Led by researchers from Alaska, the group took ground measurements at several points along two transects on opposite sides of the pole – the North American Arctic Transect and the Eurasian Arctic Transect. This data was then compared with greenness values measured by satellites, which are expressed as the normalized difference vegetation index (NDVI).

The team found that, despite large differences in environment and vegetation between the transects, the correlation between the NDVI and the ground data was almost identical for both transects.

"This shows that there is a consistent relationship between NDVI and tundra biomass that can be used to map circumpolar tundra biomass and examine long-term biomass trends from space," said Skip Walker from the University of Alaska Fairbanks, US. "This is the first time this amount of data has been collected in this area in such a consistent way. This is an important baseline study which can be used to monitor change in the future."

The North American Arctic Transect (NAAT) stretches 1500 km from the Alaskan/Canadian border towards the North Pole whereas the Eurasian Arctic Transect (EAT) stretches from Russia across the Arctic Sea towards the pole. The two transects both cross all five Arctic bioclimate subzones, but are very different to each other. The NAAT has a continental climate while the EAT has a maritime climate.

Despite these differences, Walker and his colleagues were surprised to find that the NDVI was highly correlated with above-ground biomass.

"We were amazed that despite major differences in topography, geology, soil, disturbance regime, vegetation composition and structure, the NDVIs from two transects at opposite sides of the globe correlated so well with ground data," said Walker. "This close correspondence gives us increased confidence that the Advanced Very High Resolution Radiometer (AVHRR) satellites are providing data that can be used to map the distribution of tundra biomass and to monitor circumpolar changes in tundra biomass."

The data collected by the team has already been used as the basis of several research papers (see links on left) and Walker hopes there will be more to come. He also thinks a third transect would help verify the results. "We would like to analyse a transect that runs the entire length of Greenland, North to South," said Walker. "This has never been done before and we believe a third transect such as this would help provide a more complete analysis of the relationship between vegetation and NDVI in the Arctic."