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Soybean plants ready for harvest
Soybean plants ready for harvest. Credit: ghornephoto/iStock/Getty Images.
FOOD AND FARMING
19 January 2017 10:00

High temperatures to hit staple crops in the US this century, study says

Robert McSweeney

Robert McSweeney

01.19.17
Robert McSweeney

Robert McSweeney

19.01.2017 | 10:00am
Food and farmingHigh temperatures to hit staple crops in the US this century, study says

Staple crops grown in the US could see their yields drop substantially by the end of the century as daily temperatures regularly soar past 30C, a new study finds.

Annual yields of maize, soybean and wheat could decline by 49%, 40% and 22%, respectively, compared to yields under today’s temperatures.

While irrigation systems could help offset these losses, the researchers say, the boost to plant growth from higher levels of atmospheric CO2 “only weakly reduce” them.

High temperatures

Wheat, maize and soybean are three of the world’s most important crops. They are grown on 62% of all the cropland in the US and 33% globally.

But yields of these staple crops are threatened by climate change, says the new study published in Nature Communications. The researchers look specifically at the impact of days where temperatures exceed 30C.

High temperatures can affect crop yields in various ways: they cause water stress as the soil and plant lose moisture, direct heat can damage plant tissue, and faster aging of the plant reduces how much time and energy it has to put into growing seeds.

Annual yields of wheat, maize and soybean can drop by 6% for every day that temperatures exceed 30C, the paper says.

Glossary
RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases. RCP8.5 describes a future where emissions are not curbed and atmospheric CO2 rises to around 1,370 parts per million… Read More

The research team used a collection of crop models to assess the impact of rising temperatures on the three crops by the end of the century. Their simulations assume a business-as-usual scenario, called RCP8.5, where emissions aren’t curbed and global average temperature rise is likely to hit 5C by 2100.

The results suggest that with more days surpassing 30C in future, high temperatures will increasingly cause harm to crops – lowering yields by 49%, 40% and 22% for maize, soybean and wheat, respectively.

Note, the study only considers temperature; it doesn’t factor in changes to rainfall or the frequency of extreme weather events.

Rising CO2

The study also reveals two important findings relating to increasing atmospheric CO2 and the use of irrigation.

Scientists know that higher levels of CO2 in the atmosphere cause a boost to plant growth by speeding up photosynthesis. This is known as “CO2 fertilisation”. More CO2 in the atmosphere also tends to reduce water stress in plants as they lose less moisture during photosynthesis.

The researchers tested this effect by running their models with CO2 at present levels and then estimated for the end of the century if emissions aren’t reduced.

They found that the elevated end-of-century CO2 levels only slightly reduced the yield losses caused by high temperatures. Lead author Bernhard Schauberger, a research assistant at the Potsdam Institute for Climate Impact Research, explains to Carbon Brief:

“The models suggest that increased CO2 does not prevent the high-temperature induced yield loss at the end of the 21st century. This is contrary to usual assumptions that CO2 helps plants to overcome mild water stress.”

The findings do not suggest that high atmospheric CO2 cannot boost crop yields, the paper notes, but that it has only a limited potential to buffer against the negative impacts of high temperatures.

Water stress

The second important finding is that while CO2 might not limit the crop yield losses caused by high temperatures, irrigation can.

The model simulations distinguish between cropland that relies on rainfall (“rainfed”) and land that is irrigated, explains Schauberger. This means the researchers could separate out the effects for either type of crop management.

Their results suggest that declines in yields are small for all three staple crops when irrigation is used.

This means that irrigation could be a way of adapting rainfed cropland to hotter temperatures. But it’s worth noting that irrigation systems ultimately rely on the availability of water as well, says Schauberger:

“Irrigation of crops can overcome some of the declines. But this is, of course, only reasonable in regions where there is enough water.”

As you can see in the maps below the majority of crops in the US are currently rainfed (green shading), while irrigation systems (blue shading) are only prominent in western states.

Methods of crop management for US counties, showing rainfed (green shading), irrigated (blue), mixed (yellow) and no crops grown (grey). Maps are for a) maize, b) soybean and c) wheat. Source: Schauberger et al. (2017)

Methods of crop management for US counties, showing rainfed (green shading), irrigated (blue), mixed (yellow) and no crops grown (grey). Maps are for a) maize, b) soybean and c) wheat. Source: Schauberger et al. (2017)

The study confirms what scientists already suspected about the impact of high temperatures on crops, says Prof Andy Challinor, professor of climate impacts at the University of Leeds, who wasn’t involved in the study. But the more significant finding is that the predominant way that high temperatures affects crops is water stress, he tells Carbon Brief:

“Perhaps most importantly, [the study] indicates the importance of the water stress that accompanies high temperatures. This presents a significant challenge given the projected difficulties in managing water resources in our changing climates.”

