The biological pump is a key part of Earth's carbon cycle. It moves carbon from the air into the deep ocean by carrying organic matter from the sea surface down to the seafloor. By analyzing coccoliths from the sediment core in the northeastern Indian Ocean, we discovered that biological pump was weakest during the last ice age but strengthened significantly as the climate warmed. We found that freshwater from monsoon rains is the primary factor controlling primary productivity in this region.