Forty years ago, Soviet climatologist Mikhail Budyko predicted that the result of anthropogenic carbon dioxide emissions would be a sharp surge in the development of terrestrial vegetation and the biosphere as a whole. By 2016, this was confirmed by satellite images, but scientific publications about them caused serious opposition: some scientists tried to refute the fact of landscaping. New work has shown that it not only exists, but is accelerating.
The fact that an increase in CO2 concentration in the air causes accelerated development of plants was known more than a hundred years ago. Soviet physicist and climatologist Mikhail Budyko, who was the first to declare the inevitability of global warming in 1970, spent a lot of time assessing the impact of anthropogenic carbon emissions on the biosphere. In the 1980s, he came to the conclusion that these emissions would trigger the expansion of vegetation throughout the planet, which would make life easier for animals. This finding is one of the key reasons why he assessed global warming as a “ticket to paradise lost.” He was the first researcher to formulate such a scientific position.
In the Western scientific world, Budyko's ideas were rejected for a long time. Although a number of scientists have documented the greening of the planet in satellite images, their work has been actively contested, and the idea of global greening was not generally accepted until a publication in Nature in 2016. Then a group of authors (the first among whom was a scientist from the People's Republic of China) for the first time managed to clearly show the enormous scale of the global greening of the Earth, and also established by calculations that 70 percent of it is due to CO2 emissions and another eight percent to the increase in global temperatures. Human agricultural activity was responsible for only 13 percent of this greening, they found.
However, the publication in Nature fell on unprepared ground: the Western scientific world has always been dominated by the idea that global warming is harmful to the biosphere. Since the history of the planet does not know cases when the biosphere would suffer from a sharp expansion of vegetation, which forms the basis of the food pyramid, several scientific groups tried to challenge the conclusions of the 2016 work. Doing this for the period from 1982 to 2000 was difficult because there are not many satellite data sets for it, and they all clearly show rapid greening. But whoever seeks will always find, so many attempts have been made to challenge the fact of this process for the period after 2000.
Some authors have written papers that global greening has stagnated since 2000, while others have even said that it has reversed. How could all this happen if satellites either detect an increased amount of green on the surface of the planet or not? The answer is not very complicated: if you divide the sets of satellite observations into groups, you will notice that they show slightly different green dynamics. This is not surprising: satellite hardware can vary in quality, and some have spent more time in orbit than others and have developed errors in their hardware. If the researcher did not take all data sets to average the obtained values, like the authors of the 2016 work, he could find among these sets those where greening was slowed down after 2000.
In addition, those challenging the findings of the 2016 paper tried to take into account not only the normalized difference vegetation index, but also the so-called extended vegetation index . There is a methodological difference in their calculations, since the normalized index is sensitive specifically to chlorophyll as a whole, while the expanded index is sensitive to the characteristics of the plant crown. Finally, they used another method: they took a shorter period (after 2000, or even after 2010) and interpreted the slowdown in greening on it as a general process of recent years.
All these conflicting assessments created serious information noise, behind which it turned out to be very difficult for people far from the scientific world to understand what was actually happening. Is the Earth becoming greener, as Budyko predicted and Chinese researchers stated? Or, on the contrary, are its vegetation and biosphere shrinking in the 21st century?
This question is not entirely theoretical: if Budyko is right, then the current expensive (trillion plus dollars per year) fight against global warming is at the same time a fight against greenery and the biosphere as a whole. On the contrary, if his opponents are right, it is the only salvation of humanity from the extremely painful consequences of the death of plants and the decline in the bioproductivity of the planet.
The authors of a new work in Global Ecology and Conservation, a group of scientists from China, tried to assess the situation on a more reliable methodological basis. As they point out, debating whether the planet is green or not in terms of color is less productive than focusing on a specific physical parameter: the leaf area index. To figure out leaf area and separate it from the overall green "signal" from ground vegetation, satellite data sets are used that are sensitive to the red and near-infrared parts of the spectrum.
An important feature of the work was that the authors did not select satellite data on the basis of “those that show maximum greening” or “those that show maximum plant death.” In contrast, they took into account all four global satellite data sets for the leaf area index.
