Using multiple observational and modeling datasets, a strengthening relationship between boreal winter sea surface temperature anomalies (SSTA) in the western North Pacific (WNP) and the development of the El Ni̱o-Southern Oscillation (ENSO) in the following year is documented. The increased WNP-ENSO association emerged in the mid 20th century and has grown through the present, reaching correlation coefficients as high as ~0.70 in recent decades. Fully coupled climate experiments with the Community Earth System Model version 1 (CESM1) well replicate the WNP-ENSO association and indicate that greenhouse gases (GHG) are largely responsible for this observed increase. We speculate that shifts in the location of the largest positive SST trends between the subtropical and tropical western Pacific impacts the low-level circulation in a manner that reinforces the link between the WNP and the development of ENSO. A strengthened GHG-driven relationship between the WNP and ENSO provides an example of how anthropogenic climate change may directly influence one of the most pronounced natural climate variabilities, ENSO and potentially improve the skill of intraseasonal-to-interannual climate prediction.