Friday, May 11, 2018

Crop Yields Under Global Warming

Up until now I thought that global warming would have a significant impact on crop yields, both in the US and elsewhere.

But I've gathered some numbers, and I'm so sure anymore -- at least for US farmers/farming corps.

My understanding was taken from papers like this one:
“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 2007 Environ. Res. Lett. 2 014002 doi:10.1088/1748-9326/2/1/014002
http://iopscience.iop.org/1748-9326/2/1/014002
and this one
“With a 1°C global temperature increase, global wheat yield is projected to decline between 4.1% and 6.4%. Projected relative temperature impacts from different methods were similar for major wheat-producing countries China, India, USA and France, but less so for Russia. Point-based and grid-based simulations, and to some extent the statistical regressions, were consistent in projecting that warmer regions are likely to suffer more yield loss with increasing temperature than cooler regions.”
- B. Liu et al, “Similar estimates of temperature impacts on global wheat yields by three independent methods, Nature Climate Change (2016) doi:10.1038/nclimate3115, http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3115.html 
But -- again, for the US -- these turn out to be quite small numbers, because the market sizes are so big and yields are increasing year-after-year.

So how do crop yields (again, US-only) vary with temperature? Here are some data from the National Climate Assessment (2014), Figure 18.3, p 421


These data are, admittedly, rather scattered and a straight-line trend will have errors. (I don't have the underlying data to calculate those here.) But by eye, I estimate the trends to be, for corn: -0.39 t/ha/°F, and, for soybeans: -0.08 t/ha/°F.

So what are the current yields, and how fast are they increasing? Here is the current yield and trend in corn yields, and the same for soybean yields.

So, plugging in the data, the trend in corn yields = 1.9 bu/acre/yr, and the trend in soybean yields = 0.5 bu/acre/yr. Relative to 2017, these are 1.1%/yr and 1.0%/yr, respectively.

From the same source, 2017 yields are, for corn: 176.6 bu/acre, and for soybeans, 49.1 bu/acre. "bu" is bushels.

I need to know the density of corn and of soybeans: 39.3680 bu/t and 39.7740 bu/t, respectively, from this source.

Translating into metric units: 2017 corn yield = 11.1 t/ha, 2017 soybean yield = 3.1 t/ha.

OK, now we can put things together.

Let's assume the surface warming trend is +0.20°C/decade. That's one degree Celsius in 50 years. (Results for other trends will scale linearly.)

Assuming the current trend in yields continues (iffy?), in 50 years (a long time, granted), yields will have increased by 170% (for corn), and 165% (for soybeans).

But in that time, yields will only decrease due to higher temperature by -6% (corn) and -5% (soybeans).

So agricultural technology will, even if trends continue at only a fraction of their current value, swamp any losses due to global warming.

And it won't take much increase in yields in developing countries for them to cancel out any loses due to higher temperatures, either.

--

Of course, there's no inherent reason to believe that yield increases will continue at their rate of the last 30 years for the next 50 years. Nor will warming stay linear, probably. And we'll need more food to feed ever more people, about 10 B by the middle of this century. And warming won't be limited to just one degree Celsius (we're already at that value anyway).

But I don't anymore see a big problem here. Am I missing something?

17 comments:

William Connolley said...

I think I agree with you: https://mustelid.blogspot.co.uk/2018/05/crop-yields-under-global-warming.html

David in Cal said...

Higher levels of CO2 are beneficial to plants. That's why greenhouses put in extra CO2. I do not have figures immediately at hand, but I am confident that the positive impact of extra CO2 has so far been much larger than the (possible) negative impact of higher temperatures.

Cheers

David Appell said...

David: But higher CO2 also brings higher temperatures and changes to precipitation, neither of which are good for plants. Also, the nutritional quality of plants decreases with higher CO2.

"Ask the Experts: Does Rising CO2 Benefit Plants?" Annie Sneed, Scientific American 1/23/18
https://www.scientificamerican.com/article/ask-the-experts-does-rising-co2-benefit-plants1/

David in Cal said...

David -- It's sad how Scientific American has declined from the fine magazine it used to be. The article you linked to is tendentious. It doesn't have an unbiased scientific POV.

