The authors of a new paper on how climate change could affect crop yields in the future have reacted to an article in the Times yesterday, calling the headline “very misleading”.

The Times piece suggested the new research shows climate change will boost crop yields, a conclusion the newspaper said is “at odds” with the mainstream scientific view.  But this interpretation is “fabricating controversy where none actually exists”, the authors tell us.

Extreme heat

The new study is the first to quantify the effect of future heatwaves on food production. The latest report from the Intergovernmental Panel on Climate Change (IPCC) concluded it is very likely heatwaves will get longer and more frequent this century.

Until now, studies have only looked at what effect the rise in the global average temperature might have on crops.

Writing in the journal Environmental Research Letters, the authors looked at how the extra impact of heat waves will affect yields of three major crops – maize, spring wheat and soybean.

Crops and carbon dioxide fertilisation

Heat waves have a negative impact on all three crops, with maize suffering the biggest losses, the researchers found. Their findings show heatwaves could double maize losses by the 2080s, compared to the 1980s.

As well as temperature, the researchers took into account how rising carbon dioxide in the atmosphere could influence crop growth. Experiments have shown that raising carbon dioxide levels could make plants more efficient at using water, boosting growth.

The new paper’s calculations included the possibility that this ‘carbon dioxide fertilisation’ effect could counteract some of the losses that come from higher temperatures and heatwaves.

But while the carbon dioxide fertilisation effect works for some plants, not all respond in the same way. Maize doesn’t see much of a benefit, which is why the new research shows yields decreasing under climate change.

On the other hand, spring wheat and soybeans are in the group of plants expected to see better yields with raised carbon dioxide. According to the paper’s calculations, the benefits could be such that they outweigh the losses, meaning yields increase up to the 2080s.

Expected yields of maize, spring wheat and soybean. Coloured lines are different climate change scenarios. Dashed red line is expected yield with no carbon dioxide fertilisation. Dotted red line is yield without accounting for heatwaves.  Source: Deryng et al., ( 2014)

It’s this finding the Times  article picks up on, reporting it as:

“Rising greenhouse gas emissions may result in better harvests of wheat and soya beans even after allowing for the damage caused to crops by more frequent heat waves”.

The Times goes a step further, drawing a comparison with the next report from the IPCC on climate change impacts, due for release in just over a week. Its article says:

“[The new paper’s findings] appear at odds with the IPCC, which said in a leaked draft of a report due later this month that global warming would damage yields of wheat and “erode food security”.

But the authors come to a different conclusion about how their work fits in with the IPCC’s. Co-author on the new paper Jeff Price tells us:

“The Times puts an interesting spin on our paper. It would appear that The Times’s choice of headlines seems to be trying to fabricate controversy where none actually exists.”

And lead author Delphine Deryng tells us the model her research uses is one of the ones included in the IPCC’s analysis, so the idea that their results differ “doesn’t make sense at all”.

An inexact science

The researchers tell us they were very careful to highlight the large uncertainties about how crops will respond to different elements of climate change in their paper.

Probably the most important thing is that predicting the carbon dioxide fertilisation effect is far from an exact science, says Price. He tells us:

“The carbon dioxide fertilisation effect is incredibly uncertain â?¦ for it to exist you have to assume there’s adequate water and nutrients for plants to show accelerated growth or yield.”

Price explains the next IPCC report will “highlight the uncertainty that exists in the literature about the likely magnitude of carbon dioxide fertilisation effect – just as our study does.”

Deryng adds:

“To my view the headline of the Times article is misleading â?¦ It is true that increasing carbon dioxide into the atmosphere can boost photosynthesis of wheat and soybean, which behave differently than maize â?¦ However, the actual positive effect in the field remains controversial and highly uncertain.”

To explore what would happen if carbon dioxide fertilisation turns out to have minimal effect, the team ran their models with no benefits to offset the crop losses. They found that like maize, spring wheat and soybean suffered losses of more than 20 per cent by the 2080s – a result the Times reports. (The dashed red lines in the graphs above show expected yields with no carbon dioxide fertilisation effects).

What’s more, the researchers show ambitious carbon-cutting policies could avoid more than 80 per cent of the losses expected if emissions stay high in that scenario.

Complicating factors

There are other factors that could offset any benefits from rising carbon dioxide, such as ozone and the potential for new and unfamiliar pests and diseases, Price adds. Another factor to take into account is that higher carbon dioxide levels can affect the quality of crops. Deryng says:

“[R]ising carbon dioxide changes the carbon to nitrogen ratio, which reduces the protein content of wheat. That means that while you may get a higher yield, that yield may be less nutritious.”

In some cases, the carbon dioxide fertilisation effect may well add biomass (bulk) to the plant, but it won’t necessarily increase the amount of grain, Price explains.

With so many factors influencing crop yields, different models may give quite different projections, so it’s important to take a broad view. A paper by professor Andy Challinor earlier this week combined the results from 1,700 model experiments and estimated that without adaptation, climate change will result in lower yields for rice, wheat and maize, in both tropical and temperate regions with two degrees of warming. Challinor tells us:

“I can say that both the IPCC assessment process, and my paper published earlier this week, use a very broad range of literature in drawing conclusions about food security. It is consensus in the literature that it central to making broad conclusions.”

Climate change is bad news for food security

Even if the new paper’s projections for lower maize yields but higher wheat and soybean yields until the 2080s turn out to be right, that isn’t necessarily good news for food security. You have to look at how the effects play out in different regions, say the authors.

While temperate regions like the UK may see short term benefits, a lot of developing countries – where food security is a more immediate problem – depend on maize, and will see big drops in food availability. The authors explain in the paper:

“These losses among the top five producing countries accounting for 80 per cent of global maize production could play a major role in future world supply of maize, with consequences for stability of international crop markets and higher risks of future food insecurity.”

Maps showing percentage yield increases (green) and decreases (red) for maize, spring wheat and soybean (top row, left to right). The middle row shows yields without accounting for heatwaves. Bottom row shows yields with no carbon dioxide fertilisation. Source: Deryng et al., ( 2014)

Misconceptions

Price adds that even if a country can start to grow a different crop in place of maize, for example, the transition won’t be simple. He says:

“There’s a misconception that society can easily crop shift [selectively alter what’s grown and eaten in a given country]. Sometimes the crop is part of the culture – it may not just be that you happen to be able to shift to wheat over maize if maize is part of the culture.”

As ever with climate change, looking at one aspect in isolation obscures the wider picture – it’s important to look beyond the next couple of decades and the country we live in.  As the UK’s Chief Scientific Advisor, Professor Sir Mark Walport, told a House of Commons Committee:

“While there may be trivial benefits in some parts of the world for some of the time, the long-term direction for all of us is a negative direction.”