Thursday, September 15, 2016

Global Warming's Impact on Wheat Production

Another paper has been published showing that warming reduces wheat yields.

B. Liu et al, "Similar estimates of temperature impacts on global wheat yields by three independent methods," Nature Climate Change (2016).

Their conclusion, from several different angles:
With a 1°C global temperature increase, global wheat yield is projected to decline between 4.1% and 6.4%.
They write that "Global demand for food is expected to increase 60% by the middle of the twenty-first century."

Many skeptics say "but crop yields are going up!" Which is true. Here is the annual global wheat production for 1996-2014. And it's going up, though not on a per capita basis (see the graph to the right), in a world where 800 million people "do not have enough food to lead a healthy active life." (That one of of every nine people.)

Harvests can increase for several reasons:

* more acreage planted
* better fertilizers
* genetic modification
* better farming technology and techniques
* more CO2
* changes in government farming subsidies
* higher demand
* better weather
* more desirable temperatures
* more desirable precipitation
* etc.

Saying that yield has increased says nothing about why. Nor does it say anything about the influence of each factor, or whether yields are going to keep increasing. In particular, yields can (obviously) increase even while some of these factors are contributing to a decreasing sub-trend. And that's what scientists say is happening regarding climate change, like the paper above, and this 2007 paper by Lobell and Field, two recognized experts:
“For wheat, maize and barley, there is a clearly negative response of global yields to increased temperatures. Based on these sensitivities and observed climate trends, we estimate that warming since 1981 has resulted in annual combined losses of these three crops representing roughly 40 Mt or $5 billion per year, as of 2002.”
-- “Global scale climate–crop yield relationships and the impacts of recent warming," David B Lobell and Christopher B Field, Environ. Res. Lett. 2 014002 (2007).
And there's this paper:
"We also find that the overall effect of warming on yields is negative, even after accounting for the benefits of reduced exposure to freezing temperatures."
-- "Effect of warming temperatures on US wheat yields," Jesse Tack et al, PNAS 4/20/2015.
So interpreting yields is complicated. I guess we can hope that agricultural acreage and techniques keeps at least on its trend (and hopefully increasing per capita, in order to truly feed everyone), even in the face of lower yields as temperatures rise and planting zones moving towards the poles. Or we can just ignore climate change and hope it all works out.

Added: Wheat sells for about $4 per bushel, and there are e/100 bushels in a metric ton (very close!), so the value of the global wheat crop in 2014 was just over $100 billion. So a 1°C global temperature increase would cost about $40-60 million/year $5 billion/yr in lost production.

12 comments:

William Connolley said...

I'm not sure that "With a 1°C global temperature increase, global wheat yield is projected to decline between 4.1% and 6.4%" is very exciting. Compared to changes in crop yields due to other things (mostly, improvements in farming, fertilisers, etc etc) it is small.

There are a large number of people who don't have enough to eat; but the problem there is distribution, not total food availability.

JoeT said...

Thanks for this informative post David. What I liked about it especially is how you called out some of the complicating factors that might go into global wheat production. Because of these complicating factors, the present paper by Liu, and the previous similar paper by Asseng, try to keep other factors such as water and CO2 constant in field trials and in crop models, while only changing the temperature. Figure 1 in the Asseng paper shows this comparison very nicely. He states,

"When Tmean was >28C and when there were extremely high temperatures early in the growing season with many days of maximum temperature (Tmax) > 34C, a critical maximum temperature for wheat, crops did not reach anthesis or grain set, so it was not possible to record anthesis or maturity dates and the yields were zero."

What we are seeing especially in global wheat production is that the increases are coming from the mid-latitude countries: China, Russia, Canada etc. It's India, the second largest wheat producer, that is struggling. Back in 2014,here's one article that attributes the plateauing of India's wheat production to warming nights . A more recent article claims India faces record wheat imports due to a lower-than-average monsoon

Clearly one factor that also needs to be considered is the impact of rising temperatures on the monsoon

David in Cal said...

Rising temperatures and rising CO2 concentrations will no doubt eventually hamper agriculture. But, to date these factors have helped.

Higher CO2 is good for plants. It allows them to grow with less water. Higher temperatures mean a longer growing season and also more cold areas that can now support agriculture. Also, many believe that global warming will lead to more ocean evaporation and greater rainfall over land. More evidence that climate change has, so far, been beneficial to agriculture is the greening of the earth. The area covered by vegetation has been increasing. Grain production has risen sharply in the last 40 years. Other factors also contributed to the increase, but it seems reasonable to give some credit to climate change.

Projecting the supposed decline in wheat production as a linear function of temperature increase is hard to accept, given the many unknowns. It tells me that the model is too simple.

At some point, climate change no doubt will start to reduce agricultural output. AFAIK nobody knows when that will happen.

Cheers

David Appell said...

