Coupled ocean-land-atmospheric general circulation models are the only tools to quantitatively attribute past climate change in response to human activities and to predict the magnitude of future climate change.These models are known to differ considerably in their equilibrium sensitivities to external forcing, largely because of differences in atmospheric feedbacks, especially in cloud-climate feedbacks.Yet, all models that participated in the IPCC(Inter-governmental Panel on Climate Change) Assessment Report(AR4) showed very similar simulations of the 20th century climate by these models.This paper investigates the causes and implications of this similarity.It shows three equally important physical mechanisms acting together to reduce the sensitivity of transient climate change to model differences.These include the longer time delay of climate response to forcing, more heat exchange with the deep ocean, and larger impact of oceanic upwelling in models with larger atmospheric feedbacks, but inter-model differences will show up at longer time scales.