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What Is A 1000 Year Flood?

April 18, 2023

By Paul Homewood

As is usual, Roger Pielke destroys the latest alarmist meme with solid analysis:


We’ve all heard the terminology. An extreme event happens — a flood or heat wave — and soon after it is characterized as a “1,000-year event” (or it doesn’t have to be 1,000, it could be any number). This week I watched one of the world’s most visible climate scientists, Michael E. Mann, go on national TV and in process show that he had no idea what the concept actually means.

Let’s start by correcting that climate scientist who expressed a popular misconception (about which climate scientists should know better). A 1,000-year flood does not refer to a level of flooding that comes around every 1,000 years.

As the U.S. Geological Survey explains:

The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. 

Similarly, a 2-year flood has a 50% probability of occurring in any year and a 100-year flood has a 1% chance of occurring in any year. If we are instead talking about two six-sided die, then in the same vernacular rolling snake eyes (2 ones) would be called a 36-roll event. No one talks like that in the dice games I play. No wonder people are confused.

Can you roll snake eyes in two successive rolls? Sure you can. It is certainly rare, especially if you are all by yourself — a 1,296-roll event — but it can happen. Now imagine that you have 30 people in all 50 U.S. states each rolling two dice at the same time, how many snake eyes would you expect across those 1,500 rolls? Now imagine that they roll their dice together every day for a year, or that it is 500 people not 30.

Read the full story here.

In laymen’s terms, Roger is merely pointing out that a 1-in 1000 year event is par for the course somewhere or other when there are thousands of places in the world where such events can occur.

Roger finishes by saying:

The concept is widely misunderstood and implies a set of assumptions about the climate system that are not reflective of reality. I know the concept is now widely used in so-called event attribution analyses and press releases, and that just adds to the confusion. It is also built into the policy fabric of U.S. flood policy and is used in the insurance industry.

But if a decorated climate scientist doesn’t even understand the concept, then it probably won’t work in public discourse. Let’s talk about extreme events without invoking the N-year event.

I would go much farther. This fundamentally dishonest use of statistics is actually deliberate and fraudulent. As its perpetrators know full well.

  1. April 18, 2023 9:57 pm

    Roger is absolutely correct in his assessment. Statistical theory is always a post-priori operation. From any assemblage of data, I can say events of a certain magnitude and frequency occurred. That is purely relevant to the particular circumstances inherent in that data. The wholesale expansion of that set of conclusions to totally different areas and conditions is not valid science. This appears to be what the media and some ‘climate’ scientists have done. The number of cards in a pack is 52 and its division into suits is fixed. Therefore, applying statistics to the probabilities involved is a valid excercise. But that would not apply to a pack of cards that sometimes had 56 or 49 cards. Or had a suit distribution that was different. Gamblers exploit this ‘closed’ distribution model and are safe, unless they are playing with a cheat!

  2. April 18, 2023 10:19 pm

    It was never about truth. The scare effect is all they’re looking for.

  3. MrGrimNasty permalink
    April 18, 2023 10:26 pm

    Climate scientists understand perfectly, that they can bamboozle the public with distorted statistics to spread alarm.

    • dave permalink
      April 19, 2023 8:18 am

      We are talking about ‘Poisson processes.’ How many people have the faintest idea what that is all about? In a world where most people cannot even define a percentage, I think not many.

      The psychological point of interest is that people fail to understand that ‘surprises’ are normal. If you are told to expect a package between one and two o’clock you are not surprised that it comes in that period but the actual MOMENT of the the door bell ringing will ALWAYS be ‘a surprise’ and make you jump.

  4. kjbirby permalink
    April 19, 2023 7:51 am

    The headline ‘1000-year flood’ gives the impression of a flood that lasts for a thousand years. The headline should read ‘A once-in-a-thousand-years flood’.

    • Mike Jackson permalink
      April 19, 2023 4:39 pm

      That’s not strictly correct either. As the article describes it, it is one chance in a thousand that there will be a flood of a given magnitude in any one year.
      Given that any one weather event is random though in some instances constrained by the season it is perfectly feasible to have three “thousand year floods” in quick succession.
      In a way the description is statistically meaningless. What might be a 10-year flood in India could equally be 10,000-year flood in the Kalahari desert!

      • Carnot permalink
        April 20, 2023 9:48 am

        Mike, You are spot on. But is is even worse. I have first hand experience with this nonsense. My local council devloped a flooed alleviation scheme based on a 1:100 year event. actually a 1% probability. The snag is that this is meaning less when looking at precipitation. There wondeful model looked impressive but its prediction did not match the reality. Area that should not have flooded in the model , did actually flood. No account was taken into consideration that the rate of precipitation is important. In our case we had 80mm of rain in about 2 hours which completely overwhemed the capacity of the drainage system. The expnsive flood barrier will be too small, and cannot hold up enough water in a reat event that caused the last flood.

  5. eromgiw permalink
    April 19, 2023 8:04 am

    These statistics beloved of alarmists assume Gaussian distribution, thin tails, when weather phenomenon actually follow Power Law distribution, fat tails.

  6. mjr permalink
    April 19, 2023 9:06 am

    Lies, Damned Lies, And Statistics………… oh, and the BBC

  7. Ben Vorlich permalink
    April 19, 2023 12:45 pm

    These once a whatever events are quite localised as far as I can tell for reading reports headlines etc, Although if you only read the BBC you wouldn’t think so. So we in Derby could have once in a thousand year rainfall one day and Leicester one tomorrow and Nottingham get theirs on Friday. The BBC will say, in these circumstances, climate change is getting worse.

  8. Colin permalink
    April 19, 2023 12:53 pm

    Climate scientists are so clever that they can point to increase in 50 year floods using 20 years of data.

  9. gezza1298 permalink
    April 19, 2023 2:00 pm

    There could be some research into flood management at Houston and the concept that the skyscrapers can stall weather systems and exacerbate the rainfall. But then when does proper research get funding?

    A shame to see Tim Dunn go down the global warming rabbit hole in Architecture the Railways Built with extreme weather when talking about the GWR line at Dawlish. I have no doubt no evidence of it exists but both him and the Network Rail moppet went on about it.

  10. Dave Cowdell permalink
    April 19, 2023 3:16 pm

    well,as one involved in dam construction there was one year when in Africa we had 3 1000 year floods in one season.

  11. Richard permalink
    April 19, 2023 4:33 pm

    There has been a 600% in flooding due to the increase in Urbanisation –

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