The Ian McFadyen Site
Carbon and the End of Life on Earth
There are frequent claims that increasing quantities of carbon dioxide (CO2) in the Earth’s atmosphere threaten the very future of life on the planet.
Some scientists have warned that, with rising CO2 levels, global temperatures may reach a “tipping point” where there is runaway, unstoppable warming that will make the surface of the Earth uninhabitable.
The truth is that changes in atmospheric CO2 levels can, and will, lead to the extinction of life on Earth, but not in the way these scientists are predicting. Quite the opposite: it will not be a surfeit of CO2 that extinguishes life on Earth, but its lack.
But why should the world run out of carbon?
The answer is simply because carbon is continually, and irreversibly, being lost from the atmosphere.
To explain: life as we know it came about because the Earth’s atmosphere was rich in carbon. That carbon existed only in the form of its gaseous oxide - carbon dioxide - which was a substantial part of the atmosphere for over a billion years. Around 2.3 billion years ago, a major development occurred on the planet. Bacteria and plants evolved that were capable of separating CO2 into its components - carbon and oxygen. These organisms used the carbon to build their bodies and excreted the oxygen. Over millions of years, vast amounts of oxygen were released into the atmosphere, making it possible for aerobic bacteria and oxygen breathing animals to evolve. Today, as a result, we happily inhabit the planet, breathing plant excrement.
Today our atmosphere is about 20% oxygen which gives some idea of how much CO2 there was to begin with. Since a CO2 molecule is a larger molecule that an O2 molecule (it’s a