Abstract
Abstract. Continuous atmospheric CO2 monitoring data indicate an increase in the amplitude of seasonal CO2-cycle exchange (SCANBP) in northern high latitudes. The major drivers of enhanced SCANBP remain unclear and intensely debated, with land-use change, CO2 fertilization and warming being identified as likely contributors. We integrated CO2-flux data from two atmospheric inversions (consistent with atmospheric records) and from 11 state-of-the-art land-surface models (LSMs) to evaluate the relative importance of individual contributors to trends and drivers of the SCANBP of CO2 fluxes for 1980–2015. The LSMs generally reproduce the latitudinal increase in SCANBP trends within the inversions range. Inversions and LSMs attribute SCANBP increase to boreal Asia and Europe due to enhanced vegetation productivity (in LSMs) and point to contrasting effects of CO2 fertilization (positive) and warming (negative) on SCANBP. Our results do not support land-use change as a key contributor to the increase in SCANBP. The sensitivity of simulated microbial respiration to temperature in LSMs explained biases in SCANBP trends, which suggests that SCANBP could help to constrain model turnover times.
Highlights
The increase in the amplitude of seasonal atmospheric CO2 concentrations at northern high latitudes is one of the most intriguing patterns of change in the global carbon (C) cycle
The difference between the Copernicus Atmosphere Monitoring Service (CAMS) and CarboScope inversions reflects part of the uncertainty in inversions due to their different choices in the atmospheric transport models (ATMs), the set of assimilated CO2 data, the prior fluxes, and the a priori spatial and temporal correlation scales and is comparable to the uncertainty of the linear fit due to interannual variability. This finding is corroborated by two further analyses of inversion uncertainties: 1. While both inversions assimilate atmospheric CO2 measurements from Point Barrow, CAMS increasingly assimilates many other sites in the NH as they become available, helping to better constrain the CO2 fluxes in mid-latitudes to high latitudes with time
A higher SCANBP trend in the period 1993–2015 is reported by both CAMS and CarboScope, which estimate very similar trends in L>40 N (19.5 and 19.2 Tg C yr−2, respectively)
Summary
The increase in the amplitude of seasonal atmospheric CO2 concentrations at northern high latitudes is one of the most intriguing patterns of change in the global carbon (C) cycle. The seasonal-cycle amplitude (SCA) of atmospheric CO2 in the lower troposphere at the high-latitude monitoring site of Point Barrow, Alaska, has increased by about 50 % since the 1960s (Keeling et al, 1996; Dargaville et al, 2002). Increasing SCA has been registered at other high-latitude sites, mostly above 50◦ N (Piao et al, 2017), and appears to be driven primarily by changes in seasonal growth dynamics of terrestrial ecosystems (i.e. net biome productivity – NBP), but uncertainty remains about the relative contributions from different continents and mechanisms. Evidence suggests that crop productivity stagnated after the 1980s in many regions in the Northern Hemisphere (Grassini et al, 2013), which is not reflected in SCA trends in recent decades (Yin et al, 2018)
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