At the same time, they found that in 2001-2020, all four sets show a statistically very significant increase in this index for the Earth as a whole. The leaf area on the planet expanded by 55.15 percent of the total area where there is vegetation, that is, sustainable greening was observed on the main part of the land. In 14.44 percent of the same zone, the opposite process to greening was observed—“browning,” that is, the retreat of plants. For the rest of the surface there was no pronounced trend in either direction.
An interesting picture emerged when the authors compared the relationships between other satellite-monitored factors and leaf area. Some details were obvious in advance: where vegetation retreated, soil moisture decreased. In those parts of the planet where CO2 concentrations were higher than usual, plants grew better. But there were also surprises: the rise in temperatures coincided in time with the greening not only in those parts of the planet where it is cool (China, Europe, including Eastern Europe), but also where it is hot. For example, this is exactly what happened in a number of regions of India.
Even more surprising was the fact that the relationship between precipitation (as well as solar radiation) and greening was very weak. That is, in regions where precipitation increased, greening was not noticeably more active, and where it decreased, greening was not noticeably weaker. It indirectly follows from this that the observed changes in precipitation have little effect on soil moisture over long periods.
Apparently, this is due to a long-known fact: where vegetation appears, soil moisture is largely regulated by it, and not by precipitation itself. This happens because when there is a lack of precipitation, plants living with high levels of CO2 in the air can severely narrow their stomata, the tiny holes in their leaves. After such a narrowing, their evaporation of water from the soil is minimized. As a result, quite a lot of moisture is retained in the ground, even if precipitation in a given year is slightly below normal.
One of the key objections of opponents of Budyko’s ideas and satellite data confirming them was that landscaping is most active in China and India – countries with rapidly developing agriculture and, accordingly, a large input of fertilizers. In this regard, there have even been claims in the scientific world that agriculture is “responsible for a third and perhaps more of the observed increase in leaf area” (that is, global greening). Of course, the not-so-good scientist went even further and began to say something that no scientists have ever claimed: that the bulk of global greening is supposedly due to the influence of agriculture.
Such objections have long raised big questions: for example, greening in China most affected deserts and semi-deserts, as well as Tibet, that is, areas where there is simply no agriculture or fertilization. 42 percent of landscaping in China generally occurred in forests – and only 32 percent in arable land. Yes, there is a program for planting trees in arid zones, but the fact is that no one waters or fertilizes these trees. Obviously, the survival of these plantings would be impossible if these zones were not already suitable for the rapid expansion of plants.
Finally, in a number of regions of the Earth, global greening has clearly manifested itself precisely during the period of reduction in agricultural areas there. For example, in Russia and Australia, since the 1980s, agriculture has left vast areas – but in both places the rate of greening is very high. And if in our country it can still be attributed not so much to the influence of CO2, but to its extremely cold climate, which softens with warming, then in Australia there is no question of this. If agriculture were the main driver of greening, the examples of these two countries – with a total area a couple of times larger than India and China combined – would be impossible to explain
Another major problem with this approach was that it did not take into account the beneficial effects of CO2 on agriculture. About a quarter of the yield of major crops in the United States is due to the influence of increased concentrations of carbon dioxide in the atmosphere – increased due to anthropogenic emissions. In other words, the growth of leaf area on arable land today is largely due to the same factors as the growth of leaf area in nature as a whole.
The authors of the new work approached the problem of “What is the real cause of global greening – anthropogenic carbon dioxide emissions or agriculture.” They compared changes in CO2 levels in a particular zone (satellites “see” carbon dioxide well) with other factors operating for certain areas of the earth’s surface. It turned out that CO2 explained the trend in the leaf area index for 75.63 percent of the earth's land area with vegetation. This is even slightly higher than the assessment of the authors of the 2016 work (70 percent).
The authors asked a very logical question: is it possible, with the current level of satellite observations, to understand whether global greening is now slowing down? This is an interesting question because it is traditionally assumed that global greening should be inhibited by increasing atmospheric water vapor deficiency.
“Water vapor pressure deficit,” although measured in pascals, is not an absolute concept, but a relative one. If the relative humidity of a cubic meter of air remains unchanged as the temperature rises, then the amount of water vapor in it will increase rapidly. However, in practice, in warmer parts of the world, the ratio of the actual amount of water vapor per cubic meter of air to the theoretically possible amount at a given temperature is lower than in colder parts. To understand what we are talking about, just look at the map below. In the Amazon, it is impossible for a person from Russia to breathe at first – the air is so humid. However, the formal deficit of water vapor there is stronger than in Russia, and incomparably stronger than in Greenland, Antarctica or Tibet (this is easy to see on the map below).
