Table 3.

Toolik and Zackenberg long-term monitoring results

Variable	Characteristics	Toolik	Zackenberg
Air temperature	Useful for long-term series (decades) of moderate warming Useful for shorter-term series(<decades) in case of intense warming	No significant trend because of short record and high annual variability	Significant trend related to large annual warming
Active layer thickness	Is reset every year so closely related to summer temperature	No long-term significant trend in thickness	Significantly thicker
	Changes in surface water alkalinity are evidence of deeper penetration of water into soil previously permanently frozen	Doubling of alkalinity in lakes is evidence that water is penetrating deeper into soil as active layer increases	No change in surface water chemistry
Permafrost temperature at 20 m	As temperature signal moves deeper into the soil, the high-frequency noise is filtered out	Increase of 0.8 °C over long-term	No measures made at depths >1.2 m
Plant and animal abundance with point-frame and species counts	Abundance of individual terrestrial and aquatic species or of communities	Shrubs, graminoids increase	Some species increase, most decrease but grasses and lichens decrease
NDVI satellite measure of peak greenness at annual or seasonal periods	Estimate of peak annual or seasonal biomass or photosynthesis. Integrates over various areas	Steady increase 1982–2014	Increase 1992–2004, instrument changes to 2008