Weakened uptake of carbon in the Arctic; Scientist warn
A warming Arctic will give an extended growing season particularly due to an earlier spring, and it has been believed that this will give a greater uptake of CO2 in plants and an increased carbon sink. An impact potentially offsetting some of the anthropogenic increases in greenhouse gases. A new study suggest that the “warmer spring, bigger sink” hypothesis may no longer hold.
The Arctic plays an important role in the carbon cycle, and when the cold regions with tundra warms up and causing the permafrost to thaw there will be released carbon to the atmosphere.
It has been believed that a longer growing season will be a mechanism to increase carbon uptake and offset the negative effect of the carbon releases. This hypothesis is now challenged of an international research team in an new article in Nature Climate Change.
– For the past decade, boreal forests have been a net carbon sink; but the sink-or-source status of arctic tundra cannot be deduced from current observations, says John F. Burkhart, one of the researchers in the new study.
Cold regions and CO2-uptake by plants
The effect of temperature on spring CO2 uptake by plants has been identified as the main mechanism explaining the year-to-year variations of atmospheric CO2 in spring. In the Arctic this is a particularly strong process due to the distinct seasonal cycling over the course of the year.
Several studies have shown that at the Point Barrow atmospheric measurement station in north Alaska, the periodical spring drawdown of atmospheric CO2 occurs earlier in the year when spring temperature is warmer.
However, until recently, decadal trends of this process have not been investigated. A recent analysis of 34 years of data from Barrow, has shown that the 'warmer spring, bigger sink' hypotheses may not hold, and in fact is breaking down in recent decades
Climatological analysis of transport pathways
The study, with researchers from all over the world, show that previously reported strong correlation between the timing and magnitude of carbon drawdown and spring land temperature was found in the first 17 years of measurements, but disappeared in the last 17 years.
To determine whether this change in relationship could be related to altered transport pathways and source regions of the measurements, rather than a change in the NPP, Burkhart conducted an analysis of the Barrow region atmospheric transport.
The results confirmed that altered transport or source regions for the measurements could not explain the shift observed in the measurements.
Improved models give better answers
– The use of an advanced Lagrangian Particle Dispersion Model (LPDM) allowed us to evaluate the source region which is representative for the observations. In this case, we were able to show conclusively that the source region for Barrow has not changed over the past decades, and that changes in observed concentrations of CO2 must be related to local processes, explains Burkhart.
Calculations of air mass history were developed for Barrow, Alaska using the LPDM FLEXPART (v. 8.2). The simulations are available for the period of 1985 – 2009 inclusive, at 3-hourly resolution.
Backward in time, or so-called “retroplume” calculations were made to provide a 'footprint' map of the Barrow measurements. Every 3-hours, 40 000 particles are released from the measurement site location (Barrow) and followed backwards in time for 20 days.
Integrating time at the lowest model output layer (0-100 m) over the 20-day period provides a map of Potential Emission Sensitivity (PES) for the measurement site. Post processing of the results was conducted to evaluate the variability of the mean footprint source region for Barrow, Alaska. The 3-hourly results were averaged into monthly values, and subsequently into seasonal, annual, and decadal values. Mean PES datasets were produced for further calculation with the ORCHIDEE land surface model.
About the research team
The research team behind the article is led by scientists from the Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing.
One of the contributors to this research is also the Land-ATmosphere Interactions in Cold Environments - LATICE research group with the contributions from John Faulkner Burkhart, an Associate Professor at Department of Geosciences.
Reference article in Nature Climate Change:
Piao, S., Liu, Z., Wang, T., Peng, S., Ciais, P., Huang, M., Ahlstrom A., Burkhart, J.F., Chevallier, F., Janssens, I., Jeong, S., Lin, X., Mao, J., Miller, J., Mohammat, A., Myneni, R.B., Penuelas, J., Shi, X., Stohl, A., Yao, Y., Zhu, Z., & P.P Tans. 2017. Weakening temperature control on the interannual variations of spring carbon uptake across northern lands Nature Climate Change, doi:10.1038/nclimate3277