Thursday, July 26, 2012

Drought vs Heat

When you read the news these days about US agriculture, the headlines generally start with "US Drought."  My guess is that when a reporter sees a wilted plant, the first reaction is that precipitation must have been below normal.

Below is a graph that plots cumulative precipitation for the areas where corn is grown in the US. Similar to the previous post about extreme heat, the red line is for 2012, while the grey lines give the historic data for 1960-2011.

Precipitation has been below normal this year, although, it isn't at a new record low.  So what is more important, extreme heat or lack of precipitation? Well, both matter, and they usually occur in sync, as it only gets very hot when it is dry. When it is wet, evaporation results in cooling (if you have ever been to Arizona, you might have seen the mist sprays that keep outdoor seating at restaurants cooler).

In the debate about extreme heat versus low precipitation, I'd put my money on extreme heat, as it has been a much better predictor in statistical studies. Why does it matter? Well, pretty much all climate models predict an increase in temperature, but the effect on precipitation is more debated.

Drought is a relative concept. Here's a (bad) example what I mean: on some days I force my old body to go for a jog. If you ever run, you might have recently seen a warning that you need to drink more because it is hot. Well, crops aren't that different: if it gets hot, their water requirement is going up. So even if precipitation is at the historic average, hotter temperatures might give you wilted plants as the historic average precipitation is not enough for the increased water requirement of the plant.  To stick with the previous example: it is like running in really hot weather and drinking as much as you usually do, which will get you dehydrated.

So what is going on this year? Well, corn is getting the double-whammy. Remember the 2008 Chicago Marathon. It was very hot that day, and some water stations closed because they ran out of water - it wasn't a good day to run: in the end one person died and 250 were hospitalized.

Extreme Heat in US

There has been a lot of media coverage in the news lately about the drought in the US. My colleague  Michael Roberts has blogged extensively about this.

Here's another way to look at this: Michael and I have written a paper that looks at how weather impacts corn yields (Here's a link for those interested in reading more). We found that degree days above 29C (84F) is the best predictor of yields. Degree days are just a truncated temperature variable that only counts temperatures above 84F for each day of the growing season. All temperatures below 29C (84F) count as zero, while temperatures above 84F get counted as the difference to 29C (e.g., a temperature of 30C gives 1 degree day, a temperature of 31 gives 2, etc). We then sum this measure over all days of the growing season, which we set at March 1 - August 31st.

The following graph gives an update for 2012 that runs through July 23rd (red line). It shows the cumulative sum over time (summing all days from March 1st until the current day). The grey lines are historic data for the 52 years 1960-2011.

July has indeed been very hot: the red line has taken off. There are only three years on record where the season total at the end of the August was higher than what we have experienced this year already by July 23rd (since this is the cumulative sum, a line can only go up): These were 1980, 1983, and 1988 (the historic maximum was 1988)

We'll have to see what the rest of July and August will look like. If you are wondering how this compares to projections found in climate models: well, they predict a lot more warming. Since degree days are truncated, even moderate increases in temperatures can lead to a very large relative increases in degree days.

Finally, what is the impact on yields? If you believe our model, each degree day above 29C on average decreases yields by roughly 0.7%. The historic average season total is 34, and we already reached 51 this year, meaning that yields should be 12% below normal for the US. That is assuming that we won't go above 84F for the rest of July or August,which seems unlikely, so the end result should be even lower.

Technical notes: The degree day measure shown is averaged over the entire United States. We follow a three step procedure: (i) construct a daily measure of minimum and maximum temperature on a 2.5x2.5 mile grid as outlined here; (ii) Average all grids in a county by the cropland area in a grid; (iii) average all counties using expected production along a trend as weights (An impact on high-production areas impacts national yields more).