Plains CO2 Reduction (PCOR) Partnership

Carbon and CO2 on Earth – Things Have Changed!

Carbon is a naturally occurring element on Earth and in other places in the universe. The amount of carbon today is the same as when Earth was formed. Carbon Dioxide dissolves into the ocean Over time, the carbon atoms have been relocating and recombining, and the result has been major changes to Earth’s atmosphere, biosphere, and surface.

Scientists tell us that Earth started out about 4.5 billion years ago as a ball of hot gases. About 3.5 billion years ago, Earth was cool enough to have a solid crust as well as a primitive atmosphere. At the same time, CO2 was being formed deep in the earth by geologic processes, and this CO2 was being released to the atmosphere through volcanic eruptions. During the subsequent 2 billion years, biological activities began developing, evolving, and expanding, fed by the CO2 in the atmosphere. As biological production exploded over the last billion years, much of that atmospheric CO2 was absorbed by plants and animals and buried in ocean sediments.

Prehistoric Atmospheric CO2 Concentrations
Estimated atmospheric CO2 concentrations over the most recent 600 million years (up to 1950).
Modified from

CO2 Sequestered in Limestone

According to scientists, the atmosphere of half a billion years ago contained about 20 times the CO2 it contains now. This was just at the time that aquatic animals were developing hard shells. Carbon was the major chemical building block in animals. Animals developing the ability to make hard structures used carbon dioxide dissolved in the oceans. For example, tiny coral animals combined the CO2 with calcium to make their skeletons. Over time, massive accumulations of these skeletons made thick, carbon-rich deposits we call coral reefs. Many of these reefs were buried, preserving them in the geologic record; we know them as limestone. A lot of the carbon that was once in the atmosphere is trapped in the limestone rock now buried deep underground.

Coral Garden, Great Barrier Reef, Cairns, Queensland, Australia

CO2 Sequestered in Coal

Plants live by absorbing carbon dioxide using the carbon (with water and energy from the sun) as food and building materials—photosynthesis. As plants became plentiful on the land, they were removing more and more CO2 from the atmosphere. Geologists call the time period of 400 million years ago the Carboniferous Period (“full of carbon”) because so much carbon was taken from the air by plants. This carbon was sequestered when the plants were buried and eventually preserved in deposits of carbon-rich coal.

Carbon-Rich Coal.

CO2 Sequestered in Oil and Gas

The oceans teem with tiny organisms like algae, bacteria, and plankton. When they die, their bodies sink to the bottom of the ocean and are buried in sediments. When conditions are right (for example, a very low level of dissolved oxygen in the water), they may be preserved. If buried under more than 10,000 feet of sediments and with sufficient temperature and pressure, the bodies of these marine animals may eventually be transformed into oil and gas. Although the process continues today, most of the oil we produce comes from organisms that lived and died either 400 million or 60 million years ago. Major deposits of oil and gas are distributed across our planet.

Sequestered CO2 Returns to the Atmosphere

Scientists tell us that the amount of CO2 in the atmosphere has ranged considerably over the last half billion years. At the beginning of the age of mammals about 60 million years ago, CO2 levels were about twice what they are today.

The long-term trend has been a decrease in carbon in the atmosphere. Over the last 200 years, CO2 levels have been increasing, and human activity may be part of the reason. When humans burn fossil fuels, heat limestone to make lime for cement, or plow land for farming, carbon sequestered long ago is released to the atmosphere. These practices have made humans a participant in the carbon redistribution game. Scientists are now attempting to address the increasing CO2 concentrations in the atmosphere by looking at the natural processes that lead to carbon storage and developing sequestration strategies as part of the larger effort to address concerns about climate change.1

  1. Pacala, S., and Socolow, R., 2004, Stabilization wedges—solving the climate problem for the next 50 years with current technologies: Science, v. 305, p. 968–972.