The integrated effects of warming are obvious in the physics, chemistry, and ecology of tundra at the arctic Alaska LTER site despite no significant warming trend since climate measurements began in 1989

Background/Question/Methods

An overarching goal of the Arctic LTER project on the North Slope of Alaska is to measure changes in climate and their effects on the ecology of tundra, streams, and lakes.  The research site is at Toolik Lake, 200 km south of the Arctic Ocean.  At the site, detailed climate measurements have been made since 1989 and other measures expected to be directly related to climate, such as lake temperature, algal growth (concentrations of chlorophyll in the lake), and thickness of the annual thaw (active layer), have been made.  Another series of measurements were of integrated effects of warming.  These include the temperature of permafrost 20 m below the surface, the alkalinity and strontium isotopes in stream water as an indicator of weathering, and measures of the growth of plants.  Growth is measured as changes over time in satellite greenness indices and by measurements at 156 point-frame plots of height and density of canopy of shrubs and other plants (inventoried at five to seven year intervals beginning in 1989).

Results/Conclusions

There were no significant trends (1989 to 2011) in air temperature, precipitation, and thickness of the active layer, and no trends from 1975 to 2011 in the summer lake temperatures, and in the biomass of the plankton.  In spite of this apparent lack of change, integrated measures did show changes: permafrost temperatures at a depth of 20 m increased by 1.0 ^oC in 26 years, satellite biomass values (as peak NDVI) increased from 0.55 to 0.61 in 20 years, and plant abundance in the point frame plots increased over 20 years (graminoids by 25.5%, herbaceous dicots by 24%, and shrubs by 13%).  Further evidence for warming comes from stream and lake waters where the alkalinity has doubled and strontium isotope ratios changed both indicators of weathering of previously frozen soil.  We conclude that ecological changes have been occurring as a result of warming since 1989 despite no significant trends in average air temperature.  We hypothesize that we are able to measure changes in the ecosystem, such as in plant growth, with greater sensitivity than our ability to measure changes in climate.  A more useful measure of warming is likely to be the total degree days above freezing.