The release to the atmosphere and oceans of hundreds of billions of tons of carbon from fossil biospheres, at the rate of >2 ppm CO2 per year, is unprecedented in geological history of Earth, excepting events such as asteroid impacts which excavated and vaporized carbon-rich sediments, interfering with the carbon and oxygen cycles, which led to mass extinction of species.
The emission since 1750 of over 320 billion tons of carbon (GtC) from buried early biospheres, adding more than one half of the original carbon inventory of the atmosphere (~590 GtC), as well as the depletion of vegetation, are triggering a fundamental shift in the state of the atmosphere, tracking toward conditions which exceed interglacial temperatures over the last 400,000 years and are analogous to conditions of the mid-Pliocene ~2.8 billion years ago, the last decade 2000-2010 being the warmest since instrumental measurements commenced (Figure 2).
As stated by Joachim Schellnhuber, director of the Potsdam Climate Impacts Institute, “we're simply talking about the very life support system of this planet”.
Lost all too often in the climate debate is an appreciation of the delicate balance between the physical and chemical state of the atmosphere–ocean–land-cryosphere system and the evolving biosphere, which controls the emergence, survival and demise of species, including humans. By contrast to Venus’ thick blanket of CO2 and SO2 atmosphere, which exerts extreme pressure (90 bars) at the Venusian surface, and unlike Mars’ thin (0.01 bar) CO2 atmosphere, the presence in the Earth’s atmosphere of trace concentrations of well-mixed greenhouse gases (GHG) (CO2, CH4, NxO, O3), has modulated surface temperatures during most of the Holocene within the range of -89 and +57.7 degrees Celsius and a mean of 14°C, allowing the presence of liquid water and thereby of life. By contrast to the long-lived GHG, water vapour has a short atmospheric residence time (9 days) and low concentrations over arid climate zones and the polar regions.
Forming a thin breathable veneer only slightly more than 1000th Earth’s diameter, and evolving both gradually as well as through major perturbations with time, the atmosphere acts as a “lungs” of the biosphere, allowing an exchange of carbon gases and oxygen with plants and animals, which in turn affect the atmosphere, for example through release of methane and photosynthetic oxygen.
As shown by numerous proxy-based paleo-climate studies, when the concentration of CO2 in the atmosphere rises above a critical threshold, the climate shifts to a different state. Any significant increase in the level of carbon gases triggers powerful feedbacks, including ice melt/warm water interaction, decline of ice reflection (albedo) and increase in infrared absorption by exposed water. Further release of CO2 from the oceans and from drying and burning vegetation shifts global climate zones toward the poles, warms the oceans and induces ocean acidification [3, 10].
The essential physics of the infrared absorption/emission resonance of greenhouse molecules, indicated by observations in nature and laboratory studies, is expressed by the relations between atmospheric CO2 and mean global temperature projections.
Increased evaporation in warming oceans results in enhanced, often abrupt, precipitation events and floods, as indicated by current trends since about 1980
During most of Earth’s history the oxygen-poor composition of the atmosphere resulted in a major role of reduced carbon species in the air and the oceans, including methane and carbon monoxide, allowing mainly algae and bacteria to exist. It is commonly held that, from about 0.7 billion years ago, in the wake of the Marinoan glaciation (so-called ‘Snowball Earth’), oxygenation of low-temperature water allowed development of new oxygen-binding proteins and thereby of multicellular animals, followed by development of a rich variety of organisms — the “Cambrian explosion”..
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