Leonardo Calle

Land use change and El Niño-Southern Oscillation drive decadal carbon balance shifts in Southeast Asia

An integrated understanding of the biogeochemical consequences of climate extremes and land use changes is needed to constrain land-surface feedbacks to atmospheric CO2 from associated climate change. Past assessments of the global carbon balance have shown particularly high uncertainty in Southeast Asia. Here, we use a combination of model ensembles to show that intensified land use change made Southeast Asia a strong source of CO2 from the 1980s to 1990s, whereas the region was close to carbon neutral in the 2000s due to an enhanced CO2 fertilization effect and absence of moderate-to-strong El Niño events. Our findings suggest that despite ongoing deforestation, CO2 emissions were substantially decreased during the 2000s, largely owing to milder climate that restores photosynthetic capacity and suppresses peat and deforestation fire emissions. The occurrence of strong El Niño events after 2009 suggests that the region has returned to conditions of increased vulnerability of carbon stocks.The carbon balance in Southeast Asia is highly uncertain. Here, the authors show that land use changes and occurrence of strong El Niño control decadal shifts in the carbon balance of this region.

Plant regrowth as a driver of recent enhancement of terrestrial CO2 uptake

The increasing strength of land CO2 uptake in the 2000s has been attributed to a stimulating effect of rising atmospheric CO2 on photosynthesis (CO2 fertilization). Using terrestrial biosphere models, we show that enhanced CO2 uptake is induced not only by CO2 fertilization but also an increasing uptake by plant regrowth (accounting for 0.33 ± 0.10 Pg C/year increase of CO2 uptake in the 2000s compared with the 1960s–1990s) with its effect most pronounced in eastern North America, southern-eastern Europe, and southeastern temperate Eurasia. Our analysis indicates that ecosystems in North America and Europe have established the current productive state through regrowth since the 1960s, and those in temperate Eurasia are still in a stage from regrowth following active afforestation in the 1980s–1990s. As the strength of model representation of CO2 fertilization is still in debate, plant regrowth might have a greater potential to sequester carbon than indicated by this study. Plain Language Summary The recent enhancement of CO2 uptake by the terrestrial biosphere is slowing down an acceleration of the atmospheric CO2 increase. A stimulating effect of rising atmospheric CO2 on plant photosynthesis (CO2 fertilization) provides the most pronounced impact on the enhanced CO2 uptake. However, the question remains on how much of the enhanced uptake CO2 fertilization accounts for and a possible contribution from past land use change. Here using multiple terrestrial biosphere models, we demonstrate that despite a large contribution from CO2 fertilization, the enhanced CO2 uptake in the 2000s cannot be fully explained without an increasing uptake by land use change, in particular, plant regrowth. The regrowth effect is most pronounced in North America, Europe, and temperate Eurasia, and they account for 94% of the global total CO2 uptake enhancement by plant regrowth. The strengthening trends in both CO2 fertilization and plant regrowth suggest that the deceleration of the atmospheric CO2 increase continues in the future.