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

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.

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

  1. "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"

    During the Holocene Warm Period, temperatures were much warmer than they are now. It is called a "Climate Optimim" because plants were more productive then, not less.

  2. Mike,

    Not to get too far off the topic of statistics, but two points:

    1. "Much warmer" is contextual; e.g., North American temperatures during the Holocene Climate Optimum were, IIRC, ~2 degrees C warmer than today, which is on the low side of the temperature change predicted for 2100.

    2. It's called a climate optimum because it was a local maximum (in time) of temperature, not because it had "optimum" plant productivity. Plant productivity depends greatly on which plant species is being considered (they each have their own preferred growing temperature) and where they are being grown. It is not the case that warmer temperatures automatically translate into increased plant productivity everywhere and for all species. The crop yield and temperature data discussed in this paper suggest otherwise. (But the influence of atmospheric CO2 on plant productivity is also a factor which this work doesn't appear to take into account…)

  3. A few more considerations, from the article summary:

    -Cotton prefer warmer temperatures than corn. Might this mean that cotton production will migrate north, so that the areas becoming too warm for corn will switch to cotton (or another crop) which previously found the climate too cold?

    -What about other crops? The Central Valley of California is among the most productive agricultural regions of the world, and it routinely has summer temperature in excess of 35C. Other methods or crops might do very well with climate change.

  4. Anonymous at April 5, 2007 1:18 PM,

    Wolfram discussed these two points at his talk today.

    – Regarding the idea of switching crops, he said that this is possible, but that there hasn't been so much switching yet, which suggests that it's costly to do so.

    – He excluded California from his analysis because its crops are massively irrigated, I think at a scale that would not be feasible to extend throughout the country.

  5. how is technological impact? I noticed the author said technological progress would not change breakpoint temperature, while did he include the effect of the yield increase in the middle/long term by technology?

  6. Mike,

    You've missed the point made above, which is that you cannot generalize from generic proxy data on "plant biodiversity" to the temperature sensitivity of specific crops, grown in specific regional climates. In order to understand the latter, you have to collect data on those crops in those climates, which is what the work Andrew discusses did.

  7. We are talking about a very, very slow moving train, with global warming. Why could a farmer not switch crops? Does he have an engineering study to prove this? Heck, many of them have 3-4 crop rotations already.

    Also, does the study take into account any benefit from CO2 fertilization?

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