Major climatic-environmental transitions and ecological shifts occurred during the mid-Early Pleistocene, a time when external forcing conditions were relatively stable. To help elucidate their driving mechanisms, we perform linear-nonlinear analysis on global climate and oceanographic records. A synchronous change in these records, starting at 1.9–1.6 Ma, was accompanied by an increased obliquity signal and coeval climate regime shifts. We posit that bipolar cooling and an extension of terrestrial ice sheets across marine margins in subpolar regions increased the sensitivity of the global climate system to obliquity variations at this time. The resulting strengthening of meridional and zonal temperature gradients and shoaling of the tropical thermocline would cause enhanced aridification of East Africa and Arabia, and meanwhile a stronger East Asian winter and summer monsoon. Our study highlights the likely contribution of subpolar sea ice expansion on critical transitions in global climate and ecosystem evolution via its ability to modify the sensitivity of the Earth system to orbital variations.