Thursday, October 1, 2015

Hurricanes and floods - USA

Sou | 4:13 AM Go to the first of 11 comments. Add a comment
As a follow up to my last article, there's one topic you won't read much of at WUWT, and that's extreme weather. So far, there's not been a single mention of Hurricane Joaquin. That system is currently poised to hit the Bahamas, and is making people on the east coast of the USA a tad nervous. Most particularly because it's very difficult to know whether it will go up the coast or veer out to sea.

Source: EarthWindMap

Coincidentally, the hurricane (as a tropical low) appeared about the same time as a new paper in PNAS, about how rising sea levels and changing tropical cyclones are increasing the risk of flooding in New York City. In the abstract, the authors write how "flood risk has greatly increased for the region; for example, the 500-y return period for a ∼2.25-m flood height during the pre-anthropogenic era has decreased to ∼24.4 y in the anthropogenic era."  500 years to 24.4 years is a huge change. You can read about the paper at The Carbon Brief, where Robert McSweeney writes:
Rising sea levels and changing tropical cyclones are pushing New York City coastal floods to new heights, says a study published today.

Floods hitting the city are now more than a metre higher than before humans had an influence on the climate, the research shows, increasing the risk of coastal defences being overwhelmed.

Read the full article - it's detailed and well written.


Andra J. Reed, Michael E. Mann, Kerry A. Emanuel, Ning Lin, Benjamin P. Horton, Andrew C. Kemp, and Jeffrey P. Donnelly (2015) "Increased threat of tropical cyclones and coastal flooding to New York City during the anthropogenic era", Proceedings of the National Academy of Sciences,doi/10.1073/pnas.1513127112


  1. It's not like the United States has been affected by any kind of unusual heat and/or precipitation-related events this year. What would Mr. Watts or his guests have to write about?

    "Lovely sunset, Willis."

    "Yes, Anthony, it was."

  2. "In the abstract, the authors write how "flood risk has greatly increased for the region; for example, the 500-y return period for a ∼2.25-m flood height during the pre-anthropogenic era has decreased to ∼24.4 y in the anthropogenic era." 500 years to 24.4 years is a huge change."

    In the past you could use past observations and some math to estimate how high the 500-year return level is. Now that we are changing the climate there is no other way to estimate this as climate models. The mitigation sceptics who claim to be against modelling make our society more dependent on models. While they hate scientists, their communities increasingly depend on scientists to keep them save because of the inaction they advocate for. If you want scientists to be less important, you'd better be a conservative and not mess with the climate.

  3. I've requested a copy of this paper as the USACE just recently completed a study for the North Atlantic Division (NAD), with the 2nd volume just released in August.

    One number that 'could' be grossly misinterpreted is this part of the abstract ...

    "We find that mean flood heights increased by ∼1.24 m (due mainly to sea level rise) from ∼A.D. 850 to the anthropogenic era, a result that is significant at the 99% confidence level."

    From the carbon brief PR figure ...

    The blue dots from 850AD to 1800AD look to be suspiciously close to the 1.24 meter SLR number as quoted in the abstract.

    1240 mm /(1800 - 850) = 1.305 mm/yr (versus whatever the current much larger rate is)

    I'm sort of hoping that no one else would look at that, in quite that way, without reading the entire paper.

    See also one of Kerry Emanual's recent papers ...
    Risks of Coastal Storm Surge and the Effect of Sea Level Rise in the Red River Delta, Vietnam

    Figure 5 in particular (that study used simple linear superpositioning of SLR + storm surge) where the maximum SLR was set to 0.5 m in 2050. You will also note some 'odd' behavior in the ranked series (the last three data points are for return frequencies of 1/3000, 1/1500 and 1/1000).

    From the abstract ...
    "Our analysis finds that sea level rise through 2050 could increase the effective frequency of the current 100-year storm surge, which is associated with a storm surge of roughly five meters, to once every 49 years."

    So an ~2X increase in frequency for 0.5 m SLR (100/49 = 2.0408).

    The current study looks as if 2 meters of SLR has occurred since 850AD (it appears to be ~1.9 m).

    So 2.0408^(1.9/0.5) = 15.04 and 500/15.04 = 33.2 year return period versus 24.4 year return period as stated in the current paper (yeah, some gross assumptions are being made here, but the return period is plotted as semilog, it is a very rough estimate, but that should give everyone some idea of how return period is affected by a SLR offset).

    I could also discuss another paper that uses something called the 'flood index' if anyone else is interested.

  4. I'm in the path, outside of DC. Cleaning gutters and hoping I don't have 12in of rain with high winds...

    1. I'll be thinking of you, KR. Stay safe.

    2. Yeah, I just looked at the cone, and it has changed drastically since yesterday. Good luck, the storm surge in the Chesapeake will be bad, bad, bad.

    3. Good luck. Stay safe.

    4. The forecast discussion last night indicated the cone was an inaccurate representation of the probabilities: the forecasts are binodal -- either out to sea (according to one model), or to the Carolinas (according to the others). They don't have tools to express that kind of uncertainty.

      Not that it changes the main message for DC: you're going to get wet. Stay safe! (Or travel far away, safe.)

    5. Current predictions for my area are 3-5 inches of rain, Charlotte South Carolina is expected to get 12-18.

  5. The current track looks like Sandy MkII. Ugh.

  6. Some of the climate science "truthers" were exclaiming about how the lack of major hurricanes hitting the U.S. coast in the past 8 years and Joaquin's projected change in path showed that global warming was not an issue. Funny, but the "truthers" are not discussing the 1000 year rain and flooding that is predicted for South Carolina, which is in large part due to a Joaquin produced "Predecessor Rain Event".


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