Increase the Probability of a 100 Flood – It Can’t be Done!

Global warming is a real issue. The rise of sea level as a consequence of global warming is a natural consequence. I am not going to debate the reality of climate change or the reasons for climate change. Here I need to talk about how in describing a real consequence those who believe in global warming sometimes get the statistics wrong. No matter which side of the issue someone is on, it is always best to get the facts right or else they diminish credibility.

Such was the case this morning when I spotted an article in the Washington Post titled: “Risk of sea-level rise outlined.” Take this one quote from the piece:

Along the shores of Virginia, sea level rise multiplies the risk of a 100-year flood by 2030 by more than a factor of three, from 9 percent to 29 percent.

Now wait a minute a one year flood risk is a statistical statement of the expected frequency of floods within the 100-year flood plain. So I went to the source document at climatecentral.org. Their report, titled “Surging Seas”, outlines the consequence of rises in sea level resulting from global warming. The fact sheet for the state of Virginia states:

Odds of a 100-year flood or worse by 2030, with sea level rise from global warming: 29%

Table 2 on page 9 of the report is even more confusing. It provides “projected odds by 2030 for Historic ‘Century’ Floods.” for various locations. For example the number given for Atlantic City, NJ is 30%. I have no idea what that statement means. Does that mean there is a three in ten change of a 100 year flood in Atlantic City in 2030?

The authors attempt to finesse the issue with a footnote saying:

A 100-year flood is defined as a flood reaching a fixed elevation so high that it is expected to take place with only a 1% chance in a year, here assuming a water elevation baseline at 2009 sea level. But because sea level is rising, the odds of floods reaching any fixed elevation become higher over time.

Here the first phrase is correct. And the second sentence is true. But the statement “assuming a water elevation baseline at 2009 sea level” attempts to change the rules. What they are doing is reevaluating the risk of a given flood within currently defined geographic 100 year flood plains. They are then trying to say that these areas will be flooded more frequently if sea level were to rise. That is a true statement. However if there is a sea level rise the 100 year flood plain will expand and the statistical definition holds constant. There is still a 1% chance of a flood of that level each year.

The shame is that in what seems to be well done work they have created confusion with the mixture of definitions and logic. The case they have made still holds true – flooding will be worst in the presence of sea-level rises.