Part 3 of this series about the impacts of climate change on global agriculture was centred around a climate model study that indicated major global crop belts could experience production declines as a result of increased heat stress. These authors found strong results for rice, maize, soy – but not for wheat.

That got us digging. Never trust good news, when it comes to climate change.

Beautiful graphs showing a worrying pattern: three different calculating methods all project significant net productivity declines (on average around -5%) for all the world’s major wheat belts – under just 1 degree Celsius average temperature rise…

Well, long live those hard-working climate scientists, because they’ve actually looked at the impacts of climate change on wheat productivity specifically – and this time very thoroughly: with a group of no less than 62(!) crop & climate scientists, led by Bing Liu of Nanjing Agricultural University.

In 2016 this international research team published their results in Nature Climate Change under the title ‘Similar estimates of temperature impacts on global wheat yield by three independent methods’ – indicating this is more than a single model study.

The authors used both ‘grid-based and point-based simulations’ and compared those model outcomes with statistical regressions from historic records. These three methods “produce similar estimates of temperature impact on wheat yields at global and national scales”.

Significant production declines for an increasingly important crop

So what is that impact? For a one degree Celsius temperature rise, a net productivity decline of global wheat yields between 4.1 and 6.4 percent.

That is actually a lot, if you compare to for instance the previous study we discussed that looked at the effects of a global average warming of over 3 degrees – still a very realistic figure for our current century, considering the actual global emissions trajectory since the Paris climate agreement.

As the above diagram shows projected wheat yield declines under a 1 degree temperature rise occur in all the world’s major wheat belts, though the impact does seem to differ somewhat per either region (see China) or used method (see Russia).

Any wheat yield decline is also very significant for the global food supply. Most climate impacts studies still show stronger declines for tropical agriculture (where other staples are impacted) and combined with a steadily growing human population and their grain-consuming livestock. As a background trend more and more people become import-dependent on the global wheat belts, that are apparently also heat-stressed by climate change.

The thermal-sensitive (filling) period of wheat – and the impacts of the recent hot European summers

We’ve asked climate researcher Rik Leemans, a specialist in ecological and agricultural impacts of climate change at Wageningen University (not involved in the above study) to help us better understand the specific vulnerability of wheat to heat stress:

“This heat stress is important for wheat when it occurs during what’s called the ‘filling period’ – the developmental phase in which the plant fills the grain with starches. This critical, thermal-sensitive period starts after the growth, flowering and pollination.”

“If during this period, either at the beginning or the end stage, temperatures peak to an extreme (“heat stress”) this filling stops. You can actually end up with sufficient biomass on the fields (straw) but no grain. That’s why wheat yields in Europe were so low during the hot summers of 2003 and 2018.”

The European heat wave of 2018 was more of a northern European phenomenon, whereas the extremely hot summer of 2003 had its epicentre in France. Shown on the map are places where absolute heat records (since 1950) were broken during the 2018 summer – mostly in Scandinavia and the Netherlands. Now is heat the main problem (2003) or is it drought (2018)? Well, we also found another interesting publication that looked at the impacts of climate change on European wheat production specifically, published in Nature’s Scientific Reports in 2011. It predicts that for wheat productivity too heat stress is probably the bigger concern that drought. (The title of that study is very convincing: ‘Modelling predicts that heat stress, not drought, will increase vulnerability of wheat in Europe’.) But if we look at field data from 2003 and 2018 it might also well be that both are a problem:

Yes, lest we forget – much of Europe just experienced the driest summer on record, and in some places also the hottest, tied with that other very extreme summer in recent memory, the prolonged heat wave of 2003 – by now a well-studied climate extreme. In 2003 for instance the wheat production in France was about 20 percent below normal (with more dramatic declines in Ukraine) – and in 2018 German wheat production showed a similar ±20 percent decline, with also lower production in for instance France, Britain and Poland. In both 2003 and 2018 EU average wheat declines were smaller.

It makes sense. We’ve already passed one degree warming. Welcome to the future.

© Rolf Schuttenhelm | www.bitsofscience.org