The Story of our 4.5-Billion-Year-Old Atmosphere(s)
By: Donald MacPhail
In the beginning, there was no climate. The Earth was without atmosphere and void of oceans. Our planet was too hot to touch and incapable of supporting life.
As the Earth crusted over, volcanic activity became intense. Volcanoes, as they do today, spewed far more gas and steam than lava. These gases formed our early atmosphere and the steam, as it condensed, formed our oceans. But this atmosphere was not the comfy cushion of gases we take for granted today.
Our atmosphere today, if we ignore a dozen gases that make up about 1% of it, is 80% nitrogen and 20% oxygen. But volcanoes primarily spew carbon dioxide, methane, water vapour and minor amounts of nitrogen.
So how did our atmosphere get to be like it is today?
It evolved. Just as we have evolved, our atmosphere has evolved. Recent research into our planet’s past, coupled with what we now know about the atmospheres of other planets, indicates a fascinating history. Maybe surprisingly, life and our atmosphere have heavily influenced each other; they appear to have co-evolved.
It wasn’t long after our oceans formed that life first appeared in them. All life for the first couple of billion years was single-celled and microscopic. It thrived in the oceans that were kept warm by the dense carbon dioxide and methane-rich atmosphere and the green- 13 NB Naturalist house-like conditions those gases created. In fact, it is likely that without these greenhouse conditions the Earth’s temperature would not have been within the so-called “habitable zone” where life is possible.
Starting about two and a half billion years ago, certain living organisms began to have an impact on the atmosphere that had enabled them to exist. These were not modern, large, highly complex organisms; they were microscopic, aquatic organisms. These first climate changers were the blue-green algae, now known as cyanobacteria. Around three billion years ago, half a billion years after life first appeared on Earth, they “invented” photosynthesis.
Photosynthesis was a good idea, but, like many good ideas, it had consequences. The major consequence is that photosynthesis produces oxygen as a waste product. For half a billion years or so, the oxygen was absorbed by the abundant iron that was found on the surface of the Earth or in the water. Eventually though, with the cyanobacteria continuing to thrive and produce oxygen, but with no more iron to oxidize, something had to change. With nowhere else to go, the oxygen stayed in the atmosphere, increasing slowly to about 20% of the atmosphere like it is today. This oxygenated air became our present Earth’s atmosphere – but triggered what biologists refer to as the Great Oxygen Catastrophe.
It was a catastrophe because oxygen was lethal to most organisms and many went extinct. Of course, other species, including our direct ancestors, adapted and thrived on the new opportunities provided by the oxygenrich environment.
The basic ingredients of this atmosphere have not changed since the addition of oxygen starting more than two billion years ago. What has varied considerably is the relative abundance of the gases. Two of those gases – oxygen and carbon dioxide – have been critical to the development of life.
Oxygen, a waste product of photosynthesis, has been increasing for more than two billion years, but only slowly. Even 500 million years ago, at what has become known as the Cambrian Explosion when multi-cellular life became abundant, the oxygen content of our atmosphere was just 15-20% of its current level. Cambrian-aged New Brunswickers would have had less oxygen to breathe than those who get to the top of Everest today.
Many scientists believe that large complex life could not move onto the land until oxygen levels in the air increased even further. The level of oxygen in Earth’s atmosphere which was required to support the metabolism of such creatures was reached “just” 350 million years ago – approximately the age of the oldest known amphibian fossils.
Carbon dioxide levels have varied too, but have been more variable than oxygen. Comprising as much as 25% of our atmosphere at one time, carbon dioxide has been on a long, generally downwards, decline towards the trace amount of around 0.04% today. Even within the last few million years, carbon dioxide levels have, at times, been several times higher than they are today. Much field research is going on in this area now, but there is evidence from 30 million years ago that there were palm trees in the Arctic and from 10-15 million years ago that there was a short period of +10?C average summer temperatures in the Antarctic.
The geological record often shows that changes in carbon dioxide occurred very gradually, but it also shows instances of very rapid changes. Changes have been linked to the start or end of ice ages, variations in solar activity, movements of tectonic plates which periodically cause increases or decreases in volcanic activity, and occasional cataclysmic impacts with asteroids. And now to us as well.
For 3.5 billion years, life on Earth has been shaped by our atmosphere and oceans; in the last one-tenth of that time, life has increasingly shaped the oceans and the atmosphere in a process that is accelerating exponentially
in this industrial age.
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