A new study in Nature Climate Change claims that agricultural productivity has dropped by one-fifth since 1961 due to rising temperatures. This is absolutely amazing data: after all, earlier the same Nature showed that every fifth ton of biomass around us owes its existence to the processes behind anthropogenic global warming. How is it that the productivity of agriculture is falling due to warming, while the biomass of plants is increasing? Let's try to figure it out.
How to calculate the impact of warming on agriculture
It is known that the agroindustry is an area where accurate measurements of the influence of individual factors on the final yield are extremely difficult. It is impossible to accurately separate the influence of the weather from the joy (or negligence) of farmers, the introduction of new varieties of cultivated plants, and so on. In addition, new varieties may be more resistant to drought or, conversely, moisture-loving than the old ones. How to understand in such conditions what influenced the harvest more: weather or bad weather?
With the impact of global climate change, it is still tougher. It cannot be associated with drought or heavy rains of a particular year. First of all, because in itself it does not lead to droughts or floods - and as it develops, the frequency of droughts remains unchanged, and the level of flood peaks even falls (why - we wrote here).
Complicating the task is the fact that over the past 60 years, the world has experienced a green revolution: the productivity of the main crops has increased significantly. Mainly due to the introduction of new plant varieties bred by breeders, as well as the increase in fertilizer application in most countries. How to separate the growth due to these factors from the decline due to warming is difficult to understand.
However, the scientific world has long faced an essentially political task: in spite of everything, still to measure the impact of warming on agriculture. After all, if this is not done, then how to justify the need to combat warming? In bare words?
A group of American scientists took up the challenge. To separate the effect of warming from other factors, they took the so-called total factor productivity - how much the agrarians need to make per unit of return (crop). In theory, this separates the effects of new varieties and fertilizers from the effects of climate change.
Further, the authors collected the data that they could on the costs of farmers in different countries and laid them in a model describing the change in such agricultural productivity in 1961-2015. The influence of weather on agricultural productivity in a particular country was taken into account through two factors: the average temperature of the five most favorable months for the growing season and the average amount of precipitation at the same time.
In parallel, they simulated the same years, but minus global warming (by almost a degree over this period). The result was surprising: due to global warming, the overall factor productivity dropped by 21% during this time. And most of all in Africa, Brazil, Indonesia, that is, the warmest parts of the world.Moreover, in the original, basic version of their work, they managed to show that such productivity has fallen even in Russia.
The latest achievement cannot be overestimated. The fact is that in Russia during this period, investments (especially labor) in agriculture did not so much grow, but in places fell, but the harvests increased significantly. It turns out that the agricultural productivity and a number of types of investments in it simultaneously fell in the country, but … the harvests increased. We will not try to comment on such a complex idea, since we have no idea how to explain it. And the authors of the work themselves, alas, did not stop at such trifles.
But let's leave Russia, a country that produces a very modest share of the world's agricultural products. Let's look at the planet as a whole. How do the findings of the new work agree with other data on the effects of global warming on plants? For example, those that Africa will benefit greatly from anthropogenic CO2 emissions?
Nature vs. Nature: Who Wins?
In 2020, Scientific Reports (also part of the publishing company Nature Research) published a paper that assessed the impact of CO2 emissions and global warming on the biomass of the planet Earth. Its authors concluded that terrestrial plants convert 119 billion tons of carbon per year into their biomass. Of these, 19.8 billion are due to anthropogenic CO2 emissions that stimulate plant growth. And due to global warming - another 1.4 billion tons. It turns out that 17.8% of the green biomass, which grows every year, owes its origin to the factors behind anthropogenic global warming. More than one-sixth of the young vegetation surrounding us came from anthropogenic carbon dioxide emissions, and without them they simply could not have arisen.
It turns out strange: global warming manages to reduce agricultural productivity and at the same time … to increase the biomass of plants. Like this?
Perhaps those who wrote the new work eliminated the influence of CO2 on plants, leaving only temperature changes? No, the authors of the new work directly point out: "Our estimates do not remove the direct effects of the growing CO2 concentration on agricultural production." And they can be understood: it is extremely difficult to remove these effects, since an increase in the concentration of CO2, although it accelerates the growth of almost all agricultural crops, does it in different ways, and it is unrealistic to say for certain where what growth was provided by this particular factor. All you can get is a low, conservative estimate.
As we already wrote, an increase in the concentration of carbon dioxide in the air in comparison with the pre-industrial period, on average, should raise the yield by at least 10%. Let us emphasize again: this is a conservative estimate, because for the world, anthropogenic CO2 emissions and warming provide 17.8% of the annual biomass growth. It is doubtful that for cultivated plants the increase in yield is lower than at least one sixth, but in order not to be mistaken in the direction of overestimation, one can take 10%.
There are other non-obvious points in the text. For example, the conditions for the growth of cultivated plants for only five months a year are being analyzed - the most productive in terms of increasing green biomass. This solution is satisfactory for the temperate climate, where the authors of the work live: say, potatoes grow for only 130 days, which fits into five months. But in warm countries, everything is fundamentally different: the growth cycle of cassava (the main culture of a number of tropical countries) takes an average of 272 days. And this is clearly more than five months.
It turns out that the study inevitably gives a distorted picture of agricultural productivity for virtually all countries outside the temperate climate. Tellingly, these are precisely the states for which the authors of the work insist on the greatest drop in agricultural productivity.
