Do Increasing Temperatures Lower Crop Yields?

Guest Post by Willis Eschenbach

I keep reading these claims that we’re all going to starve because of global warming. People say it’s going to be the death of agriculture, that increasing temperatures will cause significant drops in crop yields. Here’s a typical bit of alarmism (emphasis mine):

A study by the International Food Policy Research Institute (IFPRI), indicates that climate change would hit developing countries the hardest, leading to massive decline in crop yields and production.

Whoa, a massive decline in crop yields due to increasing temperatures, sounds scary. So I thought I’d review the facts. Here is the global situation, showing the global yields of rice, corn, and wheat, along with the change in global temperature.

Figure 1. Changes in global grain yields and global temperatures 1961-2011. Data Sources: FAO, BEST, Photo 

Now call me crazy, but what I see going on there is not a global crisis. Nor is it “massive declines”. Notice that (according to BEST) the global temperature has gone up one full degree centigrade … anyone remember any thermal crises that have resulted from that one degree of warming? Since two degrees is supposed to bring untold sorrows, where are the sorrows of one degree? Where is the lethal sea level rise? Where are the disasters? ¿Où sont les neiges d’antan? And most of all, where are the decreases in yield from that one degree of warming?

Of course, you could say that this is just because it’s a global average, and not all countries produce wheat, so we wouldn’t expect good agreement between global temperature and global grain production. And you might be right. So … here’s the same chart, only this time just for the US;

Figure 2. As in FIgure 1, except for the US rather than for the whole globe. BEST US temperature data.

Again, there is no thermal related decline in yields. According to BEST the US, like the globe, has gone up about a degree since 1960 … where are the climate refugees? Where are the corpses? Where are the thermal catastrophes? And more to the current point, where are the declines in food production? I don’t see them.

Finally, I thought “Well, maybe if I detrend all of the US data and then see how well related the change in annual temperature is to the change in annual crop yields” … no joy there either. Below are the measurements for those relationships. The strength of a relationship between two variables  is measured by something called “R squared” (written “R²“), which varies from 0.0 for no relationship between the variables, up to 1.0 for perfectly related variables. Here’s the relationship of US temperature and US crop yields:

R², US BEST Land Temperature and US Maize (corn) yield : 0.001

R², US BEST Land Temperature and US Rice yield : 0.000

R², US BEST Land Temperature and US Wheat yield : 0.022

In other words, no relationship at all. I gotta confess, I don’t see what folks are screaming about. If you believe the BEST data, we’ve seen a full degree of temperature rise in the last half century, and it hasn’t done us any harm—no atolls gone underwater, no millions of climate refugees, no increases in extreme weather. And through all of that temperature rise, the crop yields have kept going up. Will they reach a maximum? Assuredly they will … but it doesn’t seem like that maximum yield is going to be much affected by the temperature.

So I fear that once again we’ll have to postpone Paul Ehrlich’s celebration. He’s been predicting the global Malthusian food crisis for decades now, to no avail. Near as I can tell, according to the Malthusian philosophers like Ehrlich, the problem is that this continued increase in crop yields works in practice, but it doesn’t work in theory …

w.

Further Reading: I put up a post a while ago called “Border Transgressions“, about wheat production and temperature in Mexico. I also discussed how much food people actually have to eat in “I am so tired of Malthus“.

[UPDATE] Some people seem to have understood me as saying that because temperatures were rising and crop yields were rising as well, that the rising temperatures were causing the rising yields. I am not saying that. It may indeed be true that in a warmer world, the general yield would be better, and I see no reason it would not be better … but that’s not what I’m saying.

Some people seem to have understood me as saying that crops are not affected by temperatures above their optimum range. I am not saying that. All crops have preferred temperatures, above or below which they do not produce as well.

People are over-thinking this. What I am saying is simple. It is the answer to the question in the subject of the post—do increasing temperatures lower crop yields? I say no.

Note that I am not saying that increasing temperatures increase crop yields, although they may do so. Instead, I am falsifying the alarmists forecasting things like “massive drops” in crop yields. I’m not saying yields will or won’t go up if it gets warmer … I’m saying they won’t go down.

Here’s what lowers crop yields. Bad weather forecasts lower crop yields. If the farmer knows it will be colder next year, don’t worry, she’ll make money, she’ll plant later, use a different variety, plant beans instead of corn, get a bumper crop, be the envy of her neighbors. Same thing in reverse if she knows it will be hotter, she’ll plant early and have her crop in while the neighbors’ crops are wilting in the field.

But a bad forecast, she puts in hot weather seed and it turns out to be a cold year, the yield will go down.

So increasing temperatures, particularly predicted increasing temperatures, particularly predicted gradual increases over a century, will be lost in the noise of the thousands of changes that farmers do each and every year to account for the much larger interannual variations and interdecadal variations. Every year, the farmers successfully deal with the fact that not next century but next year may be two or three degrees warmer or cooler than this year … do you really think a degree’s rise spread over decades will affect those farmers’ crops? It’s lost in the noise, they’ve got three degrees to think about. Here’s the part that I think many folks don’t understand.

At the end of the day, crop yield is a measure of the farmers, not of the temperature.

In evidence of this, I offer the fact that the above analysis of the detrended US temperature data and detrended US crop yield data showed only an insignificant relationship between the two.

w.

[UPDATE 2] Someone downthread asked about the yields in the poorest countries. Here is that data.

As you can see, progress has been much slower in the developing world. However, even in the worst off countries on the planet, even with the warming of the last 50 years, the yields are still rising. And it is worth noting that the worst countries are all at or above the global average yield rates in 1961. In my lifetime, the poor of the world have moved to where the global average was when I was a kid …

And obviously, of course, at this end of the spectrum even the simplest of improved methods and seeds would double the yield … which is why temperature is not the issue, and never was.