Boreal forests, aerosols and the impacts on clouds and climate

Dominick V Spracklen, Boris Bonn, Kenneth S Carslaw

Abstract

Previous studies have concluded that boreal forests warm the climate because the cooling from storage of carbon in vegetation and soils is cancelled out by the warming due to the absorption of the Sun's heat by the dark forest canopy. However, these studies ignored the impacts of forests on atmospheric aerosol. We use a global atmospheric model to show that, through emission of organic vapours and the resulting condensational growth of newly formed particles, boreal forests double regional cloud condensation nuclei concentrations (from approx. 100 to approx. 200 cm−3). Using a simple radiative model, we estimate that the resulting change in cloud albedo causes a radiative forcing of between −1.8 and −6.7 W m−2 of forest. This forcing may be sufficiently large to result in boreal forests having an overall cooling impact on climate. We propose that the combination of climate forcings related to boreal forests may result in an important global homeostasis. In cold climatic conditions, the snow–vegetation albedo effect dominates and boreal forests warm the climate, whereas in warmer climates they may emit sufficiently large amounts of organic vapour modifying cloud albedo and acting to cool climate.

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Footnotes

  • One contribution of 10 to a Triennial Issue ‘Earth science’.

  • Vegetation emissions of BVOCs at a specific temperature and light intensity are calculated using F=ED (Guenther et al. 1995), where F is the emission flux; E is the plant type emission capacity; and D is the foliar density, calculated by dividing the (two-sided) leaf area index (LAI) by the specific leaf area. Values chosen for the different ecosystems are shown in table 1.

  • For simplicity, we have here assumed that forest carbon storage and evapotranspiration do not vary with temperature, although this is not likely to be the case and will further complicate the combined feedbacks.

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