Could climate’s crop impact catch us with our plants down?

The odds that yields of maize will fall by a tenth over the next 20 years have shortened from 1-in-200 to 1-in-10. Image copyright Raman Sharma used via Flickr Creative Commons license.

The odds that yields of maize will fall by a tenth over the next 20 years have shortened from 1-in-200 to 1-in-10. Image copyright Raman Sharma, used via Flickr Creative Commons license.

With the next two decades set to see a stronger increase in demand for food than the rest of the 21st century, declining harvests would cause some serious problems. Right now crop yields are growing, but could climate trends cause them to fall by a tenth, say, over the next 10-20 years?

That’s the question David Lobell from Stanford University in California and Claudia Tebaldi from the US National Center for Atmospheric Research in Boulder, Colorado have tried to answer. They find that if the world wasn’t warming, the chance of yields decreasing by a tenth over the next 20 years would be less than 1-in-200. However, climate change has made shrinking yields more likely, shortening the odds to a 1-in-10 chance for maize and a 1-in-20 chance for wheat.

“It was surprising to see how likely it is nowadays for climate trends to significantly cut into yield progress,” David told me. “It is still more likely than not that climate will be a slight drag on progress instead of a major factor. But we can’t rule out a major slowdown, and that means we should probably think through that type of scenario to figure out how to prepare for it.”

Such near-future climate forecasts are unusual, David underlined. “Longer periods allow the signal of climate change to become clearer compared to natural variability,” he explained. “But it may simply be that most of the initial questions about climate change were about the long timescales, to decide about questions related to energy choices and emissions. Now, a lot of questions are related about how to properly adapt to the changes happening now.”

What will happen to crops is central to David’s interests as associate director of Stanford’s Center on Food Security and the Environment. “I often get asked by governments or the private sector if climate change will threaten food supply in the next couple of decades, as if it’s a simple yes or no answer,” the scientist revealed. “This was especially true of a committee I recently served on focused on social stresses from climate change in the near-term. The truth is that over a 10 or 20 year period, it depends largely on how fast things warm, and we can’t predict that very precisely. So the best we can do is put odds on things.”

Untangling the roles of climate and better farming

Stanford University's David Lobell has tried to answer the question he's often asked: will climate change threaten food supply in the next couple of decades? Image credit: Stanford University

Stanford University’s David Lobell has tried to answer the question he’s often asked: will climate change threaten food supply in the next couple of decades? Image credit: Stanford University

In an Environmental Research Letters paper published last week, whose title expresses a desire to stop us ‘Getting caught with our plants down’, David and Claudia estimate those odds. To do that, they first had to work out how rising temperatures and rainfall affect wheat and maize yields. That’s not simple to isolate, because harvests are also affected by other factors, such as improving farming methods. Lacking data for these influences, they used an approach known as ‘first differences’ used in economics when information is missing. “The sensitivities that we infer for these crops – about 6-7% yield loss per degree of warming – agree well with estimates from other approaches, including experiments or using crop simulation models,” David said.

But because Claudia and David sought to explore uncertainty, they fed 500 random values around these sensitivities into two sets of the most modern and rigorous climate models available. “It’s critical for this type of question to explore a full range of possible climate scenarios, not just a handful, since we are trying to look at outcomes that are not very likely,” David said. “Using two ensembles – one based on running lots of models and one based on running one model for lots of different initial conditions – we were able to get a better sense of the chance of these outcomes.”

Using these figures and models, the American scientists then compared scenarios where greenhouse emissions and warming continue against simulations with no emissions, where the world doesn’t warm. Their results show that because of where maize is farmed, it’s slightly more likely to experience an average 1°C temperature rise at a critical time of year than wheat.

Not knowingly over-certain

NCAR's Claudia Tebaldi highlights the need to account for uncertainties when estimating risks. Image credit: Carlye Calvin, UCAR

NCAR’s Claudia Tebaldi highlights the need to account for uncertainties when estimating risks. Image credit: Carlye Calvin, UCAR

The simulations also suggest that there’s a 1-in-10 to 1-in-5 chance of a cooling trend over the next decade, though that shrinks to around a 1-in-20 chance by the end of the second decade. Average yield impacts are similar for maize and wheat – both falling by a fortieth after one decade and a twentieth after two. However there’s a much wider spread of possibilities for maize, which means the chances of much worse harvests are higher than for wheat.

