Supernovae and Life

Part 4 - Atmosphere

Three billion years ago, the sun was 30 % less bright than it is today but methane and carbon dioxide in the Earth's atmosphere trapped enough of the sun's heat to prevent Earth from freezing over. The atmosphere consisted of nitrogen, hydrogen sulfide, methane, and 0.4 to 8 % carbon dioxide (ten to 200 times more than today's atmosphere), there was no oxygen.

There was no significant amount of free oxygen in the atmosphere or the oceans until about 2.4 billion years ago, when huge numbers of dead microbes began to be buried in the sediment at the bottom of shallow seas.   Atmospheric oxygen increased to about 10% of the present value in the Great Oxygenation Event (GOE). The oxygen combined with the atmospheric methane (a green-house gas) converting it into carbon dioxide (a much less effective greenhouse gas) and water, thereby triggering the longest period of glaciation in history, known as Snowball Earth. About the same time, atmospheric carbon dioxide did not increase substantially as the microbes were converting the carbon into more microbes and releasing more oxygen.

But for most of the past 290 million years, Earth was much warmer than it is now. Between 200 million and 45 million years ago polar ice caps were small or absent. The Eocene Epoch (between 56 and 34 million years ago) was the warmest part of the past 65 million years. The Arctic Ocean was not permanently frozen. Palm trees grew as far north as Canada and alligator relatives lived on Ellesmere Island near the North Pole.

Fossils from near the poles tell us that the yearly average temperature was as much as 25°C (45°F) warmer than it is today but the tropics were not much hotter because Earth's warmth was more uniformly distributed.

A second Snowball Earth period, about 650 million years ago, may have triggered the evolution of multicellular organisms known as the Cambrian explosion.

About 500 million years ago, the amount of oxygen in the atmosphere increased to the range 50% to 150% of the present percentage permitting the evolution of organisms that consumed oxygen, which established the current atmospheric equilibrium.

Earth's atmosphere now contains nitrogen (78%), oxygen (21%), argon (1%) with trace amounts of carbon dioxide (currently about 0.04 % (400 ppm (parts per million) by volume) and other gases.   It also contains water vapour, varying from 4% to 1% depending upon the locality. This is enough water to cover the entire land and ocean surface of the Earth with about 2.5 cm (one inch) of rain. The atmosphere is generally cooler at higher altitudes where the vapour condenses into tiny droplets of water known as clouds or fog which may coalesce into rain, ice pellets (hail) or snow.

Free molecules of oxygen normally consist of two atoms of oxygen linked together, except in the ozone layer, in the upper part of the atmosphere, where solar radiation creates molecules with three atoms known as ozone. These are unstable and are constantly created by solar radiation but benefit life on Earth's surface by preventing much of the sun's ultra violet radiation from reaching the ground.

Other traces of gas in the atmosphere, include neon, helium, methane, krypton, hydrogen, nitrous oxide, xenon, iodine, carbon monoxide, and ammonia. Helium and any free hydrogen being light gases tend to drift off into space and may be permanently escape from the atmosphere. Probably the only reason that there is any helium left on Earth is that it is constantly created by the decay of radioactive isotopes.

Water vapour represents up to 90 % of greenhouse gas volume in the atmosphere compared to a range of 9 to 26 % for carbon dioxide. Clouds act like a blanket to prevent heat from the ground radiating back into space. Conversely, they also reflect radiation from the sun back into space. Water in the form of snow and ice also reflects the sun's radiation back into space while also acting as a blanket to reduce heat loss from the ground underneath.

The oceans, which cover two thirds of Earth's surface, and atmospheric water vapour provide the planet with considerable thermal inertia which is why the planet does not freeze over at night.

The amount of carbon dioxide in the atmosphere has remained fairly stable for the past 800,000 year ranging from about 170 ppm (pars per million) during cooler periods to about 290 ppm during the warmer periods. But in the past 200 years it has increase to 400 ppm probably as a result of our use of coal, natural gas and oil.

The last ice age started about 110,000 years ago and remained until about 12,000 years ago. Sea level was lower, exposing land bridges in many parts of the world allowing Asians to cross to Alaska and begin colonizing the American continent.   20,000 years ago, the site of Canada's national capital city, Ottawa, Ontario, was buried under ice two kilometres (6,500 feet) thick. The movement of atmospheric gases and ocean currents connect all life on Earth. The molecule of oxygen you just inhaled was probably produced by a plant on the other side of the world perhaps a million years ago. The carbon dioxide and water we breath out will be used to make more plants or phytoplankton somewhere else on the planet. Similarly, the cold waters in the north Atlantic and Antarctic can absorb more oxygen and being denser sink, so that warmer water flows on top in an endless circuit as the cold water is pushed toward the equator. These slow currents move oxygen and other nutrients though out the Atlantic and across the Pacific in an endless conveyor that may take a thousand years. This permits microbes in the ocean to use sulfide or ammonium to fix carbon with oxygen that may have been released millions of year earlier.