Schauberger, B. et al. (2017) Consistent negative response of US crops to high temperatures in observations and crop models, Nature Communications, doi:10.1038/ncomms13931

  • Paul Matthews

    This year there was a record corn and soybean crop in 2016

    http://www.cornandsoybeandigest.com/blog/usda-projects-record-corn-and-soybean-crop-2016

    • MarkR

      Does that contradict any research?

      Most crop stuff I’ve read says that climate change until now (~1 C global warming) has helped some places and hurt others, but the effect is smaller than that from improved farming techniques. 3-5 C of warming, about the difference between the last glacial and today, would have much more of an effect.

      Examples:
      “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…these impacts are small relative to the
      technological yield gains over the same period…”
      http://dx.doi.org/10.1088/1748-9326/2/1/014002

      “yields increase with temperature up to 29° C for corn, 30° C for soybeans, and 32° C for cotton but that temperatures above these thresholds are very harmful…[if farms don’t adapt/move then] average yields are predicted to decrease by 30–46% before the end of the century under the slowest (B1) warming scenario and decrease by 63–82% under the most rapid warming scenario…”
      http://www.pnas.org/content/106/37/15594.short

  • KingB

    It is a common misperception amongst researchers that the RCP8.5 is a “business as usual” case. It is no such thing. RCP 8.5 is an extreme scenario chosen because it was just that: extreme. It was intended to be the “worst case scenario” and as such researchers would be better served by using RCP6.0 or RCP4.5 to conduct research. When I read that a researcher has used RCP8.5 as a “business-as-usual” case it always makes me extremely cautious about the work.

    • Robert McSweeney

      Hi KingB, thanks for your comment. While RCP8.5 does have the highest emissions pathway of the RCPs, it is described in the original paper as a “business as usual scenario” (the paper is open-access, and you can find it here: http://link.springer.com/article/10.1007/s10584-011-0149-y). Also, as this paper points out, global emissions are tracking just above RCP8.5: http://www.nature.com/nclimate/journal/v4/n3/full/nclimate2148.html

      • Paul Matthews

        The title of the paper is “RCP 8.5—A scenario of comparatively high greenhouse gas emissions”.

      • KingB

        Robert,
        As described in the literature RCP8.5 was developed to represent a high-end emissions scenario. “Compared to the scenario literature RCP8.5 depicts thus a relatively conservative business as usual case with low income, high population and high energy demand due to only modest improvements in energy intensity.” (Riahi et al. 2011). RCP8.5 comes in around the 90th percentile of published business-as-usual (or equivalently, baseline) scenarios, so it is higher than most business-as-usual scenarios (van Vuuren et al. 2011a)

        A careful read of the scenario identifies so many assumptions that are completely inconsistent with observed trends as to be meaningless, including:

        – lower trade flows, relatively slow capital stock turnover, and slower technological change;
        -less international cooperation than the A1 or B1 worlds. People, ideas, and capital are less mobile so that technology diffuses more slowly than in the other scenario families;
        -international disparities in productivity, and hence income per capita, are largely maintained or increased in absolute terms;
        -development of renewable energy technologies are delayed and are not shared widely between trade blocs;
        -delayed land use improvements for agriculture resulting in increased pollution and increased negative land use emissions until very late in the scenario (close to 2100);
        -a rebound in human population demographics resulting in human population of 15 billion in 2100; and
        -a 10 fold increase in the use of coal as a power source and a move away from natural gas as an energy source.

        Literally none of these assumptions have come to pass. There is a reason the IPCC does not refer to RCP8.5 as “business as usual” and that is because the IPCC recognized that it was not. As I wrote at my blog RCP8.5 was a pathway designed to model a set of conditions that could lead to a world where climate forcing by the year 2100 reached 8.5 watts per meter squared. It was essentially designed to provide a worst-case scenario [used in its traditional literary sense] if everything in the world went sideways or backwards.

        • Christian_Moe

          Like a world where the British vote to leave the EU and Donald Trump gets elected president of the US. I know, wacky. 🙂

      • Les Johnson

        RCP 8.5 also burns more fossil fuels than are in current reserves, but assumes low prices for fossil fuels. Its not a realistic scenario.

      • Paul Barry

        Hi Robert, the open-access link you posted above isn’t working. Perhaps you could just give the name of the paper, the year and the authors and I can find it.
        Thanks.

        • Robert McSweeney

          Hi Paul, apologies – the paper is “RCP 8.5—A scenario of comparatively high greenhouse gas emissions” in Climatic Change, by Keywan Riahi.

          Here’s the link again (it’s also the same link as the one on “RCP8.5” in the text of the article): http://link.springer.com/article/10.1007/s10584-011-0149-y

          • Paul Barry

            Thank you!


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