reduction of CO2 by formation of deposits

• coal

  • plants absorbed CO2.
  • they died and decayed
  • this layer of decaying plants was compressed to form coal
• oil and natural gas

  • plankton absorbed CO2
  • plankton died and were deposited in muds on the sea floor
  • they were covered over and compressed over millions of years
• limestone

  • shelled animals absorbed CO2 to make their calcium carbonate shells
  • the remains of these animals were compressed to form limestone