Monday, April 15, 2013

Climate Change Still Not Affecting Global Cereal Yields

A couple of years ago, I took a look at the FAO data for global cereal yields with a view to answering the question of whether climate change was yet having a noticeable impact on global food production.  At that time, the answer was unequivocally no:
So, clearly, the overwhelming story in global agricultural yields is this: improving agricultural technology has increased yields at a steady, reliable pace - they have more than doubled over the last 50 years. There just is absolutely no support in the data for the idea that climate change, or any other negative or scary factor you care to name - eroding soil, depleting aquifers, peaking oil supplies - is causing the agricultural yield curve to start bending downward. Maybe they will in the future, but it sure isn't happening yet.
At that time, the slope of the straight line through the yield data was 0.0442 tonnes/hectare/year.  The straight line explained 99.132% of the variance in the data, and a quadratic, which could potentially capture the data bending downward, only explained an additional 0.016% of the data.

So has two additional years worth of data (2010-2011) changed the picture?

No, it hasn't.  2011 was the highest yield yet, the slope of the line has increased to 0.0446 tonnes/hectare/year, it now explains 99.153% of the variance in the data, and the quadratic is down to explaining only an additional 0.001% of the variance (yes, the straight line is good to four significant figures).

I also looked at the departures from the straight line, as a percentage of the yield, to see if relative fluctuations in the total harvest were increasing.  The answer, then as now, is no:

Fluctuations, relative to the current yield level, are going down over time.

Climate change is scary, we are clearly melting the north pole, droughts and floods and heat waves are increasing over time.  However, so far, it's not hitting us where it would really hurt: in the stomach.


Aimee said...

that's great news, but I think it is not the case for certain important crops which, while not staples, are certainly "essential" in many peopl'es book. I'm thinking of coffee, but I think the same is true for many tropical crops.

barath said...

Would you also be able to post data on total yields, both of cereals and of food overall, globally? Yields without data on total harvests only tell part of the story.

Unknown said...

Good Morning Stuart,

We did not reach yet the peak food...

The Rational Pessimist said...


An interesting and informative post. I would only have one caveat: you ignore the question of price.

Climate change may be impacting on crop yields already, but you are not seeing it in volume and productivity data since the strain is being taken up by rising prices. The FAO Food Price Index has been on an upward trend for the past 10 or so years in real terms.

Your preferred productivity measures, tonnes per hectare, doesn't capture the possibility that a lot more non-land inputs are being used to increase yields. Rising prices allow more non-land inputs to be added while maintaining margins.

Of course, there are a lot of other moving parts in this; for example, expanding Indian and Chinese food demand. But I don't think you analysis is sufficient to exclude climate change from the food mix.

Anonymous said...

Rational, that's exactly the point I was about to make ... while it's encouraging that climate change hasn't noticeably affected yields of staple grains yet, extreme weather has undoubtedly impacted prices, to the great harm of the world's poorest.

Case in point, this article published just this week:

Stuart Staniford said...

barath - see today's post. If anything the picture is even a little better if one looks at total production.

Stuart Staniford said...

Rational, greenforlife:

Prices are a function of the difference between supply and demand. While the supply side is growing steadily and with less percentage fluctuation over time, the demand side has recently experienced a very sharp growth in biofuel usage, and I think that's the major cause of the recent price surge (which are still nowhere near the highs of the seventies, say).

Unknown said...

Excellent tonic for the running out of food hysteria. Your graph on trend deviations is to be expected if technology leads to constant unit increases in crop yields. There is a long debate whether trends in crop yields should be modeled as linear trends or log-linear trends. For similar evidence for US corn yields see this post:

Scott Irwin
University of Illinois

Stuart Staniford said...

Scott - many thanks, and interesting post. I'm curious what accounts for the decline in irrigation fraction in the last decade or so?