[LUM#21] Heat Waves in Cold Forests
As massive wildfires have ravaged millions
of hectares of boreal forests in recent years, scientists are studying
how these ecosystems are changing in response to global warming. Given that
changes in this massive natural carbon sink will have repercussions
on the Earth’s climate system.
Large wildfires are nothing new. For thousands of years, these fires have played essential roles in boreal forests, promoting tree regeneration and the nutrient cycle. But over the past decade, their scale and frequency have increased. In Canada, several million hectares of boreal forest burned in the Northwest Territories in 2014, and then in British Columbia in 2017 and 2018. In 2023, the country set a new record with more than 18 million hectares going up in smoke. Record-breaking fires are also ravaging the boreal forests of Siberia. During the summer of 2021, more than 18 million hectares of taiga burned (Cold Forests Are Burning, July 22, 2021, University of Montpellier). These mega-fires are affecting the health of boreal forests throughout the Northern Hemisphere and increasing carbon dioxide emissions into the atmosphere. Scientists are seeking to predict how these boreal ecosystems will evolve under the effects of global warming.
Search the terrestrial archives
“Cold forests account for one-third of the world’s forests. The future of this enormous carbon sink—stored primarily in peatlands that are thousands of years old—will have repercussions on the planet’s climate balance,” notes Adam Ahmed Ali of the Montpellier Institute of Evolutionary Sciences (ISEM)1. The paleoecologist has joined the international Cold Forests network, based at the University of Quebec in Abitibi-Témiscamingue. Paleoecology allows us to examine Earth’s archives to see how, over millennia, the climate has influenced these ecosystems. By examining sediments at the bottom of the great lakes of boreal Quebec, Adam Ahmed Ali and his team measured the amount of charcoal for each climatic period of the Holocene. And his results —published in the journal PNAS in 2012—are counterintuitive.
“One might have expected to find large fires, especially during the Holocene climatic optimum 6,000 years ago, when the climate was warmer and drier than it is today (+2 °C). However, they are instead concentrated during the ‘cold and wet’ period of the Holocene, that is, around 4,000 B.C., during the Neoglacial,” explains the researcher. This pattern could be explained by recurring episodes of exceptional spring and summer droughts around the year 1,000, during the Medieval Warm Period. An increase in the occurrence of large forest fires during this climatic warming was recorded in various parts of Canada’s boreal forest. “These data show that the processes controlling fire regimes, particularly large wildfires, are complex. Does the year 2023 foreshadow conditions similar to those recorded during the Medieval Warm Period?”
The chicken-and-egg paradox
Many uncertainties remain before such a scenario can be confirmed. First, because the rate of warming is unprecedented, and “an average increase of 3 to 4 °C in annual temperatures, as suggested by some predictions, has never been recorded in Holocene sedimentary archives, ” explains Adam Ahmed Ali. Furthermore, changes in precipitation are uncertain (Climate has determined the fire cycle, and humans have influenced the frequency of fires). Climate models are unable to predict whether boreal regions will receive more or less precipitation than they do today. More broadly, climatologists still have more questions than answers about the future climate of the Anthropocene, such as the frequency and magnitude of extreme events, or changes in ocean currents, which are powerful regulators of the climate.
Another interesting factor to consider, according to the researcher, is the increase in flammable conifers such as black spruce (Picea mariana) and jack pine (Pinus banksiana) during the cold period of the Neoglacial (A 8,500-year history of climate-fire-vegetation interactions in the black spruce and moss bioclimatic zone of the Eastern Maritimes). Did the climate change the vegetation, which in turn changed the fire regime, or did the fire regime change the vegetation? “The famous chicken-and-egg paradox, ” summarizes Adam Ahmed Ali. The nature of tree species is indeed a crucial factor in understanding the evolution of fires. In particular, the proportion of broadleaf trees in the boreal forest, as they are less susceptible to fire. However, global warming will allow deciduous trees to migrate from temperate forests to the boreal forest and should therefore help reduce the number of wildfires. While these factors highlight the difficulty of predicting their long-term scale, scientists are unanimous in saying that large wildfires will become more frequent in the coming decades.
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- Isem (CNRS, UM, IRD)
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