Let's summarize.The authors of the 2021 study, using simulations, concluded that warming, together with an increase in CO2 concentration in the atmosphere, leads to a drop in agricultural productivity. A 2020 study found that warming, coupled with rising CO2 concentrations, led to a dramatic rise in the biomass of terrestrial plants. A number of earlier studies gave the same conclusions. Including those that compared the content of carbonyl sulfide in the air from ancient ice (over the past 50 thousand years) with modern ice. Carbonyl sulfide in the pre-industrial period was produced only by plants, and even today its industrial emissions can be easily distinguished from natural ones. From these - empirical, not simulated - data, it follows that the green biomass of terrestrial plants today is 31% higher than in the pre-industrial period, before the onset of significant anthropogenic CO2 emissions.
It is absolutely clear that if the simulated calculations show the oppression of plants from something, and the empirical study of nature demonstrates their accelerated growth, then someone is grossly wrong here.
How the work went through peer review
Anyone who has worked as an anonymous reviewer for articles submitted for publication in a scientific journal understands that a normal reviewer should ask the authors of the work the same questions as we do. Until they answer them, the work should not be published, except on the preprint servers. In other words, in theory, a new study should never have been published at all. It is impossible to prove that the model of reality reflects this reality better than empirical observations of reality.
If someone has done something in front of your eyes that should not have happened, you should put yourself in his place and start thinking in the same way. Read what he is reading, reflect on what he is, in fact, for a short time, dotted line, become the one you are trying to understand. Let's do this with the authors of the work - typical scientists of our time.
It is necessary to investigate the impact of global warming on agriculture. Models are the easiest and most popular way to do this. In contrast, empirical research in this area is monstrously complex. Let's say you can compare carbonyl sulfide in samples of ancient air (from bubbles in ancient ice) and modern air, you don't even need to go anywhere (cores are brought from Antarctic expeditions by other people). But how to reliably understand what part of carbonyl sulfide is from cultivated plants and what part is from wild plants? No method, no research.
There is a modeling method, but there are simply no precise empirical methods to assess the impact of CO2 emissions on agricultural productivity. That is, the authors of the new work had a choice: either to write according to the model, or not to write at all. In the modern world, a scientist must regularly write works, otherwise he will have nothing to eat.
Could the authors of the work have taken a different path? Yes. Nothing stood in the way of comparing the model - showing a decrease in yields with increasing temperatures - with the actual picture observed in agriculture. For example, find out that when growing plants in large greenhouses, they maintain an average temperature of plus 27-29 degrees, For comparison: this is higher than the average annual temperature in Kinshasa, the capital of the Congo. Although there is an equatorial climate, hotter than in the vast majority of places on the planet.
In other words, the idea that "the rise in temperatures by itself inhibits the plants" is questionable for the modern world. Again, the authors of the work might wonder: where in the world do the most productive crops grow? Of the massively cultivated plants, the highest yields are in sweet potatoes, potatoes, bananas, and cassava. It would seem obvious that the rise in temperatures can be dangerous: potatoes at stable night temperatures from plus 25 degrees do not produce tubers at all.
However, this is not entirely correct. Think about it: one hundred grams of potatoes have only 322 kilojoules of energy, and sweet potatoes have 360.Manioc for the same mass gives 670 kilojoules: that is, in reality, it is more productive from one hectare than potatoes. This means that in terms of nutritional value per hectare, the main crops of a hot climate significantly exceed the main crops that require coolness.
This fact alone makes it clear that the current warming cannot seriously reduce the potential of world agriculture. So far, it has not created a situation anywhere where cassava (a very thermophilic culture) would suddenly begin to grow worse. This is also indicated by the fact of a sharp increase in harvests in those parts of the world where, according to the authors of the new work, labor productivity due to warming has decreased the most - that is, specifically in Africa.
The simplest empirical assessments of this kind should have turned the authors of the new work away from the conclusions they made in it. But this did not happen.
The likely reason is simple: scientists are as much a part of society as everyone else. In the press, we regularly see headlines like "Warming will make it impossible for people to live there" or "Climate change will lead to droughts." In a number of scientific works, we see the opposite: people clearly lived in the hottest regions of Africa during periods that were much hotter than the current one. Moreover, today's climate change is in no way leading to droughts.
But the press doesn't write about it. Scientists, as far as their specialization is concerned, sooner or later are infiltrated with ideas from the press - also because they themselves cannot cover all the works in scientific journals. Most likely, the authors of the new study really believe in what they wrote - and it was because of their faith that they did not consider it necessary to check empirical facts about the real impact of warming on the world around them.
There is also nothing to take from the reviewers: in order to ask the authors of the work the questions described above, they, after all, must also have an opinion that differs from those voiced in the press. And this, whatever one may say, is unsafe: any scientist who dares to say "global warming helps, not hurts agriculture" will inevitably face a negative attitude from colleagues.
One might argue: what is it? The scientific world has gone through many wrong, but fashionable theories - from "first Clovis" to "The moon came from the blow of Theia." Delusion is a normal part of the evolution of science, especially if there is no practical harm from it.
The problem is that there is such damage. If science tells policymakers that CO2 emissions reduce agricultural productivity, they must take concrete action. For example, invest heavily in agriculture, while reducing investments in a number of other sectors - for example, medicine, education, and so on.
But in reality, anthropogenic emissions dramatically increase agricultural productivity. In other words, societies begin to invest en masse where there is no point in investing. We are only at the very beginning of this path, and it is still difficult to predict how far such erroneous works will lead the modern world. Only one thing can be said for sure now: this movement will go in the wrong direction.