The Arctic and high mountain regions in the northern hemisphere share some common features: the climate is harsh and the correspondingly sparse vegetation consists partly of the same or closely related species. Scientists analyzing long-term data from East Greenland and the Tibetan Plateau have now found that the growth of shrubs — or rather dwarf shrubs — has been declining for years, for which they blame global warming. Their work was published in the journal Proceedings of the National Academy of Sciences.
Conditions in the Arctic and in alpine high altitudes in the northern hemisphere are extreme, and yet there are some shrub species that have adapted to life in the cold and thrive as dwarf forms creeping along the ground. Their distribution is highly dependent on climate, which in turn is influenced by large-scale atmospheric circulations such as the Arctic Oscillation (AO), the Atlantic Multidecadal Oscillation (AMO) and the El Niño Southern Oscillation (ENSO). Shrub vegetation therefore responds strongly to changes in atmospheric circulation.
In the study, the authors describe that shrubs advanced into Arctic and alpine grasslands during the 20th century, leading to changes in carbon storage, biodiversity and feedbacks on regional climate. Although global warming is progressing and vegetation has continued to expand, observations indicate shrubs have been declining at high elevations and high latitudes for several years, possibly related to changes in water availability.
According to the study, High Arctic shrub species in East Greenland near Ittoqqortoormiit reached their maximum distribution between 1961 and 1970, while juniper shrubs on the Tibetan Plateau reached their peak distribution between 1931 and 1940. Thereafter, shrub growth declined in both study areas. The researchers found that the spread of the shrubs correlated strongly with the AO and AMO indices.
The AO is a back-and-forth shift in atmospheric pressure between the Arctic and the mid-latitudes of the North Pacific and North Atlantic. When it is strongly positive, a strong mid-latitude jet stream directs storms northward and reduces cold air outbreaks in the mid-latitudes. Conversely, when the AO is strongly negative, a weaker, meandering jet stream dips further south, allowing Arctic air to flow into the mid-latitudes.
The AMO is described as a cyclically occurring circulation oscillation of ocean currents in the North Atlantic, although it is disputed whether it is actually a natural phenomenon or an artefact of climate change.
According to the authors, the study provides clear evidence that climate tipping points in recent decades have already reversed the formerly increasing trend in shrub expansion in these cold habitats. The shrubs’ declining distribution is likely due to drought stress associated with changes in atmospheric circulations, they say. In both East Greenland and on the Tibetan Plateau, the AMO and El Niño brought warmer and drier climates, which reduced soil moisture and thus worsened growing conditions.
Although tundra vegetation is spreading as it warms, the further spread of shrubs in particular is limited by the availability of moisture in the soil. Progressive warming could therefore affect the spread of shrubs or even cause them to die.
Evidence of the link between shrub decline and atmospheric circulations leads the authors to believe that their patterns could help predict vegetation shifts in these cold regions.
Julia Hager, PolarJournal