Agriculture after climate change: awakening a sleeping giant

In this article I would like to explain how I have arrived at the conclusion that the effects of climate change on agriculture to be drastically underestimated. A 2012 study looked at the track record of the IPCC.1 It found that rather than making “alarmist” predictions, the IPCC appears to have been systematically too conservative in its projections of the impact of climate change on our future. One potential reason for this may be that the IPCC makes assessments that serve to inform the media, the public and policy makers.

It’s known that despite there being a consensus among climatologists about climate change, large minorities of the general public are not convinced of the predictions made by clmatologists. For any of the IPCC’s projections to be too alarmist would end up as fuel on the fire of denialism. Thus the IPCC tends to be relatively conservative in its projections.

One area where the IPCC has been particularly conservative is on the topic of agriculture. In its fourth assessment report, the IPCC projected that up until three degree Celsius, global production potential would increase. Above three degree Celsius, global production potential would end up lower.

The IPCC report specifically states that the consequences of certain factors have not been adequately incorporated into its analysis. Effects that had been difficult to incorporate into the IPCC’s review are the consequences of changes in extreme weather events, the spread of diseases and pests and changes in technological developments.

In its first update in seven years, the IPCC has become more worried about the effects that climate change will have on agriculture. The IPCC now claims that the negative effects of climate change on food production are already stronger than the positive effects, even though it originally expected this to happen only above three degree Celsius.3

One aspect where the IPCC appears to have been too optimistic is in regards to the ability of technology to increase agricultural yields in the future. There are technologies ahead of us that may make food production less labor intensive, but to continue drastically increase yields is proving to be difficult. Global food production has to double by 2050 to meet global demands, but yields are not increasing sufficiently to meet these expectations.

The question we want to answer is how climate change can be expected to influence global crop yields in the future. This is obviously a massive subject, thus in this article I would like to focus here on a few factors, that I expect will work together to cause a reduction in global yields, far greater than policymakers currently anticipate:

-Effects of changes in atmospheric carbon dioxide on plant-pathogen interaction

-Effects of changes in atmospheric carbon dioxide on interactions between plants and their symbionts

There are other factors we will briefly touch, but the two factors above are very important, because we hardly ever think about them, as they are somewhat non-intuitive. It’s relatively easy for us to conceive of what could happen if parts of Africa suddenly were to receive fifty percent less rain, or if regional temperatures somewhere reach extremes that local farmers are unprepared for. It’s more difficult for us to conceive of how changes in the interaction between ryegrass and a fungus that inhabits the ryegrass could impact our milk production, yet, this is still a legitimate problem that we’re going to face in the coming decades.

Why pesticides will not save us this time

Part of the reason the problem that we’re about to face is not adequately taken into consideration is because our society does not treat plant pathogens as a legitimate threat to our food production. Fungicides and other chemical treatments exist to address the pathogen that is held responsible for the symptoms. We’ve beaten plant pathogens. After all, when was the last time you heard of anyone developing ergotism after eating rye? If climate change were to spread a particular rice pest, we expect that we can address the problem by spraying more as well as better pesticides.

There are a number of problems with this line of thinking however. First of all, pesticide application is itself not without consequences. We know the pesticides can be directly toxic to wildlife and humans. In addition, plants naturally produce chemicals that protect them against plant pathogens. Some of these, like salicylic acid, are beneficial to human health. Some of the effects such chemicals have is to reduce our risk of diabetes and cardiovascular disease. Pesticide application has been found to reduce salicylic acid synthesis in plants.5 Comparing organically grown plants to conventionally grown plants reveals that the prior contain six times as much salicylic acid as the latter.6

More important for us to consider is that we are aware certain pests can not be successfully treated with pesticides. Cabbages, broccoli, cauliflower, Brussels sprouts, radishes, turnips and many other of our favorite foods are all part of the same family of plants. These plants can be affected by a disease known as clubroot.

It is believed that ten percent of land used to grow plants in the cabbage family is infected with the clubroot organism.7 The disease causes greatly reduced yield of these plants. The problem lies in the fact that clubroot can not be successfully treated. Besides the fact that eliminating clubroot would require so much pesticides that it would violate current regulations, applying this much pesticide would not be cost-effective.

