Stern's wheat graph

For reasons that I can't quite recall, I found myself looking at Chapter 3 of the Stern report, where I noticed a graph on yield losses in wheat.

The text discussing the figure has this to say:

High temperature episodes can reduce yields by up to half if they coincide with a critical phase in the crop cycle like flowering (Figure 3.4).

The change in behaviour at 30°C seems quite distinct, but it's always good to look at the paper behind the headline. From the minute you read the title, you get a sense that all is not quite well with Lord Stern's work.

Growth and yield of winter wheat (Triticum aestivum) crops in response to CO₂ and temperature.

So it's a study on winter wheat. I don't know about you, but I haven't come across any suggestions that winter and spring temperatures in the UK might exceed 30°C, but perhaps I'm wrong.

The abstract is interesting too:

Crops of winter wheat (Triticum aestivum L. cv. Hereward) were grown within temperature gradient tunnels at a range of temperatures at either c. 350 or 700 μmol mol⁻¹ CO₂ in 1991/92 and 1992/93 at Reading, UK. At terminal spikelet stage, leaf area was 45% greater at elevated CO₂ in the first year due to more tillers, and was 30% greater in the second year due to larger leaf areas on the primary tillers. At harvest maturity, total crop biomass was negatively related to mean seasonal temperature within each year and CO₂ treatment, due principally to shorter crop durations at the warmer temperatures. Biomass was 6–31% greater at elevated compared with normal CO₂ and was also affected by a positive interaction between temperature and CO₂ in the first year only. Seed yield per unit area was greater at cooler temperatures and at elevated CO₂ concentrations. A 7–44% greater seed dry weight at elevated CO₂ in the first year was due to more ears per unit area and heavier grains. In the following year, mean seed dry weight was increased by > 72% at elevated CO₂, because grain numbers per ear did not decline with an increase in temperature at elevated CO₂. Grain numbers were reduced by temperatures > 31 °C immediately before anthesis at normal atmospheric CO₂ in 1992/93, and at both CO₂ concentrations in 1991/92. To quantify the impact of future climates of elevated CO₂ concentrations and warmer temperatures on wheat yields, consideration of both interactions between CO₂ and mean seasonal temperature, and possible effects of instantaneous temperatures on yield components at different CO₂ concentrations are required. Nevertheless, the results obtained suggest that the benefits to winter wheat grain yield from CO₂ doubling are offset by an increase in mean seasonal temperature of only 1·0 °C to 1·8 °C in the UK.

The abstract is somewhat unclear, but we seem to have a beneficial effect from CO2 fertilisation offset by temperature increases. The tentative conclusion seems to be that these broadly cancel but that more work is required to prove the point.

Stern seems to have picked up on a fall in seed head numbers. I can't see anything in the abtract to support his claim that that there is a fall in yield, but this may be in the body of the paper. It's not clear from the abstract what the counterbalancing effect of CO2 was on these seed heads.

It seems to me that it must be a perilously difficult problem to predict the effect of temperature changes on crops. The threshold demonstrated in this paper seems clear enough, but do we have the same kinds of data for other crops? Or is the 30°C threshold simply extended to other crops? Presumably you then need to assess the likelihood of the threshold being surpassed in each location that particular crop is grown. And is the good news about CO2 fertilisation factored into the impact assessments too?

Lots of questions. Perhaps readers can help find the answers.

Comments

[Streetcred]

Apr 21, 2012 at 3:04 PM | Luther Bl't - On a different matter, can anyone clarify if Lord Stern takes his name from a love of authoritarianism, or from the back end of a ship?
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Head shaped like the "back end of a ship". LOL ! Not much room for a functional brain in there.

[Tommo]

Here in OZ where a lot of wheat is grown the limiting factor is soil moisture. I've never seen temperature mentioned as a factor except for frosts during the flowering period. Cheers from soggy - again! - Sydney.

