Daily Archives: April 4, 2007 12:35 PM

Climate change is predicted to reduce U.S. crop yields by 25%-80%

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Wolfram Schlenker of our economics department is presenting this paper by himself and Michael Roberts on the effects of climate change. The talk is this Thursday, 11:30-1, in 717 IAB. Here’s the abstract:

There has been an active debate whether global warming will result in a net gain or net loss for United States agriculture. With mounting evidence that climate is warming, we show that such warming will have substantial impacts on agricultural yields by the end of the century: yields of three major crops in the United States are predicted to decrease by 25-44% under the slowest warming scenario and 60-79% under the most rapid warming scenario in our preferred model. We use a 55-year panel of crop yields in the United States and pair it with a unique fine-scale weather data set that incorporates the whole distribution of temperatures between the minimum and maximum within each day and across all days in the growing season. The key contribution of our study is in identifying a highly non-linear and asymmetric relationship between temperature and yields. Yields increase in temperature until about 29C for corn and soybeans and 33C for cotton, but temperatures above these thresholds quickly become very harmful, and the slope of the decline above the optimum is significantly steeper than the incline below it. Previous studies average temperatures over a season, month, or day and thereby dilute this highly non-linear relationship. We use encompassing tests to compare our model with others in the literature and find its out-of-sample forecasts are significantly better. The stability of the estimated relationship across regions, crops, and time suggests it may be transferable to other crops and countries.

50% declines in crop yields–that’s pretty scary! Getting to the statistics, Schlenker points out that weather can be considered as a natural experiment with effects on crop yields, but that if effects are nonlinear, you can’t just use broadly spatially- and time-aggregated weather.

My main substantive question would be about potential effects of mitigation (such as switching crops). Also here are some specific comments (bearing in mind that I haven’t had a chance to look at the paper in detail):

– I can’t believe it’s a good idea to fit 6th-order polynomials. I mean, if you want a 6-parameter family, why polynomial? I’d think a spline would make more sense.

– The tables should be graphs. Really really really. Tables 1 and 2 should be a series of line plots with temperature on the x-axis. This is a gimme. Tables 3-9 should be displayed graphically also. In addition, temperature should be per 10 degrees so that the coefs are more interpretable, also (if you must use a table) use fewer significant figs. Precip should also be on a more interpretable scale (you can see the problem by noting the tiny coef on Precip squared).

– The color scheme in Fig 1 should be fixed. In particular, it’s not clear if Florida is Interior or Irrigated. Also, the caption says “counties” but the graph seems to be of states.

– The county maps are pretty. Would be improved by either eliminating the borders between counties or making them very very light gray. As it is, they interfere with the gray scheme. Also, I’d remove the N/A counties entirely, rather than coloring them in white, which looks too much like one of the colors in the map.

Finally–and most importantly–the figures are ok but what’s missing is a check that the models fit the data. The paper makes a strong substantive claim that might very well be disputed, so I recommend trying to do some of these checks right away: I’d like to see some plots of the data, along with plots of replicated data under the model to reveal what aspects of data are not being captured.

One thing that might be helpful would be to make these model-checking plots, first for a linear model of the form implicitly fit by others, then using the current model, to see the improvement in fit.

The Bonus Army and the G.I. Bill

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Taylor Branch has a fascinating article in the New York Review of Books on the Bonus Army (the gathering of WW1 veterans in Washington in 1932) and the G.I. Bill, which paid for millions of college educations and mortages for WW2 veterans. I knew about Herbert Hoover and the Bonus Army but I didn’t realize that Roosevelt later said no to them too or that “‘Opposition to the bonus,’ Arthur Schlesinger Jr. recalled, ‘was one of the virtuous issues of the day.'” Or that the press referred to work camps for veterans as “playgrounds for derelicts” who were “shell-shocked, whisky-shocked and depression-shocked.” Or that a major motivation for the G.I. Bill was to avoid similar political controversies, or that Martin Luther King was modeling his last campaign on the Bonus Army. There are also some political issues that Branch touches briefly upon, such as the ambigous role of the American Legion in the politics of the time, and the current status of soldiers and veterans in U.S. politics.

From a parochial academic perspective, the article reminded me of why political scientists need historians.

P.S. I’m not surprised that this is the first book by Paul Dickson to have been reviewed by the New York Review of Books. Dickson is, among other things, a Washington, D.C. aficionado with a charming and amiable writing style, and some of his earlier books are Jokes, Names, and Baseball’s Greatest Quotations.

“Don’t get it right. Get it written, and get feedback”

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Seth writes,

One of the first managing editors of The New Yorker had a slogan: “Don’t get it right, get it written”. My philosophy with regard to the Shangri-La Diet was similar: “don’t get it exactly right, get it written, and get feedback.”

Here are some ways the Shangri-La Diet has been improved by feedback (almost all from the SLD forums):

1. It is much clearer what rate of weight loss to expect.

2. The idea of nose-clipping. Which makes any food a weight-loss food.

3. With nose-clipping, you can use flaxseed oil to lose weight. The benefits of omega-3 have become much clearer.

4. Putting the oil in water makes it much easier to drink.

Setting aside the virtues or defects of nose-clipping, etc.: Getting it out and getting feedback does seem like a good idea. In Seth’s case, a key intermediate step was the publication of the Behavioral and Brain Sciences article on self-experimentation, which gave potentially critical readers some data and theories to chew on.