David expects the details of the study will draw criticism from other scientists who think there are better ways to estimate these odds. “That’s all for the better if more people start focusing on the issue of near-term risks and getting better numbers,” he said. “For people working on issues related to food security in governments, NGOs, and the private sector, hopefully the general point of the paper will help to motivate more thinking about how to ensure continued progress in crop yields. This paper won’t be very useful for people who want to know what will definitely happen. But there’s a whole community of people used to dealing with risk and planning for contingencies.”

Claudia adds that their paper is part of a greater effort among climate scientists to account for natural variability and other sources of uncertainties, like those in climate models. She points to a paper published in the journal Nature last year examining the forthcoming effects of climate change on ecological systems that didn’t do this. Last week a different group of researchers warned the original authors hadn’t been through the uncertainties in enough detail, and therefore presented forecasts as more precise than they really were.

“Our study took advantage of large ensembles of simulations in order to avoid just that –misrepresenting results to be too precise or too certain,” she explained. “We’re trying to get at the odds and risks, accounting for the fact that the climate system, even when undisturbed by greenhouse gas emissions, shows some odds of the events we are concerned about. We want to characterise the enhanced risks as best we can, when we see them in our data.”

The distributions of yield impacts Claudia and David got by applying samples of randomly generated sensitivities to climate changes as predicted by different sets of climate models. In each panel the black line corresponds to the control experiment results without extra greenhouse gas emissions or warming, the red line to the large group of simulations run on the same model (CCSM) and the yellow line to results from a large group of models (CMIP5). (a), (b) are impacts over 10 years relative to the 2008–2012 average global yields for maize and wheat. (c), (d) are yield impacts over 20 years. The higher the line gets, the more likely the change marked underneath that point on the bottom axis is. Figure copyright IOP Publishing, used via Creative Commons license.

The distributions of yield impacts Claudia and David got by applying samples of randomly generated sensitivities to climate changes as predicted by different sets of climate models. In each panel the black line corresponds to the control experiment results without extra greenhouse gas emissions or warming, the red line to the large group of simulations run on the same model (CCSM) and the yellow line to results from a large group of models (CMIP5). (a), (b) are impacts over 10 years relative to the 2008–2012 average global yields for maize and wheat. (c), (d) are yield impacts over 20 years. The higher the line gets, the more likely the change marked underneath that point on the bottom axis is. Figure copyright IOP Publishing, used via Creative Commons license.

Journal references:

Lobell, D., & Tebaldi, C. (2014). Getting caught with our plants down: the risks of a global crop yield slowdown from climate trends in the next two decades Environmental Research Letters, 9 (7) DOI: 10.1088/1748-9326/9/7/074003
Mora, C., Frazier, A., Longman, R., Dacks, R., Walton, M., Tong, E., Sanchez, J., Kaiser, L., Stender, Y., Anderson, J., Ambrosino, C., Fernandez-Silva, I., Giuseffi, L., & Giambelluca, T. (2013). The projected timing of climate departure from recent variability Nature, 502 (7470), 183-187 DOI: 10.1038/nature12540
Hawkins, E., Anderson, B., Diffenbaugh, N., Mahlstein, I., Betts, R., Hegerl, G., Joshi, M., Knutti, R., McNeall, D., Solomon, S., Sutton, R., Syktus, J., & Vecchi, G. (2014). Uncertainties in the timing of unprecedented climates Nature, 511 (7507) DOI: 10.1038/nature13523

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20 Responses to “Could climate’s crop impact catch us with our plants down?”

  1. Richard Mallett Says:

    Higher CO2 and higher temperatures (within reason) should ensure higher crop yields, as they are currently doing.
    See http://faostat3.fao.org/faostat-gateway/go/to/home/E

    • andyextance Says:

      I appreciate this comment relates to the general theme of this post, but what’s the specific relevance? What point are you trying to make? That we shouldn’t plan for events that have a 1-in-10 or 1-in-20 chance of happening? Or just that you think this study is wrong?

