Large and increasing biospheric productivity of northern ecosystems

Plants take up carbon dioxide (CO₂) through photosynthesis. How this will change with rising CO₂ concentrations in the atmosphere will strongly determine future climate change. An increase in the seasonal variations of atmospheric CO₂ in recent decades indicates a positive trend in photosynthetic carbon uptake. We combined data-driven seasonal cycles of plant productivity with carbon sinks across the range predicted by current biospheric process models to explain the seasonal variations of CO₂ at high and low northern latitudes over the past 40 years. We find that increases in seasonal variations can only be explained by a larger gross primary productivity (GPP) of northern ecosystems than most current estimates and by an increase of GPP about proportional to the increase in atmospheric CO₂, also larger than most current estimates. Our results provide an improved constraint to estimate the future behavior of the terrestrial carbon sink.