[AndrewKennett]

According to USDA figues they've seen a 160% increase in yield:inout for wheat since 1950 (see the US wheat facebook page) and hasn't it been getting hotter? I suppose the UK could move from a winter wheat to a spring wheat like in Kansas

[Filbert Cobb]

I guess Stern's researchers were scratching around for "evidence" to support the predetermined outcome. What they turned up was a thread about the potential sensitivity of wheat to brief high temperature at a critical developmental stage - and that's all. Wheat is generally unaffected in the field - it has a mechanism for temperature control which relies on the cooling effect of evapotranspiration. Like all plants.

Mealie mealie - obviously my South African friends do not know the correct name for their own porridge. I would tell them, but they prefer Corn Flakes.

[marchesarosa]

UK Winter wheat and Spring wheat cycles. Seems it's just that winter wheat is in the ground longer. They are harvested at the same time according to this webpage.

http://www.ukagriculture.com/production_cycles/wheat_production_cycle.cfm?comment=add&attributes.title=The%20wheat%20production%20cycle

[richard verney]

Can someone remind me why we grow plants in green houses.

Surely, it cannot be that the extrra warmth is beneficial?

[Sleepalot]

Why are there notes on _second year_ growth? Who grows wheat for two years - other than agenda-driven researchers?

[flaxdoctor]

@marchesarosa

Yes, but...

Winter wheat usually out yields spring wheat by quite a lot - 30% or more isn't uncommon - also it's easier to get the crop growing in the autumn where seedbeds are more reliably moist,

Why plant spring wheat at all then? Most don't, but it does mature about 3 weeks later, which means you don't need to combine all your crop at once - this is crucial for quality concerns, since if a mature crop is rained on frequently then it can start to germinate, which begins the conversion of starch in the grain to sugar. This plays havoc with breadmaking for all sorts of reasons. Also switching to spring-sown crops give opportunities for farmers to use different methods for tricky weeds which can plague winter crops, such as blackgrass, which can resist chemical control but aren't quite so resistant to being cultivated.

[TerryS]

Re: marchesarosa

Winter wheat is a "hard" wheat suitable for making breads. Spring wheat is softer and is better for making cakes.

[Filbert Cobb]

"Winter wheat is a "hard" wheat suitable for making breads. Spring wheat is softer and is better for making cakes."

Oh dear ... take a look at the HGCA Recommended Lists for winter and spring wheats (these are for varieties available in UK) and then come back and say whether you still believe this.

[AndrewKennett]

Winter wheat can be soft or hard, winter just means it is planted at the start of winter - often it likes to be cover in snow but not here in Australia where all our wheat is winter. Spring wheat is planted at the end of winter and can be soft or hard as well

[Anon]

I have downloaded the original paper because I was interested to see whether the authors included the 'dog leg' in their paper. I haven't read the paper in full, but the graphs suggest that Stern has indeed been highly selective in what he shows. The data from the figure shown in your post appear to come from figure 11b of the Wheeler et al. paper. There is no 'dog leg' in the original paper. However, there are two sets of points in figure 11b. The ones shown in the Stern report are for 380-390 ppm CO2, but even here there appears to be a point edited out in the Stern report at about (42C, 25 grains per ear) which would suggest a less precipitous decline with temperature. But in the context of AGW, why would the temperature rise much with only 380-390ppm CO2?

The second set of points in the Wheeler et al figure 11b but missing from the Stern graph are for 684-713ppm CO2. And guess what? This second set of points show a steady increase in number of grains per ear from around 30 grains at 20C to around 40 grains at around 37C. There is no data above 37C.

The findings of the Wheeler et al paper appear to be quite complex taken as a whole, and shouldn't be read as endorsing higher CO2 as an unmitigated good. However, it is evident that Stern has cherry-picked the results that are themselves incoherent in the context of AGW - high temperatures but low CO2 - and deleted from the original graph the results which are coherent in the context of AGW - high temperatures and high CO2 - because they show steadily improving yields. A clear case of hiding the incline.

