Satellite images show the Arctic has become greener as temperatures in the far northern region rise three times faster than the global average.
Some theories suggest that this “greening of the Arctic” will help counter climate change. The idea is that since plants absorb carbon dioxide as they grow, rising temperatures will mean that arctic vegetation will absorb more carbon dioxide from the atmosphere, thereby reducing warming greenhouse gases. the planet.
But is this really the case?
I am a biologist who focuses on the response of ecosystems to climate change, including tundra ecosystems. For the past five years, my colleagues, students, and I have been tracking vegetation changes in remote Arctic locations to find out.
Brave bears to collect evidence in the tundra
The arctic tundra is a vast, mostly treeless region that spans the northernmost regions of North America and Eurasia. A few meters below its surface, much of the ground is frozen permafrost, but the top layer blooms with grasses and low shrubs during the short summer months.
Over the past decade, satellite studies have tracked changes in Arctic greening by measuring visible and near-infrared light reflected from vegetation. Healthy green vegetation absorbs visible light but reflects near infrared light. Scientists can use this data to estimate plant growth over large areas.
But satellites do not measure the uptake of carbon dioxide by plants.
Until recently, field studies to verify how much carbon dioxide Arctic plants were absorbing were scarce, preventing scientists from testing the hypothesis that earlier snowmelt and its impact on plants was helping control carbon dioxide in the atmosphere.
For our study, scientists braved bear territory and cold summer nights to collect in-depth carbon dioxide measurements near plants and soil in 11 arctic tundra ecosystems, including Alaska, Canada , Siberia and Greenland. We focus on less studied arctic areas, located on continuous permafrost.
Early growth, but a slowdown at the end of the season
Arctic plants currently only have about three months in which to grow and reproduce before temperatures get too cold.
When we started this study, we wanted to know what effect an early start to the growing season had on the total amount of carbon dioxide absorbed by vegetation each summer. The results surprised us: although greening was evident, overall carbon dioxide uptake did not increase significantly or only recorded minor increases.
When we take a closer look and compare the week-to-week changes, we find out why. While the previous thaw had boosted plant productivity in June, that productivity began to decline in July, normally their peak season for photosynthesis. In August, productivity was well below normal.
Bushes, sedges and other dominant plants in arctic wetlands were no longer sequestering carbon by the end of the season. It was like waking up earlier in the morning and being ready to go to sleep earlier at night.
There are still many unanswered questions about the Arctic
We still have many questions, including why plants react this way and whether the widely used index for plant growth based on changes in visible and infrared light, called NDVI, is definitely associated with increased carbon dioxide uptake. carbon. Some Arctic ecosystems showed strong correlations between NDVI and carbon dioxide uptake, while others did not. We found no evidence that plants were affected by late season water limitations.
If tundra ecosystems are unable to continue to absorb carbon dioxide later in the season, the predicted increase in carbon-sequestering plants may not materialize.
And there is another problem. Generally, tundra plants store more carbon through photosynthesis than is released by tundra, making them a large carbon sink. Long, cold winters slow the decay of plants and lock them in frozen ground. However, when permafrost containing this and other organic matter thaws, it releases more greenhouse gases into the atmosphere.
Local impact goes beyond carbon
It’s not just about plants and weather. Changes in vegetation can have far-reaching effects on other components of ecosystems, including animals and humans.
The Intergovernmental Panel on Climate Change, the UN body tasked with assessing the science related to climate change, has found that changes in snow cover have already affected food and water security. Many local Indigenous communities depend on hunting, trapping and fishing, and early vegetation development can affect the delicate balance of complex Arctic systems.
If the greening of the Arctic only changes the seasons and does not increase the global level of carbon dioxide as previously believed, it could also mean that the models currently used to assess and predict the global impact of the change climate lack important information. The result could be that a process we thought would slow or mitigate climate change is not actually working as expected.
This article was written by Donatella Zona, associate professor of biology at San Diego State University. It is republished from The Conversation under a Creative Commons license. Articles in English