Annual mean temperature map for latest-Carboniferous temperatures for 150 ppm of CO2 and a cold orbital configuration, with reconstructed contours of continents. Graphic: Feulner, 2017 / PNAS

10 October 2017 (PIK) – While burning coal today causes Earth to overheat, about 300 million years ago the formation of that same coal brought our planet close to global glaciation. For the first time, scientists show the massive effect in a study published in the renowned Proceedings of the US Academy of Sciences. When trees in vast forests died during a time called the Carboniferous and the Permian, the carbon dioxide (CO2) they took up from the atmosphere while growing got buried; the plants’ debris over time formed most of the coal that today is used as fossil fuel. Consequently, the CO2 concentration in the atmosphere sank drastically and Earth cooled down to a degree it narrowly escaped what scientists call a ‘snowball state’.

“It is quite an irony that forming the coal that today is a major factor for dangerous global warming once almost lead to global glaciation,” says author Georg Feulner from the Potsdam Institute for Climate Impact Research. “However, this illustrates the enormous dimension of the coal issue. The amount of CO2 stored in Earth’s coal reserves was once big enough to push our climate out of balance. When released by burning the coal, the CO2 is again destabilizing the Earth system.”

The study examines the sensitivity of the climate in a specific period of Earth’s deep past by using a large ensemble of computer simulations. While some of the changes in temperature at that time can clearly be attributed to how our planet’s axis was tilted and the way it circled the sun, the study reveals the substantial influence of CO2 concentrations. Estimates based on ancient soils and fossil leaves show that they fluctuated widely and at some point sank to about 100 parts CO2 per million parts of all gases in the atmosphere, and possibly even lower. The model simulations now reveal that global glaciation occurs below 40 parts per million.

Burning that same coal dangerously raises greenhouse gas concentration in our atmosphere

Today, CO2 levels in the atmosphere have reached more than 400 parts per million. Carbon dioxide acts as a greenhouse gas: the Sun warms Earth’s surface, but most of the heat radiated by the surface escapes into space; CO2 and other greenhouse gases hinder part of this heat from escaping, hence warming the planet.

“We should definitely keep CO2 levels in the atmosphere below 450 parts per million to keep our climate stable, and ideally much lower than that. Raising the amount of greenhouse gases beyond that limit means pushing ourselves out of the safe operating space of Earth,” says Feulner. “Earth’s past teaches us that periods of rapid warming were often associated with mass extinction events. This shows that a stable climate is something to appreciate and protect.”

Article: Feulner, G. (2017): Formation of most of our coal brought Earth close to global glaciation. Proceedings of the National Academy of Sciences (PNAS) [doi:10.1073/pnas.1712062114]

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Formation of coal almost turned our planet into a snowball

ABSTRACT: The bulk of Earth’s coal deposits used as fossil fuel today was formed from plant debris during the late Carboniferous and early Permian periods. The high burial rate of organic carbon correlates with a significant drawdown of atmospheric carbon dioxide (CO2) at that time. A recent analysis of a high-resolution record reveals large orbitally driven variations in atmospheric CO2 concentration between ∼150 and 700 ppm for the latest Carboniferous and very low values of 100 ± 80 ppm for the earliest Permian. Here, I explore the sensitivity of the climate around the Carboniferous/Permian boundary to changes in Earth’s orbital parameters and in atmospheric CO2 using a coupled climate model. The coldest orbital configurations are characterized by large axial tilt and small eccentricities of Earth’s elliptical orbit, whereas the warmest configuration occurs at minimum tilt, maximum eccentricity, and a perihelion passage during Northern hemisphere spring. Global glaciation occurs at CO2 concentrations <40 ppm, suggesting a rather narrow escape from a fully glaciated Snowball Earth state given the low levels and large fluctuations of atmospheric CO2. These findings highlight the importance of orbital cycles for the climate and carbon cycle during the late Paleozoic ice age and the climatic significance of the fossil carbon stored in Earth’s coal deposits.

Formation of most of our coal brought Earth close to global glaciation



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