Recent decline in tropical temperature sensitivity of atmospheric CO₂ growth rate variability

A two-fold enhancement in the sensitivity of atmospheric CO₂ growth rate (CGR) to tropical temperature interannual variability ( ${\Gamma }_{\mathrm{CGR}}^T$ ) till early 2000s has been reported, which suggests a drought-induced shift in terrestrial carbon cycle responding temperature fluctuations, thereby accelerating global warming. However, using six decades long atmospheric CO₂ observations, we show that ${\Gamma }_{\mathrm{CGR}}^T$ has significantly declined in the last two decades, to the level during the 1960s. The ${\Gamma }_{\mathrm{CGR}}^T$ decline begs the question of whether the sensitivity of ecosystem carbon cycle to temperature variations at local scale has largely decreased. With state-of-the-art dynamic global vegetation models, we further find that the recent ${\Gamma }_{\mathrm{CGR}}^T$ decline is barely attributed to ecosystem carbon cycle response to temperature fluctuations at local scale, which instead results from a decrease in spatial coherence in tropical temperature variability and land use change. Our results suggest that the recently reported loss of rainforest resilience has not shown marked influence on the temperature sensitivity of ecosystem carbon cycle. Nevertheless, the increasing extent of land use change as well as more frequent and intensive drought events are likely to modulate the responses of ecosystem carbon cycle to temperature variations in the future. Therefore, our study highlights the priority to continuously monitor the temperature sensitivity of CGR variability and improve Earth system model representation on land use change, in order to predict the carbon-climate feedback.