Attributing Past Carbon Fluxes to CO2 and Climate Change: Respiration Response to CO2 Fertilization Shifts Regional Distribution of the Carbon Sink

Over the past century, increased atmospheric CO2 concentrations have enhanced photosynthesis through CO2 fertilization across the globe. However, the increased growth has also led to greater respiration rates & mdash;both from vegetation (autotrophic respiration) and through the breakdown of plant litter and soil organic matter (heterotrophic respiration). The resulting change in carbon f lux & mdash;and its spatial distribution & mdash;that can be attributed to increasing CO2 and climate change remains unknown. We used the Carbon Data Model Framework, a model-data fusion system that assimilates global observations from satellites and other sources to create an ensemble of observationally constrained carbon cycle representations, to determine the photosynthesis and respiration fluxes that can be attributed to increased atmospheric CO2 and associated climate change from 1920 to 2015. Across the globe, the response of photosynthesis and respiration to atmospheric CO2 dominates their response to climate alone. The regional distribution of the carbon sink attributable to climate change and CO2 is strongly influenced by the 'loss ratio of carbon gained'& mdash;the fraction of enhanced photosynthesis that is lost to respiration. While the wet tropics' attributable photosynthesis flux is 1.4 times larger than that of the temperate region, the attributable flux of net carbon uptake is actually 1.25 larger in the temperate region, due to the wet tropics' greater heterotrophic respiration response to enhanced plant growth. At the global scale, the loss ratio of carbon gained is 83 +/- 0.6%. Our results highlight the importance of the respiration responses to enhanced plant growth in regulating the land carbon sink.Plain Language Summary Earth's land areas have taken up a large amount of carbon from the atmosphere over the last century. However, exactly where, why, and by how much carbon uptake has increased is uncertain. We used a modeling system informed by global observations from satellites and elsewhere to quantify how the flows of carbon changed in response to the last century of increasing atmospheric CO2. We found that increased photosynthesis stimulates greater ecosystem respiration, decreasing CO2's effect on net land carbon uptake. The fraction of increased photosynthesis that goes to respiration (rather than land carbon storage) varies by region and determines the location of the largest net land carbon uptake. Although it acts indirectly through changes in plant and soil carbon stocks, the respiration response to CO2 is a dominant component of the land carbon cycle response to human-caused emissions of CO2 and associated climate change.