The clubroot organism is currently spreading to places where it was previously not seen. It was confirmed for the first time in Saskewatchan in 2011. In 1993 it was discovered for the first time in Nepal. In 2008, a big epidemic of clubroot there reduced yields of Brassica family plants by forty percent throughout the whole country.8

Part of the reason for the spread of clubroot to such places is globally rising temperatures. Clubroot needs a minimum temperature of fourteen degree Celsius to thrive.9 In the Netherlands, cabbage is generally only grown in northern parts of the country, because these parts are generally not affected by clubroot.

However, there are other factors implicated in the spread of clubroot. One issue we face is that clubroot is affected by higher atmospheric CO2 concentrations. One study done in Japan found that at relatively higher CO2 concentrations clubroot grows better, because the PH concentration of the water decreases.10

Effects of temperature changes on agriculture

If you were to observe that global temperatures are rising, you may expect that the same plants can still grow successfully, but simply further away from the equator. However there are aspects of the climate that change fundamentally. Because of the greenhouse effect, temperature minimums increase more than temperature maximums. One study found that between 1951 and 1990, global minimum temperatures increased by 0.84 C, while global maximum temperatures increased by 0.28 C.11 In other words, the variability of temperatures is affected as well.

This might seem beneficial to plants at first, but it has to be made clear that there are associated factors that imply significant harm as well. As an example, consider the fact that five percent of global crop loss is caused by root-knot nematodes.12 These nematodes are only found in warmer regions of the globe, because they tend to die when the soil freezes. Thus, as the climate changes, the range where these type of plant predators can survive is going to expand, more than you might expect from average temperatures alone, as their limiting constraint is the soil’s minimum temperatures.

This principle applies to other issues as well. The malaria parasite’s replication is inhibited at temperatures above 30 C.13 Minimum temperatures are estimated at 14.5 C and 16.0 C for two different species respectively.14 Thus, if global maximum temperatures increase less than global minimum temperatures, we would expect the global malaria burden to increase. This is problematic, because it’s known that in places with unstable malaria presence, irrigation increases malaria transmission.

Effects of CO2 enrichment on our main crops

The effects of atmospheric CO2 enrichment on our main crops appear to be beneficial, at least under the type of isolated circumstances modern science enables us to produce. In the real world, the effects can be expected to be more detrimental. Studies find for example that powdery mildew, leaf rust and stem rust all produce more severe symptoms when CO2 concentrations are increased from 390 ppm to 750 parts per million.15

Insect predation of these plants also increases. Soybeans show increased predation by insects at higher CO2 concentrations, because the plants become less able to produce chemicals that protect them against predation. Thus leaf damage of soybeans increases.16

Competing with other plants after climate change

The plants we use have to compete with other plants. What characterizes the plants we grow is that they tend to do relatively better under lower atmospheric carbon dioxide concentrations. Maize is the most widely grown crop in America. Maize is a C4 plant. The C4 pathway of carbon fixation allows a plant to lose less water while gathering carbon dioxide from the atmosphere. This pathway evolved in multiple plant lineages independently, to thrive under the low carbon dioxide concentration our atmosphere currently has. Thus, studies tend to find that maize suffers from higher CO2 concentrations in its competition with weeds.17

Perhaps even worse for modern agriculture is the observation that many weeds seem to develop greater resistance to herbicides at higher CO2 concentrations. This is partly due to the dilution effect from the larger size of the weeds, but not entirely, as some other aspect seems to play a role as well.18

Storing food after climate change

There are other aspects of agriculture that we don’t really think about either, treating them as self-evident. One of these aspects is the storage of food. Food that’s stored can develop molds. The speed at which this happens is influenced by atmospheric carbon dioxide concentrations. At 0.5% atmospheric carbon dioxide, Aspergillus Niger, a common food contaminant that causes food to rot, begins to germinate more rapidly. A total of 70 to 90% of spores germinate within six hours, compared to just 15-20% at normal concentrations.19 Another common mold, Alternaria Alternata has also shown strongly increased growth at mildly elevated atmospheric carbon dioxide concentrations.20 The Alternaria genus is responsible for 20% of all food spoilage.

