Projected forest position in Siberia and tundra area at year 3000 CE for different climate mitigation scenarios and under potential cooling back to 20th-century temperatures after peak temperatures have been reached. The area of tundra changes over time and can only partly recover when temperatures cool and forests recede (plots next of the maps show years 2000–3000 CE). Only tundra areas above the treeline (Walker et al., 2005) are considered. Map projection: Albers Equal Area. Graphic: Kruse and Herzschuh, 2022 / eLife
Projected forest position in Siberia and tundra area at year 3000 CE for different climate mitigation scenarios and under potential cooling back to 20th-century temperatures after peak temperatures have been reached. The area of tundra changes over time and can only partly recover when temperatures cool and forests recede (plots next of the maps show years 2000–3000 CE). Only tundra areas above the treeline (Walker et al., 2005) are considered. Map projection: Albers Equal Area. Graphic: Kruse and Herzschuh, 2022 / eLife

25 May 2022 (AWI) – Due to global warming, temperatures in the Arctic are climbing rapidly. As a result, the treeline for Siberian larch forests is steadily advancing to the north, gradually supplanting the broad expanses of tundra which are home to a unique mix of flora and fauna. Experts from the Alfred Wegener Institute have now prepared a computer simulation of how these woods could spread in the future, at the tundra’s expense. Their conclusion: only consistent climate protection measures will allow roughly 30 percent of the Siberian tundra to survive to mid-millennium. In all other, less favourable scenarios, the unique habitat is projected to disappear entirely. The study was just released in the journal eLife.

The climate crisis can especially be felt in the Arctic: in the High North, the average air temperature has risen by more than two degrees Celsius over the past 50 years – far more than anywhere else. And this trend will only continue. If ambitious greenhouse-gas reduction measures (Emissions Scenario RCP 2.6) are taken, the further warming of the Arctic through the end of the century could be limited to just below two degrees. According to model-based forecasts, if the emissions remain high (Scenario RCP 8.5), we could see a dramatic rise in the average summer temperatures in the Arctic – by up to 14 degrees Celsius over today’s norm by 2100.

“For the Arctic Ocean and the sea ice, the current and future warming will have serious consequences,” says Prof Ulrike Herzschuh, Head of the Polar Terrestrial Environmental Systems Division at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “But the environment on land will also change drastically. The broad expanses of tundra in Siberia and North America will be massively reduced, as the treeline, which is already slowly changing, rapidly advances northward in the near future. In the worst-case scenario, there will be virtually no tundra left by the middle of the millennium. In the course of our study, we simulated this process for the tundra in northeast Russia. The central question that concerned us was: which emissions path does humanity have to follow in order to preserve the tundra as a refuge for flora and fauna, as well its role for the cultures of indigenous peoples and their traditional ties to the environment?”

The tundra is home to a unique community of plants, roughly five percent of which are endemic, i.e., can only be found in the Arctic. Typical species include the mountain avens, Arctic poppy and prostrate shrubs like willows and birches, all of which have adapted to the harsh local conditions: brief summers and long, arduous winters. It also offers a home for rare species like reindeer, lemmings and insects like the Arctic bumblebee.

The trajectory of the simulated treeline position relative to its current position and the maximum at the shoreline versus its climate-analogue position for four regions in Siberia: Taimyr Peninsula, Buor Khaya, Kolyma, and Chukotka. The trajectory shows a migration lag of the treeline during the first centuries (each line segment represents 25 years, and the length of the arrowhead corresponds to the step length) until the simulated treeline is limited by climate. Forests expand their area further and infilling proceeds when climate conditions cool and even overshoot in the long run with cooling back to 20th-century temperatures. The diagonal is where climate and the treeline are in equilibrium; below the diagonal, tree migration lags climate; above the diagonal is ‘overshooting’ and reaching locations actual climate would allow. For each relative concentration pathway (RCP) scenario, two are presented, one for the scenario as-is and the second for the cooling; scenario RCP 2.6* warms only at half the rate of RCP 2.6. See also Appendix 3—figure 1 for the year when the trajectory passes the equilibrium. Graphic: Kruse and Herzschuh, 2022 / eLife
The trajectory of the simulated treeline position relative to its current position and the maximum at the shoreline versus its climate-analogue position for four regions in Siberia: Taimyr Peninsula, Buor Khaya, Kolyma, and Chukotka. The trajectory shows a migration lag of the treeline during the first centuries (each line segment represents 25 years, and the length of the arrowhead corresponds to the step length) until the simulated treeline is limited by climate. Forests expand their area further and infilling proceeds when climate conditions cool and even overshoot in the long run with cooling back to 20th-century temperatures. The diagonal is where climate and the treeline are in equilibrium; below the diagonal, tree migration lags climate; above the diagonal is ‘overshooting’ and reaching locations actual climate would allow. For each relative concentration pathway (RCP) scenario, two are presented, one for the scenario as-is and the second for the cooling; scenario RCP 2.6* warms only at half the rate of RCP 2.6. See also Appendix 3—figure 1 for the year when the trajectory passes the equilibrium. Graphic: Kruse and Herzschuh, 2022 / eLife

