Climate tipping points in the boreal forest region

By: Roger Olsson

The forest of Interior Alaska is changing, rapidly and thoroughly. Over the course of a few decades, an ecosystem shift, from coniferous to deciduous forests, is taking place over an area the size of Spain. The driving force is global warming, and the transformation will most likely contribute to further warming.

The development in Alaska is highlighted in an IPCC report on the Cryosphere (the cold areas in the northern hemisphere) published last September1. The report, and the new scientific findings underlying it, shows that climate-driven shifts in northern ecosystems not only may happen faster and at much lower levels of warming than was previously expected. In fact, they are already happening.

It’s an understatement to claim that much is at stake. Permafrost areas in Northern America and Eurasia, mostly Arctic tundra, represent a frozen carbon store of about 1,500 Gt (Gigatonnes). The coniferous forest belt of the northern hemisphere – the boreal forest – holds another 500 or 600 Gt of carbon, for the most part in the soil. Since much boreal forest grows on permafrost the figures are partly overlapping, but a reasonable estimation is that these two biomes together contain more than twice the amount of carbon of the entire atmosphere.

To halt global warming at 1.5°C, future emissions of carbon dioxide must not exceed 500 Gt globally. Thawing permafrost may, even by conservative estimates, claim 20 per cent of that space. Climate-driven wildfires, pest outbreaks and vegetation changes in the boreal forest could add another 100 Gt, meaning that less than half of the global carbon budget would remain for anthropogenic emissions2.

The present development in Alaska’s forests suggests that this is where we are heading.

The annual mean temperature in Interior Alaska has increased by 1.4°C over the last century, while precipitation has decreased by 11 per cent. Warmer and drier summer climate has created more favourable conditions for wildfires in coniferous forest, where flammable debris has accumulated on the forest floor for centuries. Thus, fire frequency has increased dramatically. As a consequence, around 1990 the forest ecosystem switched from being a carbon sink to a net carbon source.

Two-thirds of all forest stands in Interior Alaska used to be black or white spruce. Twenty years from now, when the transformation phase is completed, the landscape will be dominated to the same extent by deciduous forests, mostly aspen – a normal succession stage after fire disturbances in boreal forest. These forests are far less likely to burn than old-growth coniferous forests, which means that fire frequency will decrease. However, this does not mean that the forests of Alaska will return to their former state of a net carbon sink. On the contrary, the change may have a positive feedback effect on global warming. One reason for this is higher soil temperatures, speeding up the decomposition of organic matter.

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Boreal forest and tundra – close to tipping?

Permafrost areas and boreal forest are two of the so-called tipping points identified by the IPCC. Tipping points are “climate bombs” with the potential to reinforce global warming through powerful positive-feedback loops. Crossing one or several tipping points may cause uncontrollable further warming.

When the concept of tipping points was introduced twenty years ago general understanding was that the risk of crossing critical thresholds would occur at around 5°C of warming. New facts summarized in the IPCC’s Special Reports on Global Warming of +1.5°C (part of which is presented in this article) now suggest that tipping points could be exceeded even between 1 and 2°C of warming.

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There are studies indicating that the transformation now seen in Alaska is also underway in other parts of the boreal region, for example in Russia3 and north-eastern China. Obviously, climate is changing in a similar manner all over the region, and so is the frequency and extent of wildfires. Across the North American boreal region the total burned area grew by a factor of 2.5 between the 1960s4 and 1990s. In fact, there are studies indicating that fire frequency in the boreal region is higher now than anytime over the last 10,000 years. The IPCC estimates that wildfire is projected to increase for the rest of this century across most tundra and boreal regions. As a result of this and other changes, the boreal forest may have started transitioning from a carbon sink to a source on a global scale. Some regions such as western Canada and Siberia may already be emitting more carbon than they capture5.

Changes in fire frequency and other disturbances are not the only cause of large-scale shifts in northern forest and tundra biomes. Climate change also affects ecosystems directly, since temperature and precipitation are important environmental factors for trees and other vegetation. Drought-induced mortality has already been reported in several boreal regions, and is predicted to increase regionally6.

Furthermore, winter snow cover is of crucial importance to boreal ecosystems, and it has changed dramatically. Since 1981 the area covered by snow in June has decreased by more than 10 per cent per decade in the Arctic region (north of 60° N).

One may think that boreal forests as well as other ecosystems would simply migrate northwards as temperatures rise, but numerous model studies show that the reactions will be more complex, and far more threatening from a human perspective. An obvious reason is that climate zones are shifting northwards an order of magnitude faster than the ability of trees to migrate7.

The boreal forest will expand at its northern edge, but is not projected to colonize present tundra on a larger scale, at least not during this century. Nevertheless, far-reaching changes are underway. Woody shrubs are already projected to expand to cover 24–52 per cent of arctic tundra by 2050, resulting in an overall positive feedback effect on climate, likely to cause greater warming than has previously been predicted. While denser vegetation may decrease carbon emissions from permafrost thaw, this may be counteracted by changes in albedo (heat reflection) and increasing amounts of water vapour in the atmosphere. In addition, denser vegetation and a warmer climate will facilitate the expansion of fire into tundra, causing large reductions in soil carbon stock.

In the southern part of the boreal forest zone, climate change may cause closed forest to be replaced by open woodland or scrubland. It is documented that such changes can happen rapidly in response to changes in climate. Since the new vegetation types have lower biomass than those they replace, large amounts of carbon will be released into the atmosphere during such a transition. At the same time the total boreal forest area, and thus its capacity as a carbon sink, will be reduced. This carbon loss is likely to offset any carbon gains from projected expansion of boreal forest into tundra in the north.

The rapid and profound transition of boreal forests outlined in the IPCC report and underlying scientific papers will of course not only affect future climate. It will have severe consequences for a number of vital ecosystem services; it will jeopardize the survival of many indigenous and local communities and it will pose a great threat to biodiversity, not only in the Arctic.

Roger Olsson

  1. The Ocean and Cryosphere in a Changing Climate. IPCC 2019
  2. Lenton, t M et al 2019: Climate tipping points – to risky to bet against. Nature 575:592-595
  3. Shuman, J. K. et al., 2015: Forest forecasting with vegetation models across Russia. Canadian Journal of 47 Forest Research, 45 (2), 175-184, doi:10.1139/cjfr-2014-0138.
  4. Boreal Forest and Climate Change p 17-
  5. Gauthier et al., 2015
  6. Gauthier et al., 2015
  7. Gauthier et al., 2015

Sources

The article is based on the IPCC report “The Ocean and Cryosphere in a Changing Climate” (2019) and scientific papers cited therein. Other sources are

Lenton, T. M. et al., 2019: Climate tipping points – too risky to bet against. Nature 575:592-59

Olsson 2009: Boreal Forest and Climate Change. AirClim report 2009

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