Climate & Agriculture 3: Temperate and subtropical agriculture equally affected by heat stress

Our previous post focused on a study indicating climate change can lead to a net decline in African agricultural productivity – at least for five major food staples, with maize being the most important. The study also showed that it is not precipitation changes, but heat stress that is the main concern.

That is not a uniquely African problem, nor a problem that is constrained to the tropics. Paradoxical as it may seem, increasing heat stress is set to create comparable declines in agricultural productivity in colder (subtropical and temperate) climate regions – affecting other global food staples, like wheat, rice and soy…

Heat stress damage from climate change for 4 global food staples
Heat stress damage from climate change for major agricultural production areas for four global food staples: rice, maize, soy and wheat. Increasing heat stress from climate change is a global problem, not limited to the tropics and subtropics, but extending to Earth’s temperate climate zones.

Agricultural declines in let’s say a 3 degrees warmer world

Before the 2013 report (IPCC AR5) introduced the four ‘representative concentration pathways’ – numbered from the most ambitious (RCP2.6) to unabated emissions (RCP8.5) by the average-expected climate forcing these scenarios exert in Watts per square meter – the world used another set of climate change scenarios. These were based on 4 global development families: A1, A2, B1 and B2

These climate scenarios you may remember from the third (2001) or the fourth (2007) assessment report, or the IPCC special report on emission scenarios (SRES) from 2000 – a range that proved in the decade to follow to be an underestimation of both rapidly rising emissions and of the economic and geopolitical reality, and was replaced by this somewhat more sobering outlook based on the RCP scenarios.

Nonetheless the A1-B2 scenarios still surface in many important climate impact studies from before 2013-2014. Like the one we discuss today, a study titled ‘Global hot-spots of heat stress on agricultural crops due to climate change,’ performed by an international group of crop researchers in 2011 and then published in 2013 in the journal Agricultural and Forest Meteorology.

climate emission scenario A1B -> ±3 degrees warming
For your information the future-warming scenario is this study is ‘A1B’. The A1 storyline is based on the assumption of a future world of very rapid economic growth, global population that peaks in mid-century [which is certain by now it will not] and declines thereafter, and the rapid introduction of new and more efficient technologies. Unlike the ‘fossil-intensive’ A1FI, the A1B sub-scenario adds a ‘balanced energy mix’ to this global economical development. All in all A1B is not your worst scenario (although it probably compares better to RCP6.0 then to RCP4.5). That’s always good to be aware of, when you try to interpret quantitative results of a climate impact study: if we stay on the current track, reality will be worse…

Crop belts are like biomes – all are affected by climate change

The study investigates a possible research bias, namely that tropical agriculture is the most sensitive to the consequences of climate change [presumably because you go from hot to hotter quicker when your baseline temperature is higher – we’re speculating].

“Global hot spots of crop heat stress overlap with important agricultural regions such as Eastern China, the Northern United States, South-Western Russian Federation and Southern Canada. This indicates that agricultural production in temperate countries may suffer substantial production losses from climate change (as for the A1B emission scenario used in our study), extending the findings of previous studies that impacts would mainly occur in sub-tropical and tropical regions…”

Of course there are strong comparisons to make between agriculture and ecology, and the word’s major crop belts are in a sense like biomes. And from climate impact studies on ecology we know colder biomes – like the rapidly changing taiga and tundra – are at least as affected by climate change as tropical regions, and also temperate zone biodiversity is not exempt from (specific and complex) warming damage.

It would imply, like biomes, different crop belts are faced with their own individual challenges as both the climate-average temperatures go up and extremes increase. As is often the case, the higher likelihood of ‘old extremes’ and the emergence of a new category of super extremes, is what creates most of the damage. For crops these fall in several categories: droughts, floods, storms – perhaps you can even include plagues as ‘climate extremes’ – and heat extremes, like the increase of summer heat waves. Especially the latter is an important factor determining productivity.

But when you’re a wheat farmer in the Ukraine, there is a big difference between an August heat wave and one in June. That has to do with the ‘thermal sensitive’ period in the growing season for respective crops, a crucial period illustrated below:

Heat stress from climate change depends on thermal sensitive period for a crop
Heat stress from climate change depends on thermal-sensitive period for a crop.

And that’s what the crop researchers have tried to model for various global food staples, comparing a baseline climate between 1970 and 2000 with a warmer new climate average between 2070 and 2100, in line with the A1B emissions scenario, as the maps at the top of this article show for rice, maize, soy and wheat.

“The percentage of days, within a 30-year period, with heat stress events increased from the Base to the A1B climate scenario. This increase was most evident for wetland rice in suitable areas of Central Asia, South Asia and Central North America where a high prevalence was predicted.”

“Most affected lands were located in continental regions at mid to high latitudes, particularly in the Northern Hemisphere. The largest extension of land at risk was predicted to occur between 40 and 60◦ N.”

“Extending the findings of previous studies which concluded that mainly tropical agriculture will suffer from climate change, our results indicate that global food supply may also be affected by heat stress in temperate and sub-tropical regions. Without mitigation measures to combat climate change or the implementation of local adaptive technologies, countries with extensive agricultural lands in continental regions at high latitudes may experience significant crop losses. Investment in local adaptive measures such as development of resistant varieties and changes in crop management are therefore necessary to minimize risks to global food supply.”

Heat stress of climate change compared for 4 crops: rice, maize, soy and wheat
Interesting little detail: rice, maize and soy all show strongly significant increases in heat stress damage, leading to net production declines. Wheat seems a bit different, not because heat stress damage is lower in a warmer climate – but because it was already equally high in the baseline climate (or even a bit higher).

Other interesting findings are that projected crop damage from heat stress is largest for rice and maize and smallest for wheat – and that “negligible heat stress effects were simulated for equatorial regions”.

Let’s see if we can find other studies that have opposing conclusions again somewhere about these effects of heat stress. Or studies that focus on other climate change impacts on agriculture. We’ve only just started.

© Rolf Schuttenhelm |

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