Thus, it is easy to notice that this concept does not reflect the potential suitability of a particular climatic zone for complex life. In Greenland and Antarctica, the air is extremely dry, so much so that without special measures it quickly damages the skin and mucous membranes, and the vegetation there is either weak or non-existent. That is, in places with low water vapor deficiency, suppression of green biomass is the norm. On the contrary, in places with a moderate deficit of water vapor pressure, the norm is the jungle of the Amazon and Congo. Only zones where this deficit is higher than 1.5-2.0 thousand pascals (that is, extremely high) are really unsuitable for vegetation.
Many climate scientists argue that as warming continues, water vapor deficits, already increasing in temperate latitudes, will become less moderate than in the Amazon and more extreme than in tropical deserts, leading to the death of vegetation and the end of global greening. And some of them are even sure that this has already happened. The authors of the new work set their sights on testing this thesis. If greening on a global scale is slowing down, it means that the lack of water vapor has really hit the plants.
Unfortunately, for most of the globe, current observations are not accurate enough: four sets of satellite images showed conflicting data on accelerating global warming for 60 percent of the land mass. Let us remember that in such works we are talking only about that part of the land where there are plants, that is, subtracting the Arctic deserts and the Sahara.
However, for 40 percent of the land mass, the acceleration of global warming was determined – and there it turned out to be generally positive. In other words, global greening accelerated slightly rather than slowed between 2001 and 2020, although the magnitude of this acceleration is not yet statistically significant. Interestingly, the rate of global greening is accelerating in the European plains (including parts of Russia), East Africa and India.
In Russia and Europe as a whole, in 2001-2020, the area of agricultural land did not increase (and specifically in our country it even fell). At the same time, in a number of regions of the North American plains and in China, global greening has slowed down somewhat. This is especially interesting because in China specifically, in 2001-2020, agriculture experienced a serious boom – yields of major crops increased. The thesis “greening is due to intensive agriculture” was once again not supported by specific data.
The researchers believe that the main reason why a number of previous studies have attempted to “undo” global greening was due to the use of controversial data sets. First of all, we are talking about the Advanced Very-High-Resolution Radiometer (AVHRR), a satellite observation system in five spectral bands. Two of them – about 600 and 900 nanometers – in theory reflect information from the normalized difference vegetation index. The authors of the new paper note that this project has long been aware of a significant accuracy problem caused by orbital drift of the satellites that supply this data.
Their work specifically eschews estimates of the normalized difference vegetation index and focuses instead on the leaf area index—derived from data from MODIS, a satellite observing system with much less problematic observational instruments that do not produce as much systematic error.
The following picture emerges: global greening is an “indisputable fact,” as the authors state in their work. On 55 percent of the earth's land that has vegetation, greening is taking place (most actively on the plains of Europe and India) and only on seven percent the vegetation is retreating. Moreover, carbon dioxide is responsible for three-quarters of greening. The effect of increasing its concentration is so great that the water vapor pressure deficit not only cannot stop greening, but does not even show the ability to stop the acceleration of global greening.
It should be said frankly: this scientific work describes the most important process taking place in the Earth's biosphere. Yes, she has limitations.
For example, leaf area grows more slowly than green biomass, as we have already written about. But still, green land plants make up more than two-thirds of the Earth's total biomass. If they show rapid expansion, the food supply for animals also grows quickly. And this, of course, is good for the biosphere as a whole.
Despite the importance of the topic, the media avoids it. The reason is simple: this is a very unpleasant topic for Western media. After the first works on global greening were published, special popular articles even appeared there, declaring that global greening in the long term is terrible.
Naturally, it was impossible to find convincing arguments that would show that the vegetation boom was “terrible” in the long term. I had to use completely unconvincing ones. Media workers don’t really like to put their signatures on bad-looking material: after that, the trust of readers decreases.
It is not surprising that the PRC, where there is no single party line on issues of global warming, can still afford scientific work on accelerating greening, while the United States can no longer do so much. In the Russian-speaking cultural space, the attitude towards anthropogenic carbon emissions is closer to the United States than to the PRC. After Budyko’s death, our scientific community basically repeats Western theses on the issue and informs society in the same spirit.
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