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The boreal climate. Chapter 7 in Vegetation, Water, Humans and the Climate: a New Perspective on an Interactive System.

The boreal ecosystem encircles the Earth above about 48° N, covering Alaska, Canada, and Eurasia. It is second in areal extent only to the world’s tropical forests and occupies about 21% of the Earth’s forested land surface (Whittaker and Likins 1975). Nutrient cycling rates are relatively low in the cold wet boreal soils. Whittaker and Likins (1975) estimate the annual net primary productivity of the boreal forest at 800 g C m–2 y–1 and its tundra at 140 g C m–2 y–1, in contrast to tropical forests averaging 2 200 g C m–2 y–1 and temperate forests at 1 250 g Cm–2 y–1. However, the relatively low nutrient cycling rates at high latitudes result in relatively high longterm boreal carbon storage rates averaging roughly 30 to 50 g C m–2 y–1 (Harden et al. 1992), a result of relatively high root turnover from trees, shrubs and mosses with relatively low decomposition rates. Over the past few thousand years, these below-ground storage processes have created a large and potentially mobile reservoir of carbon in the peats and permafrost of the boreal ecosystem. Currently, the boreal ecosystem is estimated to contain approximately 13% of the Earth’s carbon, stored in the form of above-ground biomass and 43% of the Earth’s carbon stored below-ground in its soils (Schlesinger 1991). Meridional gradients in atmospheric CO2 concentrations suggest that forests above 40° N sequester as much as 1 to 2 gigatons of carbon annually (Denning et al. 1995; Randerson et al. 1997), or nearly 15 to 30% of that injected into the atmosphere each year through fossil fuel combustion and deforestation. Given the enormous areal extent of the ecosystem, roughly 20 Mkm2 (Sellers et al. 1996b; Fig. A.45), shifts in carbon flux of as little as 50 g C m–2 y–1 can contribute or remove one gigaton of carbon annually from the atmosphere. The size of the boreal forest, its sensitivity to relatively small climatic variations, its influence on global climate and the global carbon cycle, therefore, make it critically important to better understand and represent boreal ecosystem processes correctly in global models.

In Sect. A.7.1 we will first describe the global boreal ecosystem, its landscape structure and composition and the extant factors that have shaped this landscape; its palaeo- and modern climate, and its fire disturbance history. We will pay particular attention to the effects of high-latitude climate change on snow extent and depth.

In Sect. A.7.2 we will review research relevant to energy dissipation and transport between the boreal land surface and atmospheric boundary layer. We will discuss how the boreal landscape, its albedo and biophysical control on the surface energy and water budget affects atmospheric circulation in the short term and climate at longer time scales.

In Sect. A.7.3 we will consider biogeochemical cycling by the boreal landscape, focusing on carbon uptake and release for both carbon dioxide and methane; how the cycling rates are affected by landcover type, climate, soils and surface hydrology.

Finally, in Sect. A.7.4 we will examine the projected impacts of future climate change on the land surface and potential feedbacks, including the effects of variation in snow cover extent and duration, carbon sequestration and release by the surface to the atmosphere and the effects of land-cover change.

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Hall, F.G., A.K. Betts, S. Frolking, R. Brown, J. M. Chen, W. Chen, S. Halldin, D. P. Lettenmaier, and J. Schafer, (2004): The boreal climate. Chapter 7 in Vegetation, Water, Humans and the Climate: a New Perspective on an Interactive System, A Synthesis of the IGBP Core Project, Biospheric Aspects of the Hydrological Cycle, Eds, Kabat et al., Springer, Berlin, Heidelberg, New York, pp. 93-114. ISSN 1619-2435, ISBN 3-540-42400-8.