Revisiting drivers of the enhanced amplitude of atmospheric CO2 in northern terrestrial ecosystems

Airborne and ground-based measurements have consistently shown a rise in the seasonal amplitude of atmospheric CO2 concentration since the 1960s, particularly notable in the high northern latitudes. For instance, Barrow (BRW, 71ºN) witnessed a 50% increase in CO2 amplitude from 1960 to 2011, compared to a 15% increase at Mauna Loa (MLO, 20ºN). This trend suggests significant alterations in biosphere-atmosphere interactions and a changing carbon cycle in northern ecosystems. Previous studies suggest that the enhanced amplitude of atmospheric CO2 is mainly caused by the amplified plant productivity in northern ecosystems. However, the major factors that drive the increasing CO2 amplitude in the northern ecosystems are still subjected to debate, reflecting the fact that the underlying mechanisms or processes that govern the changes still remain unclear. Our study aims to understand these changes from both modelling and observational perspectives by using long-term (1980-2018) monthly CO2 concentration records, incorporating climate data, land cover changes, fire emissions, and ecosystem carbon fluxes (including gross primary production and ecosystem respiration). In parallel, we employed the LPJmL dynamic global vegetation model coupled with the TM3 atmospheric transport model (LPJmL+TM3) to simulate the seasonal CO2 concentration shifts and investigate the cause of the increasing CO2 amplitude. Our results show that the LPJmL+TM3 successfully captures both the interannual variability and the rising trend in CO2 amplitude from 1980 to 2018. Utilizing ensemble regression modeling and Shapley value analysis, we assessed the impact of various factors on CO2 amplitude changes. Different to the prevailing view that attributes the increase primarily to photosynthetic carbon uptake, our results suggest that ecosystem respiration plays an important role in driving both the interannual variability and the rising trend in CO2 amplitude in the northern terrestrial ecosystems.