Image: Marc Macias-Fauria

As Arctic summers warm, Earth's northern landscapes are changing. Using satellite images to track global tundra ecosystems over decades, a new collaborative study involving the University of Oxford and global institutions across the world, found the region has become greener, as warmer air and soil temperatures lead to increased plant growth.

"The terrestrial Arctic has become more productive in the last decades, and summer warming is behind this increase. This strongly affects the ecology, energy budget, and livelihoods of one of the coldest and fastest-changing biomes on Earth," said Dr Marc Macias-Fauria, head of the Biogeosciences lab at the School of Geography and Environment, University of Oxford and an ecologist on the project.

The study, published this week in Nature Communications, is the first to measure vegetation changes across the Arctic tundra, from Alaska and Canada to Siberia, using satellite data from Landsat, a joint mission of NASA and the U.S. Geological Survey. Landsat data can be used to determine how much actively growing vegetation is on the ground - greening can represent plants growing more, becoming denser, or shrubs encroaching on typical tundra grasses and moss.

"This is the first time that Landsat satellite imagery has been used to assess tundra vegetation response to ongoing warming over the whole Arctic," Dr Macias-Fauria adds. "This is important because the high spatial grain of the images (30 meters) better encapsulates the irregular Arctic terrain, as seen in the agreement between the vegetation trends inferred from the images and land-based measurements."

When the tundra vegetation changes, it impacts not only the wildlife that depend on the certain plants, but also the people who live in the region and depend on local ecosystems for food. While active plants will absorb more carbon from the atmosphere, the warming temperatures could also be thawing permafrost, releasing greenhouse gasses. The research is part NASA's Arctic Boreal Vulnerability Experiment (ABoVE), which aims to better understand how ecosystems are responding in these warming environments and its broader implications.

The research team used the Landsat data and additional calculations to estimate the peak greenness for a given year for each of 50,000 randomly selected sites across the tundra. Between 1985 and 2016, about 38% of the tundra sites across Alaska, Canada, and western Eurasia showed greening. Only 3% showed the opposite browning effect, which would mean fewer actively growing plants. To include eastern Eurasian sites, they compared data starting in 2000, since that's when Landsat satellites began collecting regular images of that region. With this global view, 22% of sites greened between 2000 and 2016, while 4% browned.

Change in tundra greenness, 2000-2016

"Whether it's since 1985 or 2000, we see this greening of the Arctic evident in the Landsat record," Logan Berner, a global change ecologist at Northern Arizona University, who led the recent research said. "And we see this biome-scale greening at the same time and over the same period as we see really rapid increases in summer air temperatures."

The researchers compared these greening patterns with other factors, and found that it's also associated with higher soil temperatures and higher soil moisture. They confirmed these findings with plant growth measurements from field sites around the Arctic.

"Landsat is key is for these kinds of measurements because it gathers data on a much finer scale than what was previously used", said Scott Goetz, a professor at Northern Arizona University who also worked on the study and leads the ABoVE science team. That allows the researchers to investigate what is driving the changes to the tundra. "There's a lot of microscale variability in the Arctic, so it's important to work at finer resolution while also having a long data record," Goetz said. "That's why Landsat's so valuable."

Read the academic paper: Summer warming explains widespread but not uniform greening in the Arctic tundra biome.