One of the few positive consequences of global warming was, the scientists said, that it would cause forests to colonize the coldest parts of the planet.
However, a study now shows how pollution is dulling the atmosphere in Arctic regions, blocking the sun's rays and slowing photosynthesis.
The consequence is that thousands of square kilometers of boreal forest have already died and the darkening of the sky prevents the reinforcements from growing.
The taiga is a vast arboreal area, mainly conifers, that borders the Arctic Circle.
The name, of Russian origin, originally referred to the boreal forests of Siberia, which exceed the Amazon forest in extension.
But taiga are also the trees of the Nordic countries, northern Canada and Alaska.
Conditions are so harsh there that they survive dormant most of the year, growing only during the summer.
With rising temperatures due to climate change, the widening of this time frame was expected to accelerate forest development and its expansion to new lands.
But the taiga is regressing since the 70's. Why?
Scientists point to human-generated pollution both
and from much further afield.
They have verified this by analyzing the thickness of the rings and the density of the wood of hundreds of dead trees and a few dozen living ones.
They were all alarms or Siberian spruces that grew in a 150-kilometer radius around Norilsk.
Located in the center of Siberia, this city appears in Wikipedia for being the city of more than 100,000 inhabitants that there is more to the north of the planet.
It is also one of the largest mining complexes in the world, where metals such as nickel, copper, platinum and most of the palladium used in the world are extracted.
Its extraction and processing emitted 1.8 million tons of pollutants in 2018 alone, 98% in the form of sulfur dioxide.
The Norilsk mining complex in Siberia released 1.8 million tonnes of pollutants into the atmosphere in 2018 alone, mostly sulfur dioxide.
The results of this local work, but with global implications, show high concentrations of these metals and sulfur in the wood of dead trees.
Also the soils appear contaminated, which complicates the outbreak of new ones.
As published in the scientific journal
, mortality is higher the closer to the mines, up to 100%.
But the emissions have an even more profound and far-reaching effect: They are darkening the atmosphere.
The greater presence of aerosols generates a mist that traps or reflects a good part of the solar radiation.
In addition, the sulfur dioxide particles function as condensation nuclei, generating more and more clouds.
The result is the short circuit of photosynthesis.
"In the article we show that the dissociation between the growth of trees and the increase in temperature is due at least in part to atmospheric pollution," says in an email the ecologist from the Siberian Federal University and co-author of the study Alexander Kirdyanov .
Norilsk is only a small part of the problem.
In reality, the Arctic has become a kind of reservoir for pollutants and aerosols emitted not only by facilities in the Arctic region, but also from lower latitudes in North America, Europe and Asia ”, he adds.
Once there, the arctic wind patterns, almost circular, spread the pollution.
The extremely high mortality observed around Norilsk is a local phenomenon, recalls Kirdyanov.
"However, if you ask about the inability of the trees to follow the ongoing temperature rise, this phenomenon has been observed in many regions throughout the boreal zone," he clarifies.
“As air pollution accumulated in the Arctic due to large-scale [atmospheric] circulation patterns, we extended our research beyond the direct effects of the Norilsk industrial sector and saw that across the high northern latitudes the rest of trees are also suffering ”, says in a note from the University of Cambridge, Professor Ulf Büntgen, co-author of the study.
Büntgen knows that the Amazon rainforest receives greater media and public attention, "the ecological and climatic role of the boreal forest, the largest biome on the planet, being much less known."
But its accelerated deterioration could have consequences as great as the loss of the Amazon.
"We hope that our work contributes to raising international awareness about the harmful consequences of anthropogenic emissions in the Arctic and that their consequences can have global dimensions," he opines in an email.
The forestry researcher at the Pablo de Olavide University Raúl Sánchez, not related to the study, recalls that it was expected that “the trees in this region would grow with warming, but the increase in emissions reduces radiation, photosynthesis and, therefore , growth".
In addition, he comments that the Arctic haze "coincides with the few weeks in which they could grow and, to all this, we must add the fires."
The medium-term consequence will be the feedback of global warming: “It will alter the entire carbon cycle, the sequestration of CO₂ [carbon dioxide] that was expected with the expansion of the forest will not occur, but the release of CO₂ will occur with death. of the taiga ”.
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