Milestones in the evolution of the atmosphere with reference to climate change

The history of life on Earth is critically dependent on the carbon, sulfur and oxygen cycles of the lithosphere – hydrosphere – atmosphere – biosphere system. An Archean oxygen-poor greenhouse atmosphere developed through: (i) accumulation of CO₂ and CH₄ from episodic injections of CO₂ from volcanic activity, volatilised crust impacted by asteroids and comets, metamorphic devolatilisation processes and release of methane from sediments; and (ii) little CO₂ weathering-capture due to both high temperatures of the hydrosphere (low CO₂ solubility) and a low ratio of exposed continents to oceans. In the wake of the Sturtian glaciation, enrichment in oxygen and appearance of multicellular eukaryotes heralded the onset of the Phanerozoic where greenhouse conditions were interrupted by periods of strong CO₂-sequestration through intensified capture of CO₂ by marine plants, onset of land plants and burial of carbonaceous shale and coal (Late Ordovician; Carboniferous – Permian; Late Jurassic; Late Tertiary – Quaternary). The progression from Late Mesozoic and Early Tertiary greenhouse conditions to Late Tertiary – Quaternary ice ages was related to the sequestration of CO₂ by rapid weathering of the emerging Alpine and Himalayan mountain chains. A number of peak warming and sea-level-rise events include the Late Oligocene, mid-Miocene, mid-Pliocene and Pleistocene glacial terminations. The Late Tertiary – Quaternary ice ages were dominated by cyclic orbital-forcing-triggered terminations which involved CO₂-feedback effects from warming seas and the biosphere and albedo flips due to ice-sheet melting. Since ca AD 1750 human emissions were ～305 Gt of carbon, as compared with ～750 Gt C in the atmosphere. The emissions constitute ～12% of the terrestrial biosphere and ～10% of the known global fossil fuel reserve of ～4000 Gt C, whose combustion would compare to the ～ 4600 Gt C released to the atmosphere during the K – T impact event 65 million years ago, with associated ～65% mass extinction of species. The current growth rate of atmospheric greenhouse gases and global mean temperatures exceed those of Pleistocene glacial terminations by one to two orders of magnitude. The relationship between temperatures and sea-levels for the last few million years project future sea-level rises toward time-averaged values of at least 5 m per 1°C. The instability of ice sheets suggested by the Dansgaard – Oeschinger glacial cycles during 50 – 20 ka, observed ice melt lag effects of glacial terminations, spring ice collapse dynamics and the doubling per-decade of Greenland and west Antarctic ice melt suggest that the Intergovernmental Panel on Climate Change's projected sea-level rises (<59 cm) for the 21st century may be exceeded. The biological and philosophical rationale underlying climate change and mass extinction perpetrated by an intelligent carbon-emitting mammal species may never be known.