Great lakes dry? Why?
There's been a lot of hand-wringing over an unusually ("historically", of course) low water level in the Great Lakes. Struck me as strange, because I hadn't noticed significant drought in that area.

So, boys and girls: It's NCDC time!

Here's an official map of the Great Lakes watershed. The lakes don't have any big tributaries; they get water from smaller rivers and direct runoff in the area shown.


So I went to good old NCDC. The watershed corresponds to Climate Divisions 3,6,9 in Wisconsin; all of Michigan; Divisions 2 and 3 in Ohio; and Division 9 in New York. Here is the century precipitation record for each of those divisions:


Where's the drought? I can't find equivalent divisional records for Canadian provinces, but it seems highly unlikely that they would be acting like Oklahoma and Texas while their cross-lake neighbors are in a wet phase.

Something is very wrong with this picture. Are the Great Lakes really abnormally dry? Are they leaking? Is the St Lawrence taking out much more than usual? Is some industry or city pulling out more than usual? Whatever the problem, it doesn't look like rainfall on the US side is part of it.

Evaporation? Maybe. Michigan, which is right in the middle of the low area, had an especially warm 2012. Does 4 degrees above average evaporate enough to compensate for increased runoff? A look at this record of Great Lakes level doesn't support that idea. The previous hottest year in Mich was 1921, about the same as 2012. Precip in 1921 was consistently lower than now. If evap is the main driver, 1921 should have been the lowest level on record. But it wasn't. The next low point was 1927, with roughly average precip and temperature.

LATER: See followup here. Turns out the problem is a 'leak' of sorts: the straits of Detroit are more open than before, letting more water out of the three upper lakes.

Labels: Carbon Cult

  ¶ 3:38 AM