Climate and human evolution: past, present and future

The evolution of the atmosphere, oceans and biosphere can be traced from about ~3.8 billion years [Ga], through natural cataclysms, all the way to the Anthropocene—a geological era triggered by a mammal species which uniquely learnt to master ignition and split the atom. The histories of the atmosphere and of life are intertwined, from an initial Venus-like atmosphere dominated by greenhouse gases (GHG) and sulphur gases, and acid high-temperature oceans. The intermittent rise of oxygen released from photosynthesizing micro-phototrophs, oxidation of methane, sequestration of CO2 and build-up of nitrogen have led to repeated ice ages from at least as early as ~3.0 Ga. Further rise in oxygen in several stages, at ~2.4 Ga, 0.85 Ga and ~635–542 million years (Ma) allowed oxygen-binding proteins and emergence of the multicellular fauna. Cyclic Ice ages followed sequestration of greenhouse gases through weathering and erosion of mountain ranges and accumulation of marine carbonates, lowering atmospheric CO2 levels from several thousand ppm to less than 500 ppm. Examples are the Permian and post-32 Ma glacial era triggered by the formation of the Antarctic ice sheet. The advent of land plants from ~420 Ma led to the interfacing of carbon-rich land surfaces with an oxygen-rich atmosphere, a blueprint for development of flammable conditions on land. The process culminated with the emergence of Homo―a fire-igniting species magnifying its energy output and entropy in nature by orders of magnitude. Since the onset of the combustion of fossil organic carbon in the 18th century the rate of rise of atmospheric greenhouse gases of 2-3 parts per million CO2 per year has reached an order of magnitude similar to rates associated with mass extinctions of species, constituting a geological event horizon in the history of planet Earth. Andrew Glikson graduated at the University of Western Australia in 1968. He conducted geological surveys in central and Western Australia, studied the evolution of the early Earth crust in Australia, South Africa, India and Canada, investigated the effects of large asteroid impacts on the atmosphere, oceans and on mass extinction of species, and reviewed the relations between climate and human evolution.