Taiga
Climate change

During the last quarter of the twentieth century, the zone of latitude occupied by the boreal forest experienced some of the greatest temperature increases on Earth. Winter temperatures have increased more than summer temperatures. In summer, the daily low temperature has increased more than the daily high temperature. The number of days with extremely cold temperatures (e.g., −20 to −40 °C; −4 to −40 °F) has decreased irregularly but systematically in nearly all the boreal region, allowing better survival for tree-damaging insects. In Fairbanks, Alaska, the length of the frost-free season has increased from 60 to 90 days in the early twentieth century to about 120 days a century later.

It has been hypothesized that the boreal environments have only a few states which are stable in the long term - a treeless tundra/steppe, a forest with >75% tree cover and an open woodland with ~20% and ~45% tree cover. Thus, continued climate change would be able to force at least some of the presently existing taiga forests into one of the two woodland states or even into a treeless steppe - but it could also shift tundra areas into woodland or forest states as they warm and become more suitable for tree growth.

In keeping with this hypothesis, several studies published in the early 2010s found that there was already a substantial drought-induced tree loss in the western Canadian boreal forests since the 1960s: although this trend was weak or even non-existent in the eastern forests, it was particularly pronounced in the western coniferous forests. However, in 2016, a study found no overall Canadian boreal forest trend between 1950 and 2012: while it also found improved growth in some southern boreal forests and dampened growth in the north (contrary to what the hypothesis would suggest), those patterns were statistically weak.

A 2018 Landsat reanalysis confirmed that there was a drying trend and a loss of forest in western Canadian forests and some greening in the wetter east, but it had also concluded that most of the forest loss attributed to climate change in the earlier studies had instead constituted a delayed response to anthropogenic disturbance. Subsequent research found that even in the forests where biomass trends did not change, there was a substantial shift towards the deciduous broad-leaved trees with higher drought tolerance over the past 65 years, and another Landsat analysis of 100,000 undisturbed sites found that the areas with low tree cover became greener in response to warming, but tree mortality (browning) became the dominant response as the proportion of existing tree cover increased.

While the majority of studies on boreal forest transitions have been done in Canada, similar trends have been detected in the other countries. Summer warming has been shown to increase water stress and reduce tree growth in dry areas of the southern boreal forest in central Alaska and portions of far eastern Russia. In Siberia, the taiga is converting from predominantly needle-shedding larch trees to evergreen conifers in response to a warming climate. This is likely to further accelerate warming, as the evergreen trees will absorb more of the sun's rays. Given the vast size of the area, such a change has the potential to affect areas well outside of the region. In much of the boreal forest in Alaska, the growth of white spruce trees are stunted by unusually warm summers, while trees on some of the coldest fringes of the forest are experiencing faster growth than previously. Lack of moisture in the warmer summers are also stressing the birch trees of central Alaska.

In addition to these observations, there has also been work on projecting future forest trends. A 2018 study of the seven tree species dominant in the Eastern Canadian forests found that while 2 °C warming alone increases their growth by around 13% on average, water availability is much more important than temperature and further warming of up to 4 °C would result in substantial declines unless matched by increases in precipitation. A 2019 study suggested that the forest plots commonly used to evaluate boreal forest response to climate change tend to have less evolutionary competition between trees than the typical forest, and that with strong competition, there was little net growth in response to warming.

Climatic change only stimulated growth for trees under weak competition in central boreal forests. A 2021 paper had confirmed that the boreal forests are much more strongly affected by climate change than the other forest types in Canada and projected that most of the eastern Canadian boreal forests would reach a tipping point around 2080 under the RCP 8.5 scenario which represents the largest potential increase in anthropogenic emissions. Another 2021 study projected that under the "moderate" SSP2-4.5 scenario, boreal forests would experience a 15% worldwide increase in biomass by the end of the century, but this would be more than offset by the 41% biomass decline in the tropics.

In 2022, the results of a 5-year warming experiment in North America had shown that the juveniles of tree species which currently dominate the southern margins of the boreal forests fare the worst in response to even 1.5 °C or +3.1 °C of warming and the associated reductions in precipitation. While the temperate species which would benefit from such conditions are also present in the southern boreal forests, they are both rare and have slower growth rates.

A 2022 assessment of tipping points in the climate system designated two inter-related tipping points associated with climate change - the die-off of taiga at its southern edge and the area's consequent reversion to grassland (similar to the Amazon rainforest dieback) and the opposite process to the north, where the rapid warming of the adjacent tundra areas converts them to taiga. While both of these processes can already be observed today, the assessment believes that they would likely not become unstoppable (and thus meet the definition of a tipping point) until global warming of around 4 °C. However, the certainty level is still limited and it is possible that 1.5 °C would be sufficient for either tipping point; on the other hand, the southern die-off may not be inevitable until 5 °C, while the replacement of tundra with taiga may require 7.2 °C.

Once the "right" level of warming is met, either process would take at least 40–50 years to finish, and is more likely to unfold over a century or more. While the southern die-off would involve the loss of around 52 billion tons of carbon, the net result is cooling of around 0.18 °C globally and between 0.5 °C to 2 °C regionally. Likewise, boreal forest expansion into tundra has a net global warming effect of around 0.14 °C globally and 0.5 °C to 1 °C regionally, even though new forest growth captures around 6 billion tons of carbon. In both cases, this is due to the snow-covered ground having a much greater albedo than the forests. According to a later study, disappearing of boreal forests can also increase warming despite the effect on albedo, while the conclusion about cooling from deforestation in these areas made by previous studies results from the failure of models to properly capture the effects of evapotranspiration.

Primary boreal forests hold 1,042 billion tonnes of carbon, more than currently found in the atmosphere, 2 times more than all human caused GHG emissions since the year 1870. In a warmer climate their ability to store carbon will be reduced.
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