Weakening Temperature Control on the Interannual Variations of Spring Carbon Uptake across Northern Lands
This study examines the changes of the interannual relationship between temperature and carbon uptake and their driving mechanisms during the spring season in the northern ecosystems (50°–90°N) for the past 3 decades. The weakened interannual correlation of spring carbon uptake and land temperature was significantly identified, and this shifted response is attributable to the declining temperature response of spring net primary productivity rather than to changes in heterotrophic respiration or in atmospheric transport patterns.
Our work demonstrates the first clear evidence and drivers of weakened linkage between spring carbon uptake and temperature over northern ecosystems for the past 3 decades, and propose new metrics which could be adopted to diagnose the feedbacks between terrestrial carbon cycle and the global climate system.
We 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. Two indicators, the spring zero-crossing date of atmospheric CO2 (SZC) and the magnitude of CO2 drawdown between the magnitude of CO2 drawdown between May and June (SCC), were defined. 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 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 rather than to changes in heterotrophic respiration or in atmospheric transport patterns. Our results thus challenge the ‘warmer spring–bigger sink’ mechanism.