Monday, July 16, 2012

June Corn-Belt PDSI

The drought in the US west/midwest is starting to have severe effects on the US corn crop.  For example, Sober Look posts this chart of corn prices:

and Reuters reports that:
The worst drought in the Midwest since 1988 has done considerable damage to this year's corn crop. The USDA slashed its corn yield estimate for the world's top grower and exporter by an unprecedented 20 bushels, to 146 bushels per acre.

Half of the U.S. corn crop began pollinating in late June under triple-digit temperatures and severe rain deficits, conditions which damaged yield potential beyond repair.

Drought conditions in the Midwest worsened over the past week. A weekly U.S. drought monitor showed about a third of the nine-state region in severe to exceptional drought in the week ended July 10, up from about a quarter of the region a week ago.
Against this background, I was interested to see what the PDSI data are telling us.  It turns out that NOAA maintains a PDSI for the "Corn Belt" of the US, and I plotted the June value for it here:

The last data point is for June 2012 and you can see that indeed it's at -2: a significant drought, though nowhere near dustbowl levels yet.  However, the situation has continued into July and may yet become apocalyptic.

The relationship to climate change is clearly complicated by the fact that the data show a generally upward trend (ie wetter): that linear trend is 1.1/century, with a standard error of 0.4 - a statistically significant result.  This is consistent with the fact that the US midwest is generally trending wet in the global PDSI trend map, despite the fact that most of the planet is drying:

Whether this wetter trend continues or not is a pretty critical question for global food production.  So it will be very interesting to watch this year's drought.


HalFiore said...

Well, here things are looking pretty good. We were irrigating for a couple of weeks, but started getting a lot of good rainfall about a week and a half ago. Now the problem might be that if it doesn't dry out pretty soon, they won't be able to get the combines in there.

DonP said...

If the global climate is warming, doesn't that mean more evaporation/transpiration?

If there is more evaporation/transpiration, doesn't that mean more rainfall?

What goes up must come down.

Don P

Stephen B. said...

Only if the oceans (which are the source of most of our atmospheric moisture) rise in temperature in exact unison with the atmosphere's temperature rise Don.

I imagine after Stuart releases my comment, he'll explain more fully, along with pointing to several of his blog entries from this past winter that examine your question in depth.

Greg said...

Stuart - the historical trend is interesting.

Aiguo Dai's 2010 PDSI projections for the 21st century (which you covered so ably in October 2010) seemed to show that the midwest would become drier, i.e. that the trend is expected to reverse quite suddenly.

In your email discussion with Dai, did you ask him about this reversal and its causes?


DonP - yes, it does mean more evaporation and more rainfall: the water cycle will intensify, we expect, so wet places will get wetter and dry places will get drier.

On top of this, the Hadley cells are expected to expand, so subtropical deserts will expand away from the equator.

Douglas said...

A bit off topic, but I thought you might find this interesting Stuart:

I don't completely agree with Mass, but it does seem like a very sloppy study.

Alexander Ac said...

Hi Stuart, according to NCDC, U.S. Drought is Most Severe Since 1950s, Report Says

Stuart Staniford said...


No I don't recall asking Dai about that - but in general my sense is that climate models don't reproduce regional climate patterns all that well and predict that most of the US should have been drying already, when it's in fact been getting wetter. I guess only more data and better climate models can tell us which parts of the US are really going to get drier and which wetter. My personal best guess would be that the western half of the country will tend to get drier (and much of the forests burn) while the eastern half will tend to get more subtropical feeling.

Stuart Staniford said...


That's not inconsistent - the measure in that report is total area of the US in drought, not average dryness of the corn belt.

Emil said...

According to NPR, the drought is now the worst since 1956.

(Link here:

Still, I've read reports that other areas of the world has had record harvests(like Brazil) and that countries like China has bought a large spare capacity of food which covers almost half a year of supply so this will help in moderating the price increase going forward.

Remember to separate what the market is pushing as the 'natural' rate and what individual countries are actually settling on.