The Earth’s northernmost regions have experienced the greatest degree of climate warming in the last few decades, a trend that’s projected to continue in the 21st century. The Arctic tundra and boreal forest harbour some of the largest stretches of wilderness and carbon stocks on the planet, mostly in organic soils, so it’s important to understand how vegetation in the region responds to climate change.

The satellite data record now spans 30 years; it gives scientists a unique opportunity to monitor vegetation productivity across the globe, including in the remote polar regions. In the tundra, where conditions have historically been too cold for tree growth, our new study documents the vigorous growth of vegetation thanks to the warmer conditions of past decades. At the same time, satellite images confirm that boreal forests are increasingly feeling the heat of recent climate warming.

Since comparisons of mid-20th century and recent aerial photographs revealed the expansion of shrubs in northern Alaska, increased shrub growth is thought to have been driving the “greening” signal often observed over tundra. Now, however, a new combination of satellite data and maps of shrub cover indicates that the strong greening signal – reflecting increased vegetation productivity – occurs across the tundra, regardless of shrub presence. This ubiquitous increase in tundra productivity reflects a release of temperature limitations on growth across many different Arctic vegetation types, not just those dominated by shrubs.

In sharp contrast to the tundra and northern treeline, forested areas further south are consistently visible in satellite images as “browning”. This pattern is particularly conspicuous in the dense, coniferous forest of North America's boreal zone and indicates that the warming climate has adversely affected vegetation and tree growth here. At the same time, forest fires in the region are increasing in extent and intensity. After a fire, vegetation moves through a regeneration phase, which in boreal North America leads to rebounding productivity within a single decade. But fires also influence vegetation composition over much longer periods. Further intensification of fires thus plays a critical role in the distribution and composition of the future boreal forest.

In Siberia, forest-regeneration patterns following a fire are not yet well documented across broad regions. Given the extent of Siberia's boreal forest and recent increases in fire disturbance across the region, quantifying Eurasian post-fire vegetation composition and productivity, and its effects on the climate, is now emerging as an urgent challenge.