[HaroldW]

Semonov used a crop simulation model to speculate on the effects of two winter wheat cultivars, cv. Mercia & cv. Avalon, in "Impacts of climate change on wheat in England and Wales." While I can't speak to the reliability of the crop simulation model, and certainly would not endorse that the regional climate prediction is skillful, he predicts an increase in yield, as shown in his Figure 1.Some excerpts:

For all scenarios and both wheat cultivars, average yields are predicted to increase mainly owing to yield stimulation with rising [CO2]....Our results demonstrate that the impact of drought stress on two existing wheat cultivars in the UK is predicted to decrease with climate change. Drier and warmer summers, which are expected in the UK, do not necessarily mean additional yield losses due to water stress.

Semonov writes that the risk of encountering temperatures above 31°C just prior to anthesis is negligible, even under the future UK climate scenarios, but states that a more likely danger is temperatures above 27°C just after flowering, which "can result in a high number of sterile grains."

Semonov observes: "Owing to the complex nonlinear interactions between a plant and its environment, the assessment of impacts is not trivial."

[flaxdoctor]

HaroldW,

A friend was co-supervised by Mikhail Semenov for his PhD - I've met him and hold him in very high regard - this is a man who knows what models are for and is capable of understanding their weaknesses.

Some might say it's a shame he doesn't work in other, higher profile fields.

I don't - I'm adequately convinced that we don't have any higher priorities than food production.

[Streetcred]

Apr 22, 2012 at 9:48 AM | Filbert Cobb -- Mealie mealie - obviously my South African friends do not know the correct name for their own porridge. I would tell them, but they prefer Corn Flakes.
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Australian actually, but 26 Years in Africa is not easily dismissed, Mr Cobb.

From Wiki: Definition for mealie meal: Cornmeal is flour ground from dried maize or American corn. It is a common staple food, and is ground to fine, medium, and coarse... [en.wikipedia.org/wiki/Mealie_meal]

There is no definition for "mealie mealie" ... please get your simple facts sorted or you might put all of your knowledgeable pronouncements at question.

[Filbert Cobb]

ZAMBIA: SMUGGLING of mealie mealie and maize
SMUGGLING of mealie mealie and maize grain is a sad phenomenon which can threaten food security especially if the staple food is ferried out of the country in large quantities.

in The Zimbabwean, 20 Feb 2009

[Streetcred]

Apr 23, 2012 at 12:25 AM | Filbert Cobb -- in The Zimbabwean, 20 Feb 2009
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Mate, you'll need a better source than that ... probably a grammatic error; nothing in The Zimbabwean is remotely believable. Never heard of "mealie mealie" in 26 years living in Southern Africa ... and neither has any native South Africans that I know.

[Philip Bradley]

We grow a lot of wheat here in Western Australia. All of it winter wheat. The northern part of the wheatbelt can get 30C+ temperatures in any month of the year. I've never heard of high winter temperatures reducing yield. Rainfall is what determines yield here. And reduced transpiration from increased CO2 would seem to point towards increased yields. Although with increased evaporation operating in the opposite direct.

Here is a paper by the CSIRO on wheat yield projections in WA out to 2050. They of course predict decreased yields, but right in the middle they show a graph of actual broad acre (almost all wheat and barley) production. And surprise, surprise, actual production has doubled since 1970 when supposedly AGW started in earnest.

[Philip Bradley]

Forgot the link

http://www.agric.wa.gov.au/objtwr/imported_assets/content/lwe/cli/climate21_session1_foster_farre_asseng_.pdf

[Richard Betts]

This is a very interesting topic to me.

I've commented on this at the new, related thread "Stern's wheat graph redux", and also asked for pointers to papers. Please see my post and question there, and send more info if you have it :-)

Cheers

Richard

[mangochutney]

Apr 21, 2012 at 11:51 AM | SayNoToFearmongers

I think the Eden biomes display something important already - the big one showcases a humid tropical environment (held around 30°C) whilst the smaller of the pair houses a warm temperate environment (Mediterranean-ish, a few degrees warmer than ambient). Guess which one the staff spend much of their time hacking back so that visitors don't need their own machetes?

Wouldn't this depend on the species present and how fast they grow in their natural environment or are the species the same in both environments?