Do Mediterranean Crop Yields Show Climate Stress?

Yesterday, we were discussing global crop yields (focussing on cereals as the most widespread and nutritionally significant crops).  I was pointing out that in the overall global data, there just really isn't room to see a climate change signal.

But, if we were in the early stages of climate change related agricultural problems, you might expect the signature to show up in particularly impacted regions long before it shows up in the global data.  Readers who remember the discussion in the Future of Drought series will recall that the Mediterranean tends to show up at the top of any list of particularly strongly impacted climate regions.  It was fairly dry to begin with, there are clear drying trends in recent decades, and climate models show it getting worse still in the future.  So if anywhere shows a climate change signal in the yield data, this region would be it.

For example, this figure shows the first principal component in the global data for the Palmer Drought Severity Index

If you don't feel like clicking the link above to figure out what that means, you can just think that the orange/pink/red regions are places that are tending to get drier over time (in an agriculturally significant sense).  As you can see, the western Mediterranean is pretty much all pink or red.

Borrowing this map from Google Maps,

I selected Portugal, Spain, France, Italy, Greece, Morocco, Algeria, Tunisia, and Libya as the sample here.  I excluded Egypt, Turkey, and points east since the drying trend is much more equivocal there.  I excluded the Balkans due to potential confounding effects from the political instability in those countries in recent decades.

So then I went to the UN Food and Agricultural Organization again, for country-specific yield data.  Here is the raw data (together with the world average from yesterday):

Since that's rather noisy, I applied a seven year centered moving average just to remove some of the short term weather noise and make the graph easier to read:

I think for the most part, this raises cause for concern.  The biggest and most developed agricultural economies (France, Italy, Spain, Greece) all show very clear signs of a slowdown in yield gains in the 1980s or 1990s.  The data for the less developed countries are harder to interpret, I think.  For example, in Portugal, you see clear signs of the transition to democracy in the late 1970s resulting in a rapid rate of economic development, and some catchup in agricultural yields to those of its neighbors.  It's going to be hard to see a climate signal in that line.

The most obvious other explanation is that France, Spain, Italy, and Greece are running into some kind of technological limitations in raising yields.  I think the best way of looking at that would be to compare trends with some regions that aren't encountering drought stress, and I'll try to look at that soon.

Update: An alternative explanation for the shape of the yield curves in the developed countries here is that Europe effectively banned GM crops in 1998. So one possibility is that most of the yield improvement elsewhere has been coming from crop genetic improvements, and Europe has cut itself off from those.