Ongoing spring warming allows the growing season to begin earlier, enhancing carbon uptake in northern ecosystems; but the sink-or-source status of arctic tundra cannot be deduced from current observations.
In a new study in the journal Nature Climate Change researchers use 34 years of atmospheric CO2 concentration measurements at Barrow, Alaska (BRW, 71 N) to show that the interannual relationship between spring temperature and carbon uptake has recently shifted. They use two indicators: the spring zero-crossing date of atmospheric CO2 (SZC) and the magnitude ofCO2 drawdown between May and June (SCC).
The previously reported strong correlation between SZC, SCC and spring land temperature (ST) was found in the first 17 years of measurements, but disappeared in the last 17 years. As a result, the sensitivity of both SZC and SCC to warming decreased. Simulations done in the study with an atmospheric transport model coupled to a terrestrial ecosystem model suggest that the weakened interannual correlation of SZC and SCC with ST in the last 17 years is attributable to the declining temperature response of spring net primary productivity (NPP) rather than to changes in heterotrophic respiration or in atmospheric transport patterns.
“Several mechanisms could explain the apparent weakening response of NPP in spring to interannual temperature variations. A first possible mechanism is that winter warming comes with a loss of chilling, other possible mechanisms are the possible limitations of shorter day lengths when the growing season progressively advances earlier into the spring, and increasing occurrence of extreme events. Further studies are needed to verify these potential mechanisms”, said Prof. Josep Penuelas from CREAF-CSIC Barcelona.
Prof Penuelas notes that their results show that the linkage between spring carbon uptake and temperature is not a stable property of northern ecosystems. However, because of the relatively short run of CO2 observations, it remains uncertain whether the observed decrease in interannual correlation of spring carbon uptake with temperature reflects decadal variability, or a long-term shift in the ecological response of boreal and arctic regions to warming.
This study received support from the European Research Council Synergy grant ERC-2013-SyG-610028.
Journal Reference: Piao, S., Liu, Z., Wang, T.,Peng, S., Ciais, P., Huang, M., Ahlstrom, A., Burkhart, J., Chevallier, F., Janssens, I., Jeong, S., Lin, X., Mao, J., Miller, J., Mohammat, A., Myneni, R., Penuelas, J., Shi, X., Stohl, A., Yao, Y., Zhu, Z., Tans, P. 2017. Weakening temperature control on the inter-annual variations of spring carbon uptake across northern lands. Nature Climate Change, doi: 10.1038/nclimate3277.