become less frequent, less intense, or shorter, for example, in central North America and northwestern Australia.[91]
For the U.S., the National Climate Assessment (NCA) concluded: “There has been no universal trend in the overall extent of drought across the continental U.S. since 1900.”[92] The NCA did point to regional differences, with some regions experiencing more periods of drought, such as the U.S. Southwest, and others less, like the Midwest.
The IPCC recently summarized its findings on drought, reaching a conclusion of “low confidence” on either detection or attribution:
There is not enough evidence to support medium or high confidence of attribution of increasing trends to anthropogenic forcings as a result of observational uncertainties and variable results from region to region (Section 2.6.2.3). Combined with difficulties described above in distinguishing decadal scale variability in drought from long-term climate change we conclude consistent with SREX that there is low confidence in detection and attribution of changes in drought over global land areas since the mid-20th century.[93]
In addition, the IPCC also concluded that recent drought in the western United States—that highlighted by the U.S. NCA—could not be attributed to human-caused climate change:
Recent long-term droughts in western North America cannot definitively be shown to lie outside the very large envelope of natural precipitation variability in this region (Cayan et al., 2010; Seager et al., 2010), particularly given new evidence of the history of high-magnitude natural drought and pluvial episodes suggested by paleoclimatic reconstructions.
There is little evidence provided by the IPCC to support claims that drought has become more frequent globally on climate timescales. Further, there is also little evidence in support of claims of attribution of causes for trends in regional drought.
At the regional level consider, for example, property damages resulting from bushfire in Australia. Bushfire disasters are, unfortunately, a common occurrence in Australia and like many extremes they are often attributed to human-caused climate change.
In 2009 I was part of a team of researchers who applied a normalization technique to historical losses from bushfires.[94] You can see the results of that research in the next set of graphs.
The top panel shows how many buildings have been destroyed in bushfires from 1926 to 2009. However, over that time period there have been many more buildings built in locations prone to bushfires. The bottom panel shows losses after they have been normalized to 2009. There is no evidence in the normalized data of an increasing trend in losses. In the logic of Figure 3.1, neither detection nor attribution has been achieved.[95]
Normalization based simply on changes in number of buildings of course does not provide a complete picture of societal changes during this period (for example, we did not try to estimate changes in the fire-resistance of buildings). Nonetheless, the normalized data are consistent with known patterns of climate variability, specifically the El Niño Southern Oscillation and the Indian Ocean Dipole. That is, after normalization, we see a good correspondence between bush fire losses and climate variability. As I discussed earlier, when normalized social trends match up well with independently observed climate trends, it gives us confidence that our normalization assumptions are reasonable.
Figure 5.8: Building Damage from Australian Bushfires (1925-2008)
Normalized Building Damage from Australian Bushfires (1925-2008)
Source: R.P. Crompton, K.J. McAneny, K.P. Chen, R.A. Pielke, and K. Haynes, “Influence of Location, Population, and Climate on Building Damage and Fatalities due to Australian Bushfire: 1925-2009,” Weather, Climate, and Society 2 (2009): pp. 300-310.
Conclusion
This short volume has sought to answer a straightforward question:
Have disasters become more costly because of human-caused climate change?
Only one answer to this question is strongly supported by the available data, the broad scientific literature, and the assessments of the IPCC:
No. There is exceedingly little evidence to support claims that disasters have become more costly because of human-caused climate change.
Of course, a lack of evidence does not prevent people from believing in God, aliens, or for that matter, a small celestial teapot orbiting the Sun in the asteroid belt. People may indeed have very good reasons for believing in any of these things for which data and observational evidence are unsupportive, unavailable, or inconclusive. The issue of disasters and climate change will be no different.
And of course, science evolves. There may be future research which overturns present understandings. If and when that happens, our assessment of what the science says should change accordingly. Thus, the conclusions presented here should be interpreted as an indication of the current state of scientific understandings, and not a prediction of what a future scientific assessment might say in the years to come.
Nonetheless, one point should be abundantly clear. The evidence available today points to a clear answer to the central question at the focus of this short volume: Human-caused climate change has not led to a detectable increase in the costs of disasters.
But the climate is changing. It would be a mistake to conclude that because the evidence shows that human-caused climate change has not led to demonstrable increases in the costs of disasters that (a) climate change is not occurring, or (b) we need not worry about it.
In this regard, those advocates for action who claim to see the influence of climate change do themselves no favors by stretching and sometimes going beyond what science can support. Sure, you can get attention and news coverage with assertions that changes in climate are leading to more disasters. But over the long term, are such strategies worth the risk of exaggerating what science can actually show with evidence?
6
What About Climate Policy and Politics?
So what? It is a question that my students get tired of hearing. So what if the science of disasters and climate change is exaggerated in public debates and by some scientists? As one climate scientist observed to me after Hurricane Katrina ravaged the Gulf Coast, if people think that today’s disasters are caused by or linked to human-caused climate change, isn’t that a good thing if it motivates the support for the right policies?
I have two answers to the “so what?” question.
One is that whatever passionate advocates and partisans may say in political debates, upholding scientific integrity means that someone must take responsibility for scientific accuracy. This might include journalists (both the regular sort and those who characterize themselves as the data sort), but it definitely should include leading scientific organizations like the IPCC and authorities. The public places great trust and credibility in the scientific community, which could easily be put at
risk.
A second response to the “so what?” question is that an approach to climate policy centered on associating disasters with greenhouse gas emissions is unlikely to succeed.
Today, the approach to climate policy favored by many leaders of the climate movement centers on extreme events. The logic, it seems, is that associating past and future disasters with human-caused emissions of greenhouse gases will convince the public to demand or at least accept higher costs of energy in order to help motivate the rapid abandonment of energy technologies that emit greenhouse gases.
For instance, in 2014 when the Obama Administration rolled out its climate agenda focused on extremes, a reporter noted to John Holdren, the president’s science advisor, that the public prioritizes jobs and the economy, with climate lagging far behind. Holdren’s response was a prediction based on the new strategy: “I think you are going to see the polls change.”[96]
As noted in the Introduction, the White House website claims that droughts and floods are getting more frequent.[97] Indeed, buried deep in the National Climate Assessment, which is purported to support claims of increasing climate disasters, the Obama Administration admits:
· “There has been no universal trend in the overall extent of drought across the continental U.S. since 1900.”
· “When averaging over the entire contiguous U.S., there is no overall trend in flood magnitudes.”[98]
That the Administration can state scientific conclusions which are clearly not supported, even by its own scientific assessment, without fear of challenge by the scientific community or the media suggests that the climate science community appears to have succumbed to “noble cause corruption.” The ends seem to justify the means.[99]