There Are No Such Things As 100-Year Floods: Global Warming Isn’t Making Floods Worse

There Are No Such Things As 100-Year Floods: Global Warming Isn’t Making Floods Worse

Reader Chuck Lampert writes:

Hi Briggs! I got into a kerfluffle with some Climate Apocalypse true believers who said the frequency of 1/50, 1/100, 1/500, and 1/1000 yr floods is increasing because of climate change. Please correct me if I am wrong, but these intensity/frequency figure are almost entirely arbitrary and unless we have 1000’s if not millions of years of data are almost meaningless when applied to the 20th and 21st centuries only.

You are not in need of correction. There are no such things as 1/50, 1/100 and such like floods. Therefore, they cannot be increasing. Nor decreasing. They can’t be anything.

The phrasing of the “hundred year flood” is old, and used to be more common. When it originated, the phrase was natural and even sensible. It was a way to express uncertainty in unusual events. It was not taken nearly as literally as we, in our hyper-numeric age, take it.

All it meant was “This is a big flood. Don’t get these very often. Maybe once a century.”

The speaker’s mind was also not addled by thoughts of “climate change” and the expectation that large floods must necessarily become more common because of the evils of men.

Why, incidentally, should floods become more, instead of less, common in a changing climate? There is only one answer to that.

Stop me if you’ve heard this one before, but there is no such thing as probability. It doesn’t exist: it isn’t alive: it is not a substance. Which is why there can be no such things as hundred year floods.

Probability is instead a measure of uncertainty of a proposition given some set of premises which must be specified. Different premises give different probabilities.

Pick as a proposition, “Flood of a large size this year”, where “large” is known to speaker and listener, and where the locality of the flood, its geographic extent, I mean, is also understood. Then you might gather premises that allow you, conditional on them, to quantify the uncertainty of this flood.

Say this quantification is 0.01, or 1%. That’s per year. Inverting that gives a “100 year flood.”

Easy, yes?

Only it’s not, because there are no fixed premises. Floods have causes. If we knew all the causes, and could with no error project them, then we’d know with certainty whether this flood will occur this year in this place or not.

If we don’t know the causes, we have to correlate. And we all know that correlation isn’t causation. Unless, of course, it’s your correlation. Then you’re happy to forget you knew. Thus does over-certainty grow.

Which exact correlates must we use to calculate the probability of our flood? There aren’t any. Let me repeat that: there aren’t any.

That means opinions can differ about the probability of the flood, because opinions differ over what best correlates (premises) to use.

Naturally, those touting global warming will include premises that insist floods must increase. Their probabilities will therefore necessarily be larger. It would not be a “discovery” that floods will increase: it was a premise. All models, which include probability models, only say what they are told to say.

That’s the best we can do about future floods: bicker over the best correlates and causes to include as premises. Then make projections and see how well these do. Problem is, large floods are rare, so it’s difficult to assess how well our premises (i.e. model) did in forecasting it.

We can do much better with the past. Ignoring measurement error, which is larger the further one goes back in time—remember, people weren’t as numerically obsessed as we are—we can at least see how many floods of a certain size there were. This becomes exceedingly difficult with large floods, because they are rarer.

Plus, we not only have to consider the floods themselves, but the changing reactions to them. People build against now better than before, etc.

Anyway, with all that said, there isn’t much going on with floods. I have some statistics quoted on this in two papers I did for the Global Warming Policy Foundation: this and even more in this. Quoting from the second:

A paper in Nature Communications helps us. It is ‘Trends in flood losses in Europe over the past 150 years’, by Dominik Paprotny and others, published in 2018. The authors calculate that floods in Europe killed about 21,000 people between 1870 and 1899. That fell to about 14,000 dead between 1900 and 1929, 12,000 in 1930–1959, about 6,000 in 1960–1989, and just over 2,000 between 1990 and 2016. This is an unambiguous improvement in climate-related fatalities. But it’s even better than it seems, because before 1900 there were fewer than 300 million people in Europe, and there are 746 million now. That means the rate of fatalities has not just fallen, it has, remarkably, plunged. As more people live in the same space, fewer in total and in proportion are dying from floods.

Also look to “A critical assessment of extreme events trends in times of global warming” by Gianluca Alimonti and others. They say:

This article reviews recent bibliography on time series of some extreme weather events and related response indicators in order to understand whether an increase in intensity and/or frequency is detectable…The analysis is then extended to some global response indicators of extreme meteorological events, namely natural disasters, floods, droughts… None of these response indicators show a clear positive trend of extreme events. In conclusion on the basis of observational data, the climate crisis that, according to many sources, we are experiencing today, is not evident yet.

You have to love that “yet”. Saves them from being canceled.

A popular source is Bjorn Lomborg’s “We’re Safer From Climate Disasters Than Ever Before“. He concludes, “Though it receives little mention from activists or the media, weather-related deaths have fallen dramatically.” Here’s his picture:

But as this series explained in regard to flood costs, only measuring the total damage of natural disasters over time misses the important point—there’s much more stuff to damage today than there was several decades ago.

