Scientists measure the greenness of a landscape using light-reflection data gathered by weather satellites. Green plants tend to reflect the near infra-red wavelengths, but absorb red wavelengths for photosynthesis. Using near-infrared and red-wavelength data scientists calculate a "normalized difference vegetation index" (NDVI), where higher numbers represent greener landscapes. This measure provides a good reference for year-to-year change of the amount of tundra productivity over the past 25 years.
To better understand the trends in tundra productivity and how this might be related to the dramatic changes in sea ice that are occurring in the Arctic Ocean, Skip Walker from the University of Alaska and his colleagues have studied 25 years' worth of sea-ice data, summer land temperatures and NDVI values from the Yamal Peninsula, in north-west Siberia.
The Yamal region is an ideal place to understand NDVI because it is relatively flat and simple geologically, the vegetation structure is straightforward and it is a hotspot for Arctic land-cover change. What's more, there is a strong north-south climate gradient along which it is possible to study the effects of climate on greenness patterns.
By studying the satellite data the team found that there had been a 7% increase in NDVI over the Yamal Peninsula between 1982 and 2007. Over the same time period the researchers saw a 37% decrease in summer sea-ice concentration around the peninsula, and a 4% rise in summer land temperatures.
Previously Walker and his colleagues had hypothesized that the melting sea ice might be causing a rise in land temperatures and encouraging vegetation to grow but this appeared to be only part of the story on the Yamal peninsula. "The linkages between sea-ice decline, land temperatures and NDVI are not as strong on the Yamal as they are in other areas of the Arctic, which led us to examine other factors such as disturbance," Walker told environmentalresearchweb.
To further investigate the changes in greenness, Walker and his colleagues carried out field campaigns during the summers of 2007, 2008 and 2009, making aerial and ground observations.
They concluded that extensive gas-field development on the peninsula is unlikely to be having an impact on the entire region because the developments currently cover relatively small areas compared with the large size of the peninsula. Instead Walker and his colleagues noticed an association between landscape erosion and greening. "Most of the greenness is in valleys, where water and snow irrigate the land and nutrients accumulate," explains Walker, whose findings are published in Environmental Research Letters as part of a Focus Issue on Climatic and Environmental Change in Northern Eurasia.
Rapid erosion of permafrost and landslides stood out as a particularly important factor affecting greenness patterns. Solid ice lies at shallow depth beneath much of the upland surfaces over large areas of the Yamal Peninsula; this ice is being rapidly eroded, causing landslides and intricate drainage patterns. The vegetation on the eroded surfaces and the valley bottoms is considerably greener than on the un-eroded surfaces.
As yet it isn't clear if erosion rates have increased in the Yamal region over the study period but it is possible that warmer temperatures and increased permafrost melt have enhanced erosion over the years.
Another unknown is the impact of reindeer grazing. Walker and his colleagues haven't yet been able to monitor reindeer but they hope to work with herders in the future to see what impact that reindeer grazing is having on the greening patterns.
In addition Walker's group will continue to examine whether climate change is exacerbating the erosion and how this erosion is affecting other components of the system, including its effect on reindeer grazing patterns.
Walker and colleagues regard the NDVI as a single number that integrates the total effect of all the myriad factors that affect tundra vegetation productivity, including changes in climate and a wide variety of disturbance factors. "Although we cannot at this time determine the contribution of each individual factor to the total change over time, we have determined that the vegetation on the Yamal has become generally greener over the 25 years of the study, and we were able to compare the changes that are occurring on the Yamal with the changes that are occurring elsewhere in the Arctic," he says.