Polar ice records preserve climate versus human impact after the Little Ice Age

Pollen can help scientists track changes in vegetation over time as they respond to climate moderations, whether glaciations or deglaciations with transitions in and out of ice ages. Additionally, it can help elucidate the interaction between climate and the impact of exploitation of the natural world by early human settlements on forests.

Pollen dispersal has evolved various mechanisms over millennia and can be transported over distances ranging from hundreds to thousands of kilometers. Indeed, modeling shows that ice cores collected in southern Greenland preserve pollen from boreal and mixed conifer forests from more than 3,500 km away in eastern Canada at a resolution of five to 20 years .

New research, published in Geophysical research lettersgenerated an 850-year pollen record (including the Medieval Warm Period, ~950-1350 CE, and the Little Ice Age, ~1350-1750 CE) of Greenland to determine the role of climate and humans in changing the boreal ecosystems, particularly high. -the latitude regions most sensitive to warming.

Dr Sandra Brugger, from the Desert Research Institute, US, and colleagues used advanced techniques to isolate pollen from an archived ice core recovered from 2,120m above sea level in 2011. Individual pollen grains were counted and identified under a light microscope at 400x magnification. , revealing their microscopic size. Additionally, the presence of spheroidal carbonaceous particles is a key marker of human occupation in ice cores, being a distinct form of black carbon derived from fossil fuel combustion.

During the survey, scientists identified 111 pollen taxa in the ice core, many of which are unique occurrences, a characteristic of high-altitude ice cores that record a large catchment area of ​​pollen grains blown by wind and other aerosols. However, the most abundant taxa of boreal conifers included pine (Pinus), spruce (Picea), fir (Abies), and larch (Larix), while ragweed (Ambrosia) was used as an indicator of landscape disturbance, as it is a flowering plant that grows in clearings.

The research team identified distinct changes in boreal forest pollen coinciding with the transition from the Medieval Warm Period to the Little Ice Age, with an increase in the population of pine, spruce, fir and birch ( comprising approximately 80% of the pollen assemblage at ~1400 CE). at the start of this glacial event, when the climate became cooler and drier overall.

The proliferation of pine in particular is important because it is a pioneer species, meaning it was one of the first to colonize relatively arid landscapes and therefore likely expanded to humid areas which dried up at the start of the Little Ice Age. This conifer pollen is linked to lakes in southern Quebec and Ontario, today surrounded by mixed conifer forests.

Subsequently, records show a notable decline in conifer pollen to around 40% from around 1650 CE, but this occurred as Little Ice Age conditions continued/proliferated, so Dr. Brugger and colleagues suggest instead that the human impact of logging the forest for firewood and agriculture had a much more noticeable effect.

This continued until around 1760, when a significant change in pollen to that of the relatively modern 20th century occurred. Here, pine dominated pollen grains (~43%), while spruce and birch (Betula) were only present in small abundances of around 2–3%.

The remainder of the assemblage’s composition included tundra shrubs and herbaceous plants, which together indicate that the trees were felled to produce clearings where weeds like Ambrosia could grow, so that human impact has exceeded that of climate in the northern hemisphere.

Previous research in eastern Canada supported this hypothesis by demonstrating a decline in white pine, as it was a major target of logging by European settlers, so much so that by 1850 CE, the Most of it had been irrevocably harvested in the region.

After the end of the Little Ice Age, as warming began, the research team did not identify a concomitant increase in the number of boreal conifers, as would be expected if the landscape was modulated solely by climate. Instead, a combination of continued permafrost, wetland expansion, and human-induced logging may have kept pine abundance low after 1760 CE.

This research is important as climate change continues to rapidly alter Arctic ecosystems, with plant species migrating north into increasingly warm ecological niches. Such polar warming is exacerbated by positive feedback mechanisms, such as ice albedo, whereby melting ice sheets and glaciers reduce the “white” surface area to reflect incoming solar radiation, causing the oceans and Comparatively “darker” lands absorb more solar radiation and warm the surrounding environment. , leading to increased ice melting and so the loop continues.

Dr Brugger and colleagues therefore suggest that such feedbacks prolong droughts in high-latitude regions, thereby increasing the risk of wildfires and biomass burning, which in turn contribute more carbon dioxide to the earth. atmosphere and worsen global warming. Our boreal forests are unlikely to return to the once thriving ecosystems before the northward expansion of settlements since the Medieval Warm Period and modern industrialization.

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