E.g., it asserts that climate change will bring drought, even though that consequence is not agreed-upon. In fact, there's a plausible theory that global warming might bring more ocean evaporation leading to higher rainfall. The article lacks numbers. How large is the effect of warming and the effect of higher CO2? The article doesn't address the advantage of a longer growing seasons in cooler areas. Nor does it adjust for the possibility that farmers will change their crop mixture to accommodate higher temperatures.

Cheers

David Appell said...

"It's sad how Scientific American has declined from the fine magazine it used to be."

That's bullshit. Just because you don't like what it says, you smear it. Deniers feel the need to do that a lot.

Yes, it is known that AGW will bring more drought. Even with more evaporation (there is already known to be more water vapor in the atmosphere from AGW.)

Why does it matter, in an article like this, how much more warmer it will get? The fact is, no one knows, because no one knows how much CO2 we will admit, or the precise value of ECS.

David Appell said...

"The article doesn't address the advantage of a longer growing seasons in cooler areas."

"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

David in Cal said...

David -- Your link doesn't fully refute my points. It looks at US only and wheat only, thus not dealing with two of the possibilities that I mentioned:

1. As the the US warms, some farms will switch from wheat to crops that do better under warmer weather.

2. Farming in very cold areas like Canada and Russia will surely benefit from a somewhat warmer climate.

I think I'm a fair amount older than you. I loved and admired Scientific American as a high school student in the late 1950's. You can choose not to believe me, but IMHO it is not the same magazine today.

Cheers

George Montgomery said...

Over time, crop yields aren’t solely linked to climate conditions. Yields also vary according to international demand for crops. Farmers plant in anticipation of future demand, contracts for grains, ...
Yield is an ‘imprecise’ term to the extent that it also includes the production of seeds collected for planting. Rule of thumb: aim for three or more seeds being collected for every seed planted. Crop yields are affected by improved farming methods, improved strains of crop developed for characteristics more suited to growing conditions or with greater output of grain/plant.
Patterns of crop yields are changing – static around equatorial regions through to increasing towards polar regions.
Patterns of rainfall are changing – while rainfall increases with warming, it is not spread evenly over time or across crop cultivations. When it does rain, it can be torrential, causing flooding, with little or no follow up rain. And weeds with their short seeding cycle also benefit, more than crops, from warming and rain.
And on, and on, ...

David Appell said...

David in Cal said...
David -- Your link doesn't fully refute my points. It looks at US only and wheat only, thus not dealing with two of the possibilities that I mentioned:

Where is your own data proving your claims?

1. As the the US warms, some farms will switch from wheat to crops that do better under warmer weather.

So people will just no longer need the crops for which the climate becomes too warm to grow?

2. Farming in very cold areas like Canada and Russia will surely benefit from a somewhat warmer climate.

And those in the south who grow the same crops won't go out of business?

David Appell said...

David in Cal, you didn't address this:

"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

David Appell said...

David, re: droughts:

Guest post: Climate change is already making droughts worse, Benjamin Cook, CarbonBrief,
https://www.carbonbrief.org/guest-post-climate-change-is-already-making-droughts-worse/

David Appell said...

George: Good points, thanks.

The yields discussed here for corn and soybeans are increasing about 1.0-1.1%/yr. World population is increasing just a little faster (1.2%/yr) than that. US population is increasing about 0.7%/yr.

David Appell said...

David in Cal said...
I loved and admired Scientific American as a high school student in the late 1950's. You can choose not to believe me, but IMHO it is not the same magazine today.

Everyone thinks things were better in their childhood. Memory isn't very objective.

David Appell said...

Robert Hass — "Nostalgia locates desire in the past, where it suffers no active conflict and can be yearned toward pleasantly."

OnymousGuy said...

This will exhaust my knowledge of plant biology, but at the American Chemical Society meeting, there was discussion of the different response to climate change of C3, C4, and CAM plants. See www.cropsreview.com/c4-plants.html

David in Cal said...

David --

1. I did address, "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." I addressed it by accepting the results of that paper, that the overall effect of warming on US wheat production would be negative.

2. Droughts: you quote one paper. My view of climate change beliefs comes from the IPCC scientists. Please correct me if I am wrong, but AFAIK the IPCC has not stated as an agreed-upon scientific conclusion that global warming has already caused an increase in droughts or that it will definitely cause an increase in future droughts.

Cheers

OnymousGuy said...

Oops, wrong link.
C4 Plants, Examples, and C4 Families