Influence of high-latitude warming and land-use changes in the early 20th century northern Eurasian CO2 sink

Abstract

While the global carbon budget (GCB) is relatively well constrained over the last decades of the 20th century [ 1 ], observations and reconstructions of atmospheric CO _2 growth rate present large discrepancies during the earlier periods [ 2 ]. The large uncertainty in GCB has been attributed to the land biosphere, although it is not clear whether the gaps between observations and reconstructions are mainly because land-surface models (LSMs) underestimate inter-annual to decadal variability in natural ecosystems, or due to inaccuracies in land-use change reconstructions. As Eurasia encompasses about 15% of the terrestrial surface, 20% of the global soil organic carbon pool and constitutes a large CO _2 sink, we evaluate the potential contribution of natural and human-driven processes to induce large anomalies in the biospheric CO _2 fluxes in the early 20th century. We use an LSM specifically developed for high-latitudes, that correctly simulates Eurasian C-stocks and fluxes from observational records [ 3 ], in order to evaluate the sensitivity of the Eurasian sink to the strong high-latitude warming occurring between 1930 and 1950. We show that the LSM with improved high-latitude phenology, hydrology and soil processes, contrary to the group of LSMs in [ 2 ], is able to represent enhanced vegetation growth linked to boreal spring warming, consistent with tree-ring time-series [ 4 ]. By compiling a dataset of annual agricultural area in the Former Soviet Union that better reflects changes in cropland area linked with socio-economic fluctuations during the early 20th century, we show that land-abadonment during periods of crisis and war may result in reduced CO _2 emissions from land-use change (44%–78% lower) detectable at decadal time-scales. Our study points to key processes that may need to be improved in LSMs and LUC datasets in order to better represent decadal variability in the land CO _2 sink, and to better constrain the GCB during the pre-observational record.

Keywords

• Eurasia
• carbon-cycle
• land-use change
• climate variability