Slow terrestrial carbon cycle. Animation of some of the processes that form the terrestrial geological and industrial carbon cycle, part of the larger overall carbon cycle for the whole Earth. This part of the cycle sees the carbon taken up by trees and other plants accumulate on the ground as they die. Over long periods of time this carbon is buried deeper and deeper, where the high heat and pressure convert it first into peat, then lignite, and then coal (black). Coal has been used as a fuel for thousands of years, but the Industrial Revolution saw its extraction and burning increase enormously. More than seven billion tons of coal are burned annually, making it the primary source of energy for electricity generation. Burning coal produces carbon dioxide (CO2), which is released into the atmosphere. The burning of aeons-old plant material causes an imbalance in the carbon cycle, with plants unable to take up all the extra CO2 released. This has led to a rise in the atmospheric level of CO2, from around 280 parts per million in the pre-industrial era, to some 400ppm today.