Enhanced Pacific meridional and zonal temperature gradients from ~2 to 1 million years ago (Ma) and the subsequent shift of glacial cycles in the Mid-Pleistocene Transition (MPT) from 1.2 to 0.7 Ma are remarkable in Pleistocene climate history. However, their influence on Indo-Pacific Warm Pool hydroclimate is unclear despite its global importance. Here we study the response of Papua New Guinea precipitation to changes in climate background conditions during these transitions using 1.68 million years of records of leaf wax deuterium and carbon isotopes, and pyrogenic polycyclic aromatic hydrocarbons from the Bismarck Sea. Dry climates dominate at peak interglacials through the ~1.7 My record, likely in response to higher CO2 and smaller continental ice sheets that shifted the tropical rain belt northward. Interglacial fire activity intensified at ~1.25 Ma, indicating an increased severity of droughts possibly due to intensification of El Niño events.