For their simulation, Ulrike Herzschuh and AWI modeller Dr Stefan Kruse employed the AWI vegetation model LAVESI. “What sets LAVESI apart is that it allows us to display the entire treeline at the level of individual trees,” Kruse explains. “The model portrays the entire lifecycle of Siberian larches in the transition zone to the tundra – from seed production and distribution, to germination, to fully grown trees. In this way, we can very realistically depict the advancing treeline in a warming climate.”

The findings speak for themselves: the larch forests could spread northward at a rate of up to 30 kilometres per decade. The tundra expanses, which can’t shift to colder regions due to the adjacent Arctic Ocean, would increasingly dwindle. Since trees aren’t mobile and each one’s seeds can only reach a limited distribution radius, initially the vegetation would significantly lag behind the warming, but then catch up to it again. In the majority of scenarios, by mid-millennium less than six percent of today’s tundra would remain; saving roughly 30 percent would only be possible with the aid of ambitious greenhouse-gas reduction measures. Otherwise, Siberia’s once 4,000-kilometre-long, unbroken tundra belt would shrink to two patches, 2,500 kilometres apart, on the Taimyr Peninsula to the west and Chukotka Peninsula to the east. Interestingly, even if the atmosphere cooled again in the course of the millennium, the forests would not completely release the former tundra areas.

“At this point, it’s a matter of life and death for the Siberian tundra,” says Eva Klebelsberg, Project Manager Protected Areas and Climate Change / Russian Arctic at the WWF Germany, with regard to the study. “Larger areas can only be saved with very ambitious climate protection targets. And even then, in the best case there will ultimately be two discrete refuges, with smaller flora and fauna populations that are highly vulnerable to disrupting influences. That’s why it’s important that we intensify and expand protective measures and protected areas in these regions, so as to preserve refuges for the tundra’s unparalleled biodiversity,” adds Klebelsberg, who, in collaboration with the Alfred Wegener Institute, is an advocate for the establishment of protected areas. “After all, one thing is clear: if we continue with business as usual, this ecosystem will gradually disappear.”

Siberian tundra could virtually disappear by mid-millennium

Treeline migration trajectories in four regions of Siberia, Taimyr Peninsula, Buor Khaya, Kolyma, and Chukotka. Numbers are the first year when the simulated treeline position is equal to or farther north than the modern climate-analogue position. Colour of line segments ranges from yellow for year 2000 to blue in 300 CE. Graphic: Kruse and Herzschuh, 2022 / eLife
Treeline migration trajectories in four regions of Siberia, Taimyr Peninsula, Buor Khaya, Kolyma, and Chukotka. Numbers are the first year when the simulated treeline position is equal to or farther north than the modern climate-analogue position. Colour of line segments ranges from yellow for year 2000 to blue in 300 CE. Graphic: Kruse and Herzschuh, 2022 / eLife

Regional opportunities for tundra conservation in the next 1000 years

ABSTRACT: The biodiversity of tundra areas in northern high latitudes is threatened by invasion of forests under global warming. However, poorly understood nonlinear responses of the treeline ecotone mean the timing and extent of tundra losses are unclear, but policymakers need such information to optimize conservation efforts. Our individual-based model LAVESI, developed for the Siberian tundra-taiga ecotone, can help improve our understanding. Consequently, we simulated treeline migration trajectories until the end of the millennium, causing a loss of tundra area when advancing north. Our simulations reveal that the treeline follows climate warming with a severe, century-long time lag, which is overcompensated by infilling of stands in the long run even when temperatures cool again. Our simulations reveal that only under ambitious mitigation strategies (relative concentration pathway 2.6) will ∼30% of original tundra areas remain in the north but separated into two disjunct refugia.

Regional opportunities for tundra conservation in the next 1000 years