The latest climate models realistically simulate the present-day climate Indo-Pacific warm pool (IPWP) area, but there is considerable inter-model spread in future projections. Here we show a strong negative relationship between the present-day of IPWP area and its future change. When using this “emergent relationship” to constrain future IPWP area change, inter-model spread of future projections is reduced by more than half compared to raw projections. This emergent constraint is found to be due to the negatively skewed distribution of sea surface temperature (SST) over the Indo-Pacific Ocean as well as a negative SST-shortwave radiation feedback in the western Pacific, which leads to a large IPWP expansion in climate models having a small IPWP area in the present-day climate. This result indicates that a more accurate simulation of the IPWP area in the present-day climate is crucial to reliable future projections of the tropical climate.