New Scientist has a very strange report about an article published in Nature Reports Climate Change. It's about what it calls "premature ageing" in wheat in India.
Satellite images of northern India have revealed that extreme temperatures are cutting wheat yields. What's more, models used to predict the effects of global warming on food supply may have underestimated the problem by a third.
Golly. Sounds serious, doesn't it?
But wait a moment. A satellite image is bit of an odd way to measure grain yields, isn't it? I had always thought dear old-fashioned scales and weighbridges were quite an acceptable way of doing this kind of thing. Let's take a look at how this tech-based approach to measuring grain yields works:
David Lobell of Stanford University in California used nine years of images from the MODIS Earth-observation satellite to track when wheat in this region turned from green to brown, a sign that the grain is no longer growing.
He found that the wheat turned brown earlier when average temperatures were higher, with spells over 34 ºC having a particularly strong effect. He then inferred yield loss, using previous field studies as a guide.
I'm not quite sure how you tell a crop that has yield problems from one that is merely ripening a bit earlier - I'm also not sure about the distinction between "premature ageing" and early ripening.
Another question that occurs is whether Prof Lobell has sense-checked his findings. I mean, how about this headline from the Times of India?
Highest-ever wheat yield may rot away
The reason it's going to rot away is lack of storage. Now there are all sorts of other factor that might affect the size of the harvest - the acreage given over to wheat being an obvious one. Another possibility is the introduction of new varieties and I wonder if he has controlled for this.
Without access to the paper it's hard to say more, but the idea that the warming of recent decades has affected yields seems on the face of it somewhat implausible.