Tuesday, 29 August 2017

Q: Is Houston flooding a sign of global warming?

Q: Is Houston flooding a sign of global warming – or what the fellow travellers now call “climate change”?
A: No. No, it’s not.

To explain why not, here’s climate scientist Roy Spencer:

The flood disaster unfolding in Houston is certainly very unusual. But so are other natural weather disasters, which have always occurred and always will occur.…

His first point: “Floods aren’t due to weather.”

Wait, what? How’s that again? Floods aren’t due to weather?? Well, floods aren’t just due to weather.

Major floods are difficult to compare throughout history because the ways in which we alter the landscape. For example, as cities like Houston expand over the years, soil is covered up by roads, parking lots, and buildings, with water rapidly draining off rather than soaking into the soil. The population of Houston is now ten times what it was in the 1920s. The Houston metroplex area has expanded greatly and the water drainage is basically in the direction of downtown Houston.

Ah, I see. Still, this was Houston in the great flood of 1935, when the water level at Buffalo Bayou in Houston topped out at 16.6m:


This latest Monday morning (Texas time) saw the water level at that same location at just 11.6m – still bad enough and, as Spenser says, certain to rise. But given this has happened before, it does makes you wonder why the city isn’t better prepared to deal with such a disaster.

So, the inevitable next question: Is the recent rainfall unprecedented?

Even that question is difficult to answer. The exact same tropical system moving at, say, 15 mph might have produced the same total amount of rain, but it would have been spread over a wide area, maybe many states, with no flooding disaster. This is usually what happens with landfalling hurricanes. Instead, Harvey stalled after it came ashore and so all of the rain has been concentrated in a relatively small portion of Texas around the Houston area…. There is no aspect of global warming theory that says rain systems are going to be moving slower, as we are seeing in Texas. This is just the luck of the draw. Sometimes weather systems stall, and that sucks if you are caught under one.

Bugger. But what about those levels?

Even with the system stalling, the greatest multi-day rainfall total as of 9 a.m. this Monday morning is one metre, with many locations recording over 500mm. We should recall that Tropical Storm Claudette in 1979 (a much smaller and weaker system than Harvey) produced an 1100mm rainfall total in only 24 hours in Houston.

So, is Harvey itself unprecedented in intensity?

In this case, we didn’t have just a tropical storm like Claudette, but a major hurricane, which covered a much larger area with heavy rain. Roger Pielke Jr. has pointed out that the U.S. has had only four Category 4 (or stronger) hurricane strikes since 1970, but in about the same number of years preceding 1970 there were 14 strikes. So we can’t say that we are experiencing more intense hurricanes in recent decades
    Going back even earlier, a Category 4 hurricane struck Galveston in 1900, killing between 6,000 and 12,000 people. That was the greatest natural disaster in U.S. history.
    And don’t forget, we just went through an unprecedented length of time – almost 12 years – without a major hurricane (Cat 3 or stronger) making landfall in the U.S.

So, what does make this event unprecedented?

The National Weather Service has termed the event unfolding in the Houston area as unprecedented. I’m not sure why. I suspect in terms of damage and number of people affected, that will be the case. But the primary reason won’t be because this was an unprecedented meteorological event [but because more people now live in the affected area than ever before].
    If we are talking about the 100 years or so that we have rainfall records, then it might be that southeast Texas hasn’t seen this much total rain fall over a fairly wide area. At this point it doesn’t look like any rain gauge locations will break the record for total 24 hour rainfall in Texas, or possibly even for storm total rainfall, but to have so large an area having over 20 inches is very unusual.
    They will break records for their individual gauge locations, but that’s the kind of record that is routinely broken somewhere anyway, like record high and low temperatures.
    In any case, I’d be surprised if such a meteorological event didn’t happen in centuries past in this area, before we were measuring them.
    And don’t pay attention to claims of 500 year flood events, which
most hydrologists dislike because we don’t have enough measurements over time to determine such things, especially when they also depend on our altering of the landscape over time.
    Bill Read, a former director of the National Hurricane Center
was asked by a CNN news anchor whether he thought that Harvey was made worse because of global warming. Read’s response was basically, No
    “Unprecedented” doesn’t necessarily mean it represents a new normal. It can just be a rare combination of events. In 2005 the U.S. was struck by many strong hurricanes, and the NHC even ran out of names to give all of the tropical storms. Then we went almost 12 years without a major (Cat 3 or stronger) hurricane strike.
    Weird stuff happens

“Weird stuff happens.”

And “weather disasters happen, with or without the help of humans.”

Spenser has much more in the comments section of his post, where he answers questions.

Thoughts are wth everyone affected. Here’s the inevitable Stevie Ray Vaughan:



  1. Iwi want ownership of the water in NZ. If there was a major flood causing huge amount of damage, would they then be liable.?

  2. There's actually a pretty straight-forward way to assess the frequency of major hurricanes making landfall through time: barrier islands. These features move, and can move substantially (several feet or more) during a hurricane. This movement leaves distinct traces in the sedimentological record. When the hurricane hit New York a few years back, several geologists pointed out that while it was unprecedented in modern times, that's merely because we haven't been keeping records long enough to see it. Such storms hit New York about 3-4 times every thousand years, according to the barrier island records. We can make a similar assessment with Texas.


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