Calling it a pump is convenient. It is also slightly misleading. No piston drives carbon downward. A living network does.
Step One: Photosynthesis
Phytoplankton use sunlight and dissolved carbon dioxide to build organic matter. Some of that matter is eaten almost immediately. Some becomes particles that sink.
Step Two: The Messy Descent
Sinking particles are consumed, fragmented, and transformed. Zooplankton migrate. Microbes respire. The amount that reaches the deep ocean depends on ecology as much as physics.
Why Warming Complicates the Story
Warmer water can stratify the upper ocean, altering nutrient supply. Smaller plankton may dominate in some regions. The pump does not simply turn up or down everywhere.
