Continental movement folds the lands. Extensive forests of mosses, horsetails and tree ferns rise in massive basins during the sultry, swampy Carboniferous Period. These plants practice "giant-ism." Some of their descendants, today’s club mosses, will follow an alternate evolutionary strategy: when things get tough, get smaller.
Dead vegetation does not completely decay in these swamps. The dead organic matter accumulates in huge "carbon sinks." The burning of fossil fuel (coal, oil and gas) during the 19th and 20th centuries has already consumed a substantial fraction of the fossil fuel laid down during the 70 million years of the Carboniferous Period. This combustion has significantly raised the carbon dioxide content of the atmosphere, risking green-house warming of Earth.