As the world has gotten richer and its population has grown, the number and quality of structures in the path of floods, fires, and hurricanes have risen. If you remove this variable by looking at damage as a percent of gross domestic product, it actually paints an optimistic picture. The trend of weather-related damages from 1990 to 2020 declined from 0.26% of global GDP to 0.18%. A landmark study shows this has been the trend for poor and rich countries alike, regardless of the types of disaster. Economic growth and innovation have insulated all sorts of people from floods, droughts, wind, heat and cold.

Here’s the article Lomborg refers to: “The World Is Getting Safer From Floods“.

Though it hasn’t been well publicized, the U.N. Intergovernmental Panel on Climate Change says it has “low confidence in the human influence on the changes in high river flows on the global scale.” It expects more areas will see the frequency of floods go up than go down—a negative impact of climate change, but one that’s much less dramatic than media coverage might suggest. And as the world grows richer, infrastructure and technology are likely to drive down relative flooding costs and deaths. The data show they already are.

You get the idea. No reason to panic.

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  1. I love how folksy sayings used to describe the world around us become scientific terms used to destroy Western civilization. Oh well, takes a village and god willing and the creek don’t rise we’ll soldier on.

    On a related note don’t plan on the discussion getting any more rational any time sooner. Pretty sure the students in the local high school have done units on “climate change” each and every year that they have had a science class. At least one of the science teachers is pretty religious about it so I doubt they are actually seeing any contrary information. So all the new graduates over the past decade or so are walking down the aisle with the firmly held belief in man made climate change.

  2. Ye Olde Statistician

    As I recollect, one measures the floe rate of the river daily and over the course of the year, the maximum flow is called the “annual flood.” It needn’t be a “flood ” in the common parlance. The distribution of these annual floods are then fitted with an Extreme Value distribution. This is very different from a Normal distribution and has an extremely long positive tail (maxima) or negative tail (minima). The 0.01 cut-off is called the “100 year flood,” but this is only the probability estimated from the model for that magnitude flood in a single given year.It says nothing about the frequency of such flows over the years because, unlike most manufactured product river flow rates are not indpendent from year to year. That is, they are serially correlated (for the most part) because the relevant causes tend to persist.

    For example, my home town had terrible floods in 1903 (“the punkin freshet”), 1936, and 1955. Regarding 1903, a Perkasie newspaper said “Not in the memory of a man now living nor in history has there been such a disatrous flood in the Delaware…” But then there were three floods each the size of the 1903 Punkin Freshet in 2004, 2005, and 2006 — three years in a row. These were caused by construction in the Poconos for ex-pat New Yorkers and the concrete and asphalt increased the mountain run-off. Steps were taken to improve the drainage and the flooding ceased..

    The calculations of the model work very well in reliability engineering, where the intent is to make each unit identical; but that is not the case in nature and the main utility of “hundred year flood” estimates is to figure how big and strong to build a dam on that river.

  3. Briggs


    Yes, an engineer was asking me on Twitter about “100 year floods”, saying he had to build a tunnel against one. But this is the old-school use of the word, which meant “flood of a certain size.”

  4. Hagfish Bagpipe

    Briggs: ”No reason to panic.”

    No reason to panic?! — where’s the fun, or profit, in that?

    Scary headline:

    Global Power/Profit Lust Fueling Catastrophic Increase in 100-Year Media Panics

  5. Briggs


    Way off topic, but I finally got a pouch of “America’s Best”, which used to be “Red Man”, and which is now rarer than a thousand-year flood.

  6. Ann Cherry

    The New Pause Grows by Another Month to 7 Years 7 Months – Watts Up With That?

    “There has been no global warming – none at all – for 7 years 7 months. Yet, during that time, a significant fraction of the influence of humanity’s energetic industries and enterprises on the climate has occurred, without so much as a flicker of response from global mean surface temperature.”

  7. Douglas Skinner

    It is the physicists whom we can thank for the notion that probability is a property or a quantity like a system’s thermodynamic state. In fact they base thermodynamic states on probability as it relates to possible arrangements. Entropy, which was a thermodynamic quantity which is related to the direction of change in a system was related to something called “order.” An unstable isotope has a characteristic half life which, in turn, is somehow related to the “probability” that a observed atom will decay. In actuality no such observation–to my knowledge is actually done–partly because atoms lack markers by which they can be individuated.

    Anyway, to keep things short, I once worked on a problem concerning the probability of detecting a submarine. I realized after a while, that the number you get depends on your point of view. I maintained that my probability of detecting a submarine is unlikely to be the submarine commander’s probability of being detected. I mentioned this to some colleagues who took the attitude that if done right, both probabilities–mine and the submarine commanders–should agree. But why, I asked? They both know different things. For one thing, both depend on a particular strategy for detection which only they know and which is unknown to the other.

  8. Hagfish Bagpipe


    Even more rare–> tins of “Ice Nigger” pipe tobacco. Made of reindeer skat, aged. Good shit.