"Higher CO2 is good for plants."

Are we interested only in the welfare of plants, or of human beings?

"Higher temperatures mean a longer growing season"

"Suitable Days for Plant Growth Disappear under Projected Climate Change: Potential Human and Biotic Vulnerability,"
-- Camilo Mora et al, PLOS Biology, June 10, 2015
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002167

David Titley: Plants do better, but so do weeds. There are ag thresholds, what about water cycle, there are huge issues of ag in a changing climate.
http://rabett.blogspot.com/2015/12/senate-hearing-live-blog.html

"We also find that the overall effect of warming on yields is negative, even after accounting for the benefits of reduced exposure to freezing temperatures."
-- "Effect of warming temperatures on US wheat yields," Jesse Tack et al, PNAS 4/20/15
http://www.pnas.org/content/early/2015/05/06/1415181112

"Also, many believe that global warming will lead to more ocean evaporation and greater rainfall over land."

Why doesn't it lead to more evaporation over land?

"More evidence that climate change has, so far, been beneficial to agriculture is the greening of the earth.
"

How has this been beneficial to agricultural? (Prove it.)

David Titley: Plants do better, but so do weeds. There are ag thresholds, what about water cycle, there are huge issues of ag in a changing climate.
http://rabett.blogspot.com/2015/12/senate-hearing-live-blog.html

"Grain production has risen sharply in the last 40 years."

Did you not read my post?

--

As usual, David, your comment is simplistic and includes no science.

David Appell said...

"Projecting the supposed decline in wheat production as a linear function of temperature increase is hard to accept, given the many unknowns."

What says their function is linear?

And if it is, why is that difficult to accept? Just because you, the world renowned expert in this field, says so?

David Appell said...

Joe, thanks, that's a good point about India.

I'm going to have to dig up some time series.

David in Cal said...

Re Linearity: I may have misunderstood the comment, "With a 1°C global temperature increase, global wheat yield is projected to decline between 4.1% and 6.4%."

At first reading, I interpreted this to mean that yield would decline that amount for each 1°C global temperature increase. I.e., that each subsequent 1°C global temperature increase would also reduce global wheat between 4.1% and 6.4%. A (more or less) fixed reduction per 1°C would yield a (more or less) linear function.

However, on secend reading, that sentence may have only referred to the next 1°C global temperature increase.


Why doesn't global lead to more evaporation over land?

It does. But, the idea is that the increase in rainfall due to greater evaporation over the oceans exceeds the effect of increased evaporation over land.

I am unclear about your comment about the rapid rise in wheat production. I think it's more informative to look at actual wheat production, rather than per capita wheat production. Wheat production in 1996 was 585 million metric tons. In 2014 it was 729 million metric tons. That's 25% growth in only 18 years.

See https://en.wikipedia.org/wiki/International_wheat_production_statistics

Cheers

Dano said...

I think it's more informative to look at actual wheat production, rather than per capita wheat production.

But it is not keeping up with human population growth. That is: each person gets less wheat/annum

Best,

D

Dano said...

Higher CO2 is good for plants.

Not for graminaceous plants. The crop plants we eat actually are less nutritious under elevated CO2 regimes.

Best,

D

TheTracker said...

Wouldn't a 4-6% decline in a harvest valued at $100 billion cost $4-$6 billion, not $40-$60 million? Seems like a decimal place issue.

TW said...

This is another in a small group of tendentious papers trying to deny the obvious -- that crop yields are going up, even though the world is warming, and prove something obviously false -- that some small warming like 1C would hurt crop production. The real question is whether the warming will have good, bad, or not much effect on crop production. It is obvious that crop production has gone up a huge amount while the earth has warmed the 0.8C or so since 1880. We've already run that experiment, no problems produced.

When I read these papers, it seems so obvious that the authors are driving towards a study designed to "get" the result they want, an apparent demonstration of harm to crops from warming. Look, I can think of 5 or 10 ways to design a study that would apparently show such a result, but such studies do nothing to address the real question of what the effect on crop production will be.

Take the first Liu paper's abstract above. It describes how the study was done "without deliberate adaptation or CO2 fertilization effects". You know what every farm has? A farmer. They make lots of decisions every year, "deliberate adaptation" decisions. Without those decisions, of course crop yields would go down. It is the cumulative effect of all those decisions that have driven crop yields up. These papers are just so much more junk science in a field already drowning in junk science papers.

David Appell said...

Tracker: Yes, I think you're right.

The 4-6% number is for 1 C of warming. Warming is about 0.15-0.2 C/decade, so it will take 5-6 decades for 1 C of warming. (Rough; not counting increased positive feedbacks.) Say 5 decades. That's 50 harvests, or $5,000 B total.

4-6% of that is about $250 B, or $5 B/yr.

I'll make a change. Thanks.