New research indicates that computer models currently used to simulate how Earth’s climate will change in the future underestimate the impact of wildfires and drying climate on the world’s northernmost forests, which constitute the largest forest biome on the planet. This is an important understanding because these northern forests absorb a significant amount of carbon dioxide from the Earth.
The discovery, obtained by studying 30 years of the world’s forests using satellite imagery data from NASA, suggests that forests will not be able to sequester as much carbon as expected, making reduction efforts all the more urgent carbon emissions.
“Fires intensify and when forests burn, carbon is released into the atmosphere,” says Mark Friedl, an environmental scientist at Boston University, lead author of the study published in Nature’s climate change. “But we’re also seeing longer growing seasons, warmer temperatures, which pulls carbon out of the atmosphere. [and into plants]. More CO2 in the atmosphere acts as a fertilizer, increasing the growth of trees and plants – so scientifically there has been this big question out there: what is happening on a global scale in the forests of the Earth? Will they continue to absorb as much carbon as they do today? “
Today’s forests capture around 30 percent of all human-made CO2 emissions, which Friedl calls a “huge buffer on anthropogenic climate change.” The new study, however, reveals that scientists have so far underestimated the impact that fires and other disturbances – such as timber harvesting – have on the forests of the Earth’s north and, at the same time, overestimated the impact. ‘growth enhancing effect of global warming and increase in atmospheric CO2 levels.
“Current models of Earth systems appear to distort much of the global biosphere. These models simulate the atmosphere, oceans and biosphere, and our results suggest [the model-based simulation of northern forests] Says Friedl, professor of Earth and Environment at the BU College of Arts & Sciences and Acting Director of the BU’s Remote Sensing Center. He is an expert in the use of satellite imagery data to monitor Earth’s ecosystems on a global scale.
“It is not enough for a forest to absorb and store carbon in its wood and soils. For this to be a real benefit, the forest must remain intact – a growing challenge in a warmer climate more prone to fires”, says Jonathan Wang, lead author of the article. “The Far North is home to vast and dense carbon stores that are very sensitive to climate change, and it will take a lot of monitoring and effort to ensure that these forests and their carbon stores remain intact.”
Working on his doctorate in Friedl’s lab, Wang researched new ways to exploit data collected under the long-standing Landsat program, a joint NASA / US Geological Survey mission that carried out in-depth analysis of the Earth’s surface. from satellites for decades, to understand how Earth’s forests are changing. Wang says new computational and machine learning techniques for combining large remote sensing datasets have become much more advanced, “allowing even the most remote ecosystems to be monitored in unprecedented detail.”
He developed a method to get richer information from 30 years of Landsat data by comparing it with more recent measurements from NASA’s ICESat mission, a satellite carrying laser imaging technology, called LiDAR, which can detect the height of vegetation in a forest. Landsat, on the other hand, primarily detects forest cover, but not tree height.
Comparing the more recent LiDAR measurements with imagery data collected from Landsat over the same time period, the team then worked backwards to calculate the height and density of vegetation over the past three decades. They could then determine how the biomass in Earth’s northern forests has changed over time – revealing that forests have lost more biomass than expected due to increasingly frequent and widespread forest fires.
Specifically, Friedl says, forests are losing conifers, the iconic trees of the forests of the Earth’s north, and for good reason. “Fires come and burn, and then the more opportunistic types of species grow back first – like hardwoods – which are then replaced by conifers like black spruce,” he says. “But over the past 30 years, which is not a long time in the context of climate change, we have seen fires ravaging more forests, and we are seeing hardwoods stay longer instead of being replaced by conifers. . “
Conifers are better suited to cold climates than deciduous trees, which could potentially contribute to the decrease in overall forest biomass.
“An often-used argument against climate action is the supposed benefits that ecosystems and communities in the remote north will enjoy from increased heat,” Wang says. He hopes the study’s discovery will help people understand that the global climate crisis has serious problems for the High North as well. “It may be green, in a sense,” he says, “but in reality the climate-induced increase in forest fires negates much of the potential benefits of a warming and greening north.” .
Wang and Friedl’s findings shed light on a question that would have been difficult to answer without the help of NASA’s “eyes in the sky”.
“Fire regimes change due to climate, and many areas of the world’s forests are in uninhabited areas where the effects of intense fires are not easily noticeable,” says Friedl. “When large plots of real estate in places like California go up in flames, it gets our attention. But the forests of the north, which hold some of the largest carbon stocks in the world, are more affected by fires than we previously thought. “