Even animal husbandry is going to be affected

Grass has evolved protection against overgrazing by large herbivores like cattle and deer, in the form of endophytes, its fungal symbionts that live inside the grass. The fungus produces a toxin that ensures that animals eating too much of the grass have reduced growth, reduced fertility and reduced milk production. However, when atmospheric CO2 concentrations increase, the fungus can produce far more toxins than it normally does. 21.

Under conditions of high soil nitrogen as well as 466 ppm atmospheric carbon dioxide, endophyte infected ryegrass produced about eight times as much ergovaline as they do under current atmospheric carbon dioxide levels. Importantly, these effects are only seen under conditions of high soil nitrogen, in the type of landscape created when we practice animal husbandry, where predators do not reduce the population density of herbivores. Right now, poisoning of animals by toxins like ergovaline costs the US cattle industry one billion dollar every year, a figure that’s going to increase drastically as atmospheric CO2 concentrations start to escalate.22


I have outlined some of the issues that agriculture in the 21st century will face in this article here above. I have focused on the issues people are unfamiliar with, because they require a better understanding. Relatively little research is still done on many of the problems described here. Other problems that agriculture is going to face are better known to many people. Heat waves and droughts can reduce yields and fossil fuel depletion will inevitably cause big problems as well.

The problem I see above all however, is that everybody looks at agriculture from an anthropocentric perspective. Agriculture is seen as a project that we decided to embark upon as a species. It is hardly ever interpreted as a consequence of the unique climatic conditions of the Holocene that made an otherwise rare phenomenon possible on a global scale. This despite the observation by archaeologists that agriculture emerged more or less simultaneously in different parts of the globe, “invented” by cultures that had no contact with each other.

If we look at agriculture as a consequence of factors outside of our control, then it becomes possible to imagine a world in which agriculture can disappear again, as the unique configuration of climatic variables that gave rise to it disappears as well. Note here that I am not suggesting that agriculture itself becomes physically impossible to practice. Grain yields don’t have to drop to zero to cause a culture to abandon agriculture. Rather, a culture that does not aim to control its local ecosystem may have a survival advantage over a culture that does, whereas under present climatic conditions it has a disadvantage in most of the world’s biomes, as today’s hunter-gatherers survive in deserts, tundra and other places where agriculture is nigh impossible.

Important to note is that an end to agriculture is not the same thing as the end of humanity. Humans are adapted to a wide variety of different biomes. Inuit thrive in the Arctic, the Khoisan thrive in the Kalahari desert, Tibetans thrive in their mountainous landscape, all thanks to genetic adaptations to their local conditions, just as Europeans are adapted to the relatively low iron concentrations in their soils.

Humans also have a variety of important skills that developed before agriculture. Like other primates, we can pick parasites out of each other’s hair and skin. We can walk on two feet, which allows us to cross larger distances while spending less energy than animals that rely on four legs. We can use fire, first developed about 2 million years ago, to cook food, which allows us to make the calories and nutrients more readily available for absorption. We can use stone tools, to dig for nutritious tubers in the ground. We can regulate our temperature through use of clothing.

The Neolithic revolution happened thousands of years ago in some parts of the planet, but other large parts of the world got by until just a few centuries ago without depending on agriculture, while some people still manage to survive without practicing agriculture. Our species managed to thrive before agriculture. If agriculture proves to be unsustainable, humans will be able to survive its demise.

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7 –  Quantitative Trait Loci for Clubroot Resistance in Brassica oleracea R.E. Voorrips, M.C. Jongerius, and H.J. Kanne

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11 – A New Perspective on Recent Global Warming: Asymmetric Trends of Daily Maximum and Minimum Temperature

12 – Root-Knot Nematode (Meloidogyne Species) Distribution in Some Tomato Fields in MakurdiBem, A.A1, Antsa, R.T., Orpin, J.B, Bem, S.L, And Amua, Q.M1

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