Contrasting effects of CO<sub>2</sub> fertilisation, land-use change and warming on seasonal amplitude of northern hemisphere CO<sub>2</sub> exchange

Abstract

<p><strong>Abstract.</strong> Continuous atmospheric CO<sub>2</sub> monitoring data indicate an increase in seasonal-cycle amplitude (SCA) of CO<sub>2</sub> exchange in northern high latitudes. The major drivers of enhanced SCA remain unclear and intensely debated with land-use change, CO<sub>2</sub> fertilization and warming identified as likely contributors. We integrated CO<sub>2</sub>-flux data from two atmospheric inversions (consistent with atmospheric records) and from and 11 state-of-the-art land-surface models (LSMs) to evaluate the relative importance of individual contributors to trends and drivers of the SCA of CO<sub>2</sub>-fluxes for 1980−2015. The LSMs generally reproduce the latitudinal increase in SCA trends within the inversions range. Inversions and LSMs attribute SCA increase to boreal Asia and Europe due to enhanced vegetation productivity (in LSMs) and point to contrasting effects of CO<sub>2</sub> fertilisation (positive) and warming (negative) on SCA. Our results do not support land-use change as a key contributor to the increase in SCA. The sensitivity of simulated microbial respiration to temperature in LSMs explained biases in SCA trends, which suggests SCA could help to constrain model turnover times.</p>

Higher than expected CO2 fertilization inferred from leaf to global observations

“…A limitation of the study by Piao et al () is that it relies on an ensemble of models, many of which do not account for agricultural intensification. Bastos et al () address this limitation in their more recent study, by complementing the ecosystem model results with trend driver attribution using statistical models, confirming that the observed trends in seasonal amplitude of CO 2 are not significantly related to agricultural intensification.…”

“…Our simulated latitudinal and regional distributions of trends in GPP and the NLS (Figure a,b; Figures S5–S8) are consistent with the observed increase in the ASC of CO 2 in the Northern Hemisphere (56 ± 10% north of 45°N, 1960–2010; Graven et al, ), that is underestimated by current TBMs (Figure b), as previously reported (Graven et al, ; Thomas et al, ). Alternate explanations for the observed trend are increasing light‐use efficiency because of CO 2 fertilization (Bastos et al, ; Piao et al, ; Thomas et al, ; Wenzel et al, ) or high‐latitude warming effects on biome distribution and plant productivity (Forkel et al, ). Warming effects emerged as important in simulations using the LPJmL TBM which accurately simulates the large observed ASC trend, (Forkel et al, ), but simulates a global GPP trend ( PgC/year 2 ; 1970–2011; Forkel et al, ) that is less than half the corresponding CABLE‐simulated trend (0.53 ± 0.02 PgC/year 2 ; 1970–2011).…”

“…(c) Expansion of agricultural land area over the last century has been dominated by clearing of tropical forest—a land use conversion that is expected to contribute negatively to trends in NPP (Nyawira et al, ). (d) Recent studies using multiple models and data sets have concluded that the effect of land use change and land management on the increase in seasonal amplitude of CO 2 in the Northern Hemisphere is very small compared with the effects of CO 2 fertilization and warming‐induced lengthening of growing season (Bastos et al, ; Piao et al, ). Bastos et al () identify Eurasia as the region contributing most to increasing seasonal amplitude of CO 2 —a region that is dominated by natural ecosystems and has experienced very little land use change (Verburg et al, ) over the last half‐century.…”

“…(d) Recent studies using multiple models and data sets have concluded that the effect of land use change and land management on the increase in seasonal amplitude of CO 2 in the Northern Hemisphere is very small compared with the effects of CO 2 fertilization and warming‐induced lengthening of growing season (Bastos et al, ; Piao et al, ). Bastos et al () identify Eurasia as the region contributing most to increasing seasonal amplitude of CO 2 —a region that is dominated by natural ecosystems and has experienced very little land use change (Verburg et al, ) over the last half‐century. A limitation of the study by Piao et al () is that it relies on an ensemble of models, many of which do not account for agricultural intensification.…”

Constraining Uncertainty in Projected Gross Primary Production With Machine Learning

“…Section 2.1 reviews the method of a previously published emergent constraint on CO 2 fertilization from Wenzel, Cox, et al (2016) that we apply here to our study to rescale the climate model output (Step 1). This includes a discussion of the caveats that are associated with this emergent constraint that attributes changes in the observed increase in the CO 2 seasonal cycle amplitude at Cape Kumukahi entirely to the increase in atmospheric CO 2 concentration, yet other studies provide indications that part of this increase is due to climate and land use changes (Bastos et al, 2019; Forkel et al, 2016; Piao et al, 2018; Zhao et al, 2016). In section 2.2 the machine learning technique used in Step 2 to constrain gridded GPP projections is presented.…”