We investigate the ENSO response to greenhouse warming on the basis of a greenhouse warming simulation which was conducted with the CGCM ECHAM4/OPYC3 (MPI).
In response to anthropogenic climate change one observes changes of the ENSO phase locking characteristics, bimodal splitting of the phase space trajectory, changes of higher statistical moments, such as an increase of variance, and abrupt regime transitions. All of these features can be understood in terms of a simplified nonlinear delay equation. It is argued that the thermocline sharpness is the crucial parameter which regulates the ENSO sensitivity. Furthermore, the question is addressed as to whether the recent observed changes in ENSO variability can be understood as an expression of greenhouse warming or natural climate variability. Using multivariate optimal wavelet fingerprinting we find that the null hypothesis of natural climate variability can not be rejected at a reasonable statistical level.