      I’d also point out that ‘should’ is an imprecise word; and your link is raw data, which you’re making an interpretation of without explaining. Is that interpretation in any way based on teasing out the different impacts of technological advance, temperature and CO2, as David and Claudia have done?

      • Richard Mallett Says:

        You know that I prefer to look at the statistics of what has happened and is happening, rather than the predictions of what the models expect to happen (which can fail to be confirmed by future events, for whatever reason)

        Higher temperatures and higher CO2 levels (and other factors) are producing higher crop yields right now.

        According to the FAO, production, food supply, protein supply are all increasing. Undernourishment is decreasing. Dietary supply adequacy is increasing. Plants are currently thriving.

      • andyextance Says:

        OK, so it seems your comment isn’t particularly based on how much of the improvement comes from higher CO2, how much from higher temperature, and how much from improved technologies, which is fine. And I don’t disagree with the point that yields are now rising, but what’s the point of bringing it up? Even this blog entry makes the point that it’s most likely they’ll continue to rise, but the chance that they won’t is now much higher. Isn’t it useful to know that?

      • Richard Mallett Says:

        They talk about 6-7% yield loss per degree of warming, which (since 1850) has been occurring at a rate of 0.65 C per century; so we’re talking about a forecast yield loss of 3.9-4.55% per century or 0.039-0.0455% per year.

        If we go to http://faostat3.fao.org/faostat-gateway/go/to/browse/Q/*/E and select Production, Cereals Total, and look at Production Growth Rates by Region 2000-2012 we see a range of 0.803% for Europe to 3.647% for Africa.

        So the prediction is that this could decrease from 0.803% to 0.7575-0.764% per year for Europe, and from 3.647% to 3.6015-3.608% per year for Africa.

        Not something that is likely to keep people awake at night.

      • andyextance Says:

        Now perhaps you’ll do a full exploration of the precision and uncertainty of that estimate? ;-)

      • andyextance Says:

        I almost didn’t mention them because they seem so obvious to me and you’re so unlikely to take them on board, and you seem to be completely ignoring all of my direct questions, but this calculation has lots of potential flaws. For example your assumptions over rates of warming and continuing production growth are based on very little, as far as I can see. How are you justifying them? Have you even looked at the FAO forecasts? Plus you’re focussing on a single figure – that’s very different to trying to estimate the likelihood of different outcomes. Also, this blog entry talks about yield – but you’re talking about production growth rates, which is growing faster in Africa and Europe than yield. That’s presumably because more land is being converted to agriculture, which itself can release long-stored greenhouse gases to the atmosphere. If you can get this detail wrong, does it occur to you that some of your larger assumptions might also have problems?

      • Richard Mallett Says:

        I am really trying to answer your questions. I start with the postulate that the future is going to be somewhat similar to the past, with warming / cooling / pausing periods that I have mentioned before, and with similar production growth as the FAO has measured since 1981.

        I haven’t seen any forecasts from the FAO. Also they seem to only give figures for world production. I haven’t seen any FAO figures for world yield.

        Obviously, trying to estimate the likelihood of different outcomes requires making a number of different assumptions, some of which may turn out to be wrong. As soon as one starts using words like ‘presumably’ then one enters the realm of speculation, and I don’t want to go there. I want to stick to what I know, plus the postulates given above. So far, I see no reason to abandon those postulates.

      • Richard Mallett Says:

        From Willis Eschenbach on WUWT (which I believe is relevant here also) :-

        “On a personal level, the public peer review afforded by WUWT is of immense use to me, because my work either gets falsified or not very quickly … or else, as in this case, there’s an interesting ongoing debate. For me, being shown to be wrong is more valuable than being shown to be right. If I’m right, well, I thought so to begin with or I wouldn’t have published it, and it doesn’t change my direction.

        But if someone can point out my mistakes, it saves me endless time following blind alleys and wrong paths.”

      • andyextance Says:

        I don’t dispute this at all. However, Willis seems quite prepared to admit he’s wrong, is open to other arguments. How true is that of the rest of us I wonder? Particularly the bulk of the WUWT crowd.

      • Richard Mallett Says:

        You convinced me that Hadley CET was too local, so I’m looking for the longest temperature records with the best coverage (most years with full records) to replace it. Out of 169 records with coverage before 1850, probably 28 will qualify.

