Saturday, August 20, 2005

Flooding part 2: Global warming has already caused damaging floods

In an earlier post I said that I wasn't quite so interested in Roger Pielke Jr.'s argument that the science is indefinite as to whether climate change has caused an increase in flooding damage up to the present. I thought what mattered is the solid science that it will occur in the future. Now I'm more interested because I think he's likely wrong in making such a categorical statement.

Earlier this week, after doing a little research I added this comment to Roger's post:

I'm starting to wonder if it is also clearly a good use of science to allege an existing, present-day connection between greenhouse gases and damaging, precipitation-induced, river flooding in and near coastal areas. From the site:

"The estimated rate of sea level rise from anthropogenic climate change from 1910 to 1990 (from modelling studies of thermal expansion, glaciers and ice sheets) ranges from 0.3 to 0.8 mm/yr.

It is very likely that 20th century warming has contributed significantly to the observed sea level rise, through thermal expansion of sea water and widespread loss of land ice."

Projecting from 1990 to today gives an anthropogenic sea level rise of just under 3 cm to just under 8 cm, and near sea-level, precipitation induced, river floods should rise a similar amount. It might not sound like a lot, but every inch counts. It also might be more than that in some areas, especially depending on how high tides move that extra water volume into bays. I understand that near sea level river flooding is at its worst in high tides.

(ADVISORY: Apparently, the rest of this post is stupid. Read it only if you want to waste your time or feel like piling on some more in the comments.)

No response from Roger just yet, but maybe that will happen later. Since posting that comment, I noodled around some more on the issue of how local topography can amplify the high tide effect of a rise in mean sea level. Wikipedia says that the local topography of two bays in Canada transforms an average one-meter ocean tide into 16 and 17 meter local tides. In those cases, a 5 cm mean sea level rise results in approximately one meter rise in sea level at high tide. Obviously, those are extreme examples, but I would expect that 2-times or 3-times amplification of local tides would be common enough. Those areas should have already experienced 6 to 24 cm increase in local high tides from global warming. When the high tides coincides with river flooding, the precipitation-induced flooding damage that Roger is talking about should have already become significantly worse that would otherwise have been the case.

Interestingly, the International Panel on Climate Change's chapter on sea level rise doesn't appear to discuss how future increases in mean sea level could be amplified by local topography to produce much higher sea levels at high tide. I'm guessing that means either that I've missed something and the effect isn't real, or that the next IPCC revision needs to take on this issue.

UPDATE: some knowledgeable commentators are distinctly unimpressed with my tidal amplification argument (I hate it when that happens). I'll have to think about whether anything can be salvaged from that idea, but there probably is a valid reason why this isn't analyzed by the IPCC. I'm sticking with the 3-8 cm mean sea level rise translating into actual flooding damage, though.

key: science, global warming

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