Enhanced global carbon cycle sensitivity to tropical temperature linked to internal climate variability

The sensitivity of atmospheric CO₂ growth rate to tropical temperature (γ^T) has almost doubled between 1959 and 2011, a trend that has been linked to increasing drought in the tropics. However, γ^T has declined since then. Understanding whether these variations in γ^T reflect forced changes or internal climate variability in the carbon cycle is crucial for future climate projections. We show that doubling sensitivity events can arise in simulations by Earth system models with perturbed initial conditions but are likely explained by internal climate variability. We show that the doubling sensitivity event is associated with the occurrence of a few, but very strong, El Niño events, such as 1982/83 and 1997/98. Such extreme events result in concurrent carbon release by tropical and extratropical ecosystems, increasing the variance of the global land carbon sink and its apparent sensitivity to tropical temperature. Our results imply that the doubling sensitivity does not necessarily indicate a change in carbon cycle response to climate change.