Interhemispheric monsoon dynamics over the past 450,000 years

The global monsoon system provides freshwater to much of Earth's biota, including its human population¹. The Milankovitch theory predicts changes in Earth's solar insolation over geologic time, and these variations are expected to result in changes to the global monsoon². Internal climate forcings, such as ice cover and sea surface temperatures, may also affect the system. However, few records of monsoon variability in the Southern Hemisphere over glacial-interglacial cycles exist to test our understanding of the factors that drive variations in the global monsoon. Here we show that monsoons in both the Southern and Northern Hemispheres vary with Milankovitch forcing, and with internal boundary conditions over the past 450,000 years. We present a record of South American Summer Monsoon (SASM) variability, based on speleothem oxygen-isotope data from the Amazon Basin. South American Summer Monsoon variability and that of a major Northern Hemisphere monsoon-that is, the Asian monsoon^3,4-have been strikingly out of phase. However, we observe occasional synchronous behaviour in regions impacted by both monsoons. We identify a synchronicity threshold that is exceeded during ten events, including during glacial terminations. Comparison with palaeoclimatic forcing factors during these intervals indicates that greenhouse gas concentrations, cross-equatorial thermal gradients and sea surface temperatures in both the Atlantic and Pacific probably influence these events. This record demonstrates that the global monsoon can reorganize itself, especially during intervals of rapidly changing planetary boundary conditions.