  9. C-Marie

    Love reading all of this, and actually understand some of it!

    God bless, C-Marie

  10. Briggs


    Love it.

    Take an Expert’s word for it: Experts failing proves we have to trust them.

  11. RB Hayes

    In my locality the phrase “100 year flood” is used to help the sewer management company ignore problems. Several times a year rain would overflow our combined city sewer system and sewage would back up into our basement. They never felt like they had to fix it because these events were so rare, “100 year floods.” Except they happened monthly during the summers.

    Flooding problems are blamed on Climate Change because it’s a nebulous boogeyman that governments can’t actually do anything about, so they have no responsibility to fix the problems. Instead, the real issues (overdevelopment and century old infrastructure) are ignored. Until they effect that rich folk, that is!

  12. Uncle Mike

    I used R’s EV package to model Predicted Maximum Precipitation (PMP) for the Corps of Engineers. They used it to estimate capacity specs for flood control dams. The stats are funky, but the Corps had few or no alternatives; they had to gin up some sort of guesstimate.

    Flood control dams are handy. They mitigate. Without them we’d have a flood every year here in Slosh.

    Important point: it’s always drought or deluge. It’s never average. The average is an imaginary statistical hypothetical and thus doesn’t exist or occur in the Real World. This factoid is wonderful if your thing is hersterical global doomsterism. It’s never average. You can always have a panic about the rain, whatever it is or isn’t.

  13. Robin

    As an engineer who has worked on hydrological studies; a unit hydrograph of water levels is developed based upon the watershed area, historical rainfall intensities and durations, and estimates of infiltration. This is then used to measure flooding at various locations in the watershed area.

    The statistics come into play when examining the historical rainfall intensity and duration data. As Briggs has said, we sometimes look at the once in a century interval as a practical design guideline.

    However we also know that infiltration changes too. With development activity in a watershed area, there is generally a reduced infiltration meaning much more rain water becomes surface runoff – hence flooding worsens at downstream locations. For example, paving roads or parking lots, building structures with large roof areas, driveways, etc.

    The climate alarmists fail to take these matters into account; in fact they don’t even know about them, in most cases. The same is true in the arguments behind rising tides, rising seas etc. There are localized effects such as changes in sedimentation, introduction of coastal structures, etc that can give the appearance of worsening conditions. Again this is nothing to do with so-called climate change.

    I’ve tried to bring these considerations to the attention of climate doom mongers; it’s been a useless exercise. Kind of like explaining that the earth really really isn’t flat to someone of the medieval period. They cannot grok it.

  14. Cary Cotterman

    If you watch the local news in southern California, it’s always dire when it doesn’t rain. And it’s always dire when it does rain. There have been two truly disastrous flooding events here in recorded history, which goes back to the late 18th century–in 1862 and 1938. Both happened because there was a lot of snow in the local mountains, followed by a long period of warm rain. These are commonly referred to as “100-year floods”, but it’s obvious that they were random, unpredictable occurrences, and under the right conditions could as likely happen every three years or every three hundred years.

  15. Dr. Weezil

    As (another, civil) engineer, I’ve been convinced for many years that what we are seeing when people claiming increases in flood “severity” or damage from which, is due to some combination of increase in overall land development and the utter failure of modern stormwater management facilities and the standard stormwater management design approach. My fluid mechanics professor in college was most based and despite the fact that he designed them on a regular basis, had utter contempt for retention basins, detention basins, dry basins, overflow channels, etc., often noting their regular failure to perform as designed and the fact that they were regulatory creations conceived to appeal to “feelings” and very little hard science or engineering mechanics.

  16. Kip Hansen

    If one wants to know about flooding, try a wiki search for “the great flood” (skip all the biblical flood links) The Great Flood of 1862, 1927, 1937, 1997, 1945, 1978, 1913, 1964, 1989, 1883, 1884. The same is true in Europe. China has been fdealing with massive deadly floods literally forever.

    Nothing beats the 1937 Mississippi Flood.

  17. Those of us whose interest and competence is history can certainly agree that thousands of more years of data is necessary to both verify and POSSIBLY determine the cause of major weather events. Archeological records seem to indicate varying flood periods in ancient Egypt, one of which may account for the prosperous years of the biblical Joseph of the bible along with subsequent lean years. There was almost certainly a climate factor though there are no records to verify this. During recorded history, events cited were attributed to everything from bad air to the will of God (or gods) and little was done about them except to attempt to adapt. Today attempts are made to attribute the cause of climate events to carbon in the air and attempt various remedies, some quite fanciful. Interesting that most remedies involve increasing government and more sacrifices from the public.

  18. Chuck Lampert

    On Twitter I recently challenged another alarmist who was prattling on about 1000 year floods when I wrote:

    “ “1000 yr weather event” is pure alarmism. Who was around to observe floods & other weather events 1000 yrs ago? Even if it was what you & other climatists called it, a “1000 yr weather event can occur 3 days in a row or 3 years in a row & still is statistically meaningless.”

    Of course he blocked me but so many are buying what he is selling.

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