Enhanced sensitivity of atmospheric CO2 growth rate variations to tropical mean temperature anomalies is driven by internal climate variability in a large climate model ensemble

The atmospheric CO2 growth rate (AGR) shows large year-to-year variations, which are mainly driven by land and ocean carbon uptake variations. Recent studies suggested an approximate doubling of the AGR regressed onto tropical mean temperature anomalies (“sensitivity of AGR to tropical mean temperature anomalies”; Wang et al., 2014; Luo et al., 2022), which was attributed to increasing drought in tropical land vegetation areas in a warming climate (Wang et al., 2014). We hypothesise that at least part of this apparent sensitivity change may instead be explained by extratropical areas and by internal climate variability.Here, we study the apparent sensitivity changes of AGR to tropical mean temperature in observations, atmospheric inversions, and a large climate model ensemble of historical simulations. First, we identify the main regional drivers of the apparent sensitivity change, including the ocean and extratropical regions in all datasets. Then, we evaluate whether these sensitivity changes can be attributed to anthropogenic forcing in a large climate model ensemble, or whether they are mostly driven by internal climate variability. Our results show that other regions beyond the land tropics contribute to the change in apparent sensitivity of AGR to tropical mean temperature anomalies in atmospheric inversions and in the period 1960 to 2006. Furthermore, the climate model large ensemble shows that such "doubling sensitivity" events can occur due to internal climate variability only. This points to the importance of distinguishing internal climate variability from forced signals when attributing causes to observed changes in the carbon cycle.Wang, X., Piao, S., Ciais, P. et al. A two-fold increase of carbon cycle sensitivity to tropical temperature variations. Nature 506, 212–215 (2014). https://doi.org/10.1038/nature12915Luo, X., Keenan, T. F. Tropical extreme droughts drive long-term increase in atmospheric CO2 growth rate variability. Nat Commun 13, 1193 (2022). https://doi.org/10.1038/s41467-022-28824-5