Shrubs in the Arctic tundra have turned into trees as a result of the warming Arctic climate, creating patches of forest which, if replicated across the tundra, would significantly accelerate global warming.
Scientists from Finland and Oxford University investigated an area of around 100,000 km2, known as the northwestern Eurasian tundra, stretching from western Siberia to Finland. Surveys of the vegetation, using data from satellite imaging, fieldwork, and expert observations from indigenous reindeer herders, showed that in 8-15% of the area willow (Salix) and alder (Alnus) plants have grown into trees over 2 metres in height in the last 30-40 years.
Models assessing the potential impact of forestation have suggested that the advance of forest into Arctic tundra could increase Arctic warming by an extra 1-2 degrees Celsius by the late 21st Century.
The change from shrubs to forest is important as it alters the albedo effect – the amount of sunlight reflected by the surface of the Earth.
Source: University of Oxford Website
The thing that gets scientists excited is the remarkable speed in which biomes alter based on slight changes in median temperatures. A degree or two either way and you go from tundra to taiga and back again. I don’t think the scientific community expected to see this happen in periods of time less than half a human lifespan.