A reliable projection of extreme El Niño frequency change in a future warmer climate is critical to managing socio-economic activities and human health, strategic policy decisions, environmental and ecosystem managements, and disaster mitigations in many parts of the world. Unfortunately, long-standing common biases in CMIP5 models, despite enormous efforts on the numerical model development over the past decades, make it hard to achieve a reliable projection of the extreme El Niño frequency change in the future. While increasing attentions have been paid to estimate possible impacts of models' biases, it is not yet fully understood whether and how much models' common biases would impact the projection of the extreme El Niño frequency change in coming decades. This is an urgent question to be solved.