The concentration of greenhouse gases in the atmosphere, predominantly CO2, depends upon the dynamics of generating sources and sinks. Volcanoes are a primary emitter, whereas sequestration is had in sediments, soils, and organisms.
The planet cooled throughout the Eocene epoch owing to tectonic movements. Volcanism subsided as seafloor spreading declined.
The shallow seaway between the Indian and Asian continents disappeared as the plates collided, causing the Himalayan mountain range to rise. The advent of this and other mountain chains removed CO2 from the atmosphere while fewer greenhouse gases were being pumped from volcanoes.