What is it about?

Combined dendrochronological and biogeochemical evidence from Norilsk, the world’s northernmost city with over 100,000 inhabitants, where mining actions and uncontrolled pollutants have expanded rapidly since the 1930s, now suggests that a reduction in surface irradiance during the second half of the 20th century has adversely affected boreal tree growth (Kirdyanov et al., 2020). When derived from the diurnal temperature range and implemented as negative forcing in a forward model of radial tree growth (Vaganov et al., 2011; Kirdyanov et al., 2020), Arctic Dimming offers a plausible, large-scale explanation for the ‘Divergence Problem’ in dendrochronology (see D’Arrigo et al., 2008 for a review): the apparent decoupling between formerly temperature-sensitive tree-ring width and density chronologies at different boreal forest sites in North America and Siberia, and rising instrumental temperature measurements since the 1970s (Briffa et al., 1998). Attributing the ‘Divergence Problem’ to anthropogenic aerosol emissions, and subsequent reductions in surface solar radiation (Liepert, 2002), is fundamental to our understanding of past climate variability (Büntgen et al., 2020), because it would refer to a relatively modern issue that leaves the Principle of Uniformity in tack as it applies to dendroclimatology (Fritts, 1976; Esper and Frank, 2009), and thus to a substantial portion of high-resolution paleoclimatology.

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Why is it important?

Considering the importance and complexity of natural (e.g., volcanic eruptions and wildfires) and anthropogenic (e.g., mining, oil and shipping industries) aerosol emissions to Arctic warming is particularly timely given the recent temperature extremes recorded at high-northern latitudes (Cohen et al., 2020; Overland and Wang, 2021). Despite our knowledge about the observed and modelled climatic effects of rising Arctic aerosol concentrations (Schmale et al., 2021), which may exhibit regional and seasonal differences and call for diverse research priorities from local to circumpolar scales, we feel that the ecological consequences of an aerosol-induced reduction in surface irradiance (i.e., Arctic Dimming) justify more thorough tree-ring investigations in the future. We argue that this is particularly true if the goal is to enhance our understanding of all aerosol generating processes that impact terrestrial vegetation and its ability to assimilate carbon dioxide from the atmosphere.


The putative reduction of tree growth in response to increased cloud cover and cloud residence time, as possibly manifest in the ‘Divergence Problem’, limits the capacity of the boreal forest to assimilate carbon dioxide from the atmosphere (Fig. 1). The contribution of the world’s largest terrestrial biome to mitigate the effects of greenhouse gases on global warming will therefore be smaller than expected (Girardin et al., 2016). An anthropogenically induced light deficiency in northern vegetation zones (Nemani et al., 2003), amplified by wildfires and sulphate-rich volcanic eruptions (Stine and Huybers, 2014; Büntgen et al., 2020), will also influence the temperature-sensitivity of treeline and tundra ecotones (Hagedorn et al., 2019). Overall, the alleged changes are likely to slow down the predicted rate of Arctic greening.

Dr. Vladimir V. Shishov
Siberian federal university

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This page is a summary of: Arctic aerosols and the ‘Divergence Problem’ in dendroclimatology, Dendrochronologia, June 2021, Elsevier, DOI: 10.1016/j.dendro.2021.125837.
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