The terrestrial pool (soil and biotic carbon) is roughly four times larger than the atmospheric pool. Deforestation and agriculture have caused the loss of 320 billion tons of carbon from the terrestrial pool in the last 10,000 years, almost half of which has been since 1850. Burning fossil carbon for energy (in the form of coal, petroleum, and natural gas) has moved 292 billion tons of carbon to the atmospheric pool – most of it in the form of CO2 – and is projected to emit another 200 billions tons in the first three decades of the 21st century.
Remember the terrestrial pool of carbon, made up of soil and plants? It has lost 320 billion tons of carbon since the dawn of agriculture, but it can reabsorb much of that. Climate change will continue for centuries even if we stop emissions now because of the persistence of the greenhouse gases we’ve already released. This overshoot is why sequestration is necessary to return to 350ppm even if we reduce emissions.
Sometimes people confuse carbon and carbon dioxide. We are looking to sequester carbon— 200 to 250 billion tons of it. That carbon is captured by oxygen to form carbon dioxide, which weighs 3.67 times more. Thus the weight of the carbon dioxide we want to remove from the atmosphere is 734 to 917 billion tons. However, we usually speak of atmospheric carbon dioxide in parts per million. Every part per million of carbon dioxide is equivalent to roughly 2 billion tons of carbon. We also look at annual carbon sequestration rate, which is expressed here (and usually) in metric tons per hectare. This is equivalent to 0.4 English ton per acre. We sometimes look at the lifetime sequestration rate, which typically looks at carbon sequestered over a 20- to 50-year period until saturation is more or less reached. Finally, the term carbon stocks refers to the total amount of carbon currently held in soil and/or aboveground biomass at a particular time.