  2. Richard Mallett Says:

    I believe that, at the moment, it’s more important to continue with my investigation of long temperature records in CRUTem4. Changes in yield of around 0.04% per year are not likely to hold my interest.

    • andyextance Says:

      That’s a shame, it makes it feel like it’s a bit of a one-way relationship we’ve got going here. I do welcome your comments here on this blog and on Twitter – it’s important that we challenge even the work of people who’ve spent their whole careers researching these subjects. But I never really get the sense that you’re open to changing your mind, even though I’ve taken on board some important shortcomings you’ve pointed out in my ideas.

      A lot of your efforts seem to be about saying ‘there’s no need to worry about this’, which I ironically find worrying. You don’t deny there’s change, you don’t deny it’s man made, true. But your focus seems to be about disproving any need for caution. There is an argument that says we shouldn’t be overcautious – but on the other hand, being undercautious would be disastrous. I know which side I’d rather err on. Which side do you fall on?

      • Richard Mallett Says:

        At the moment, I don’t foresee any climate related disasters in our foreseeable future, with temperatures rising at 0.65 C/century and sea level rising by 3.2 mm. per year. So my policy currently would be ‘steady as she goes’

        The alternative would be to deny the poor peoples of the world access to cheap energy from fossil fuels, for example, when in my opinion the probability of climate related disasters in our future are not sufficient to justify that.

        At present, the only alternative to fossil fuels are nuclear (with its associated risks) and wind and solar (which are currently very inefficient, and also intermittent, and so need backup)

      • andyextance Says:

        A prediction to start with. What I write here will do nothing to change your pre-conceived ideas, even though it challenges them.

        There is some evidence that your desire not to “deny the poor peoples of the world access to cheap energy from fossil fuels” is contrary to their desires.

        From a letter from the former chair of the Least Developed Countries (LDCs) group in the United Nations climate change negotiations to President Obama.

        “I am speaking on behalf of LDCs who are already suffering from the devastating impacts of climate change.”

        “We welcome your initiative on climate change gases in coal-fired power plants announced this week: this action was overdue, and it is an important step in bringing the US closer to the actions of the rest of the world, including those of developed and many developing countries.”

        We’re both white men in the UK, and what do we know about these countries’ position on climate change? I don’t claim to know any more about that than you. But what I do know is that we should avoid claiming to act on their behalf, and let them make their own arguments.

      • Richard Mallett Says:

        Yes of course. Solar and wind only work intermittently, so what else is there ? He doesn’t say.

  3. John Says:

    Reblogged this on jpratt27 and commented:
    Great work thanks

  4. lewiscleverdon Says:

    I’d respectfully point out that getting caught with your pants down isn’t really about averages probabilites –
    it’s about that pretty postlady having a parcel you need to sign for,
    and getting no reply at the front door she comes round the back to try and find you –
    and there you are in the outhouse with the door open and your pants round your ankles . . .

    The parallel in terms of global food security is of the rising incidence of national and regional harvest failures heading towards the point where two or more coincide to give a first global crop failure. This is plainly an entirely different outcome to that provided by estimations of yield for a given species according to a given estimate of its response to a given average levels of regional warming and of precipitation-change by a given date.

    As a farmer I find the the prospect of coincident major impacts far more troubling than long-terms trends – those impacts are what put farmers out of business, already in quite large numbers, due to rising input costs and/or to lack of resilience to successive bad harvests. And if anyone would like to advise me on adaption to an increasingly erratic climate, and to explain where I can get reliable info on what to plant next year and the year after, and who will provide me with the different farm equipment I’m going to need, I’d be glad to listen.

    Regards,

    Lewis Cleverdon

    • andyextance Says:

      Wow, it’s really interesting to hear about your first-hand experience of climate impacts. Sorry that it’s such a struggle. If you’re not getting the support you need, that’s important in itself, and deserves notice. Do other farmers feel the same? Is it getting much coverage?

      Assuming you’re in the US, I thought the USDA might be providing advice? I’m increasingly covering agricultural impacts, so do feel free to sign up for email notifications in the box on the right of the page. I only scratch the surface however. I’ll keep my eye out for information that might help you and – assuming you’re signed up for email notifications on this comment thread – will post anything I find here.


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