18 April 2011

A Decrease in Floods Around the World?

A new analysis of floods around the world has been called to my attention.  The new analysis is contrary to conventional wisdom but consistent with the scientific literature on global trends in peak streamflows.  Is it possible that floods are not increasing or even in decline while most people have come to believe the opposite?

Bouziotas et al. presented a paper at the EGU a few weeks ago (PDF) and concluded:
Analysis of trends and of aggregated time series on climatic (30-year) scale does not indicate consistent trends worldwide. Despite common perception, in general, the detected trends are more negative (less intense floods in most recent years) than positive. Similarly, Svensson et al. (2005) and Di Baldassarre et al. (2010) did not find systematical change neither in flood increasing or decreasing numbers nor change in flood magnitudes in their analysis.
This finding is largely consistent with Kundzewicz et al. (2005) who find:
Out of more than a thousand long time series made available by the Global Runoff Data Centre (GRDC) in Koblenz, Germany, a worldwide data set consisting of 195 long series of daily mean flow records was selected, based on such criteria as length of series, currency, lack of gaps and missing values, adequate geographical distribution, and priority to smaller catchments. The analysis of annual maximum flows does not support the hypothesis of ubiquitous growth of high flows. Although 27 cases of strong, statistically significant increase were identified by the Mann-Kendall test, there are 31 decreases as well, and most (137) time series do not show any significant changes (at the 10% level). Caution is advised in interpreting these results as flooding is a complex phenomenon, caused by a number of factors that can be associated with local, regional, and hemispheric climatic processes. Moreover, river flow has strong natural variability and exhibits long-term persistence which can confound the results of trend and significance tests.
They conclude (emphasis added):
Destructive floods observed in the last decade all over the world have led to record high material damage. The conventional belief is that the increasing cost of floods is associated with increasing human development on flood plains (Pielke & Downton, 2000). However, the question remains as to whether or not the frequency and/or magnitude of flooding is also increasing and, if so, whether it is in response to climate variability and change.

Several scenarios of future climate indicate a likelihood of increased intense precipitation and flood hazard. However, observations to date provide no conclusive and general proof as to how climate change affects flood behaviour.
References:

Bouziotas, D., G. Deskos, N. Mastrantonas, D. Tsaknias, G. Vangelidis, S.M. Papalexiou, and D. Koutsoyiannis, Long-term properties of annual maximum daily river discharge worldwide, European Geosciences Union General Assembly 2011, Geophysical Research Abstracts, Vol. 13, Vienna, EGU2011-1439, European Geosciences Union, 2011.

Kundzewicz, Z.W., D. Graczyk, T. Maurer, I. Przymusińska, M. Radziejewski, C. Svensson and M. Szwed, 2005(a):Trend detection in river flow time-series: 1. annual maximum flow. Hydrol. Sci. J., 50(5): 797-810.

7 comments:

nearwalden said...

I often wonder about the rise of the internet in our perception of the number of tragedies and calamaties. Before the internet our awareness of these events was limited by the length of the national and local TV news and the surface area of our newspapers. Now those limits are gone, so we have the opportunity to be exposed to far more national and international news than ever before.

For those of us who's adult lives have spanned this period, it seems like it would be innevitable to experience a sense of increase in bad news. Has anyone done a study of this?

Edward Spalton said...

I think it was back in the Thirties when a competition was run for the most boring possible news headline. If I remember rightly, the winner was

"Small earthquake in Africa. Not many dead"

Now we instantly see the reality of incidents like this and many other catastrophes on TV with appeals for the victims. So, of course we think there are more of them. Previously all we saw was a few column inches of newsprint - so, of course, we feel that there are greater catastrophes and more of them.

Jan said...

This is interesting. I wonder, though, how sensitive these conclusions are for selection of timeseries. Furthermore, a recent article by Diermanse et al. (http://bit.ly/hnGSdT) tested timeseries of Rhine and Meuse rivers for any statistical evidence of upward trends. None passed the test. Does this mean no trend is present?

"However, it is also shown that the statistical tests have insufficient detection power for a relatively weak trend in the 100-year discharge series. The probability of detection of a trend by these tests will only exceed 50% for time series of more than 130 years. Furthermore, it is shown that the tests are not far from the significance level. Extension of the original data series by synthetic data demonstrates that the statistical tests will reject the null hypothesis if the presumed trend will continue over the next decades. Therefore, the fact that the discharge series of the Rhine and Meuse rivers are close to giving evidence for non-stationarity should serve as a serious warning for the possibility of a systematic increase in river discharges."

Philip said...

These claims were made shortly after the disastrous flood which hit Brisbane, Australia by warmists and the Greens leader in Australia, when a simple check of Met Bureau data, available on the net, proved otherwise. The simple facts here were:
In Brisbane, there were 8 major floods which
occurred in the Brisbane River between 1825 and 1900 compared with the 
3 major floods between 1900 and now. This included 2 in 1893 which reached 8m + at the city gauge compared to the 4.2m flood of Jan 2011.
Similar claims were made about cyclone Yasi being caused by HICC when the Met Statistics proved otherwise- cyclone frequency around northern Australia is less now than in recent history.
Yet, the mainstream media doesn't question these allegations made by the trendy "green" elite whose opinions tend to dominate the airwaves unchallenged.

Tom said...

It seems to me that building 800,000 dams over the past couple of centuries certainly should improve flood control to some extent, even with variations in precipitation.

Mike said...

"Therefore, the fact that the discharge series of the Rhine and Meuse rivers are close to giving evidence for non-stationarity should serve as a serious warning for the possibility of a systematic increase in river discharges."

Wow, talk about grasping at straws! What the data apparently say is that any systematic increase (or decrease) is negligible compared to the shorter-term "random" variability. Therefore, protection strategies should probably be based on that "random" variability and not on assumed effects of global warming...

janverkade said...

@Mike:

"What the data apparently say is that any systematic increase (or decrease) is negligible compared to the shorter-term "random" variability."

Yes, but only just.

"Therefore, protection strategies should probably be based on that "random" variability and not on assumed effects of global warming..."

And that's indeed the case.

The point that was made though, is that statistical evidence should be taken at face value. In this case, the time series just isn't long enough to be able to detect any trend -whichever direction- with any high probability.

I'll concede that the sentence you quoted does not automatically follow from the points raised prior to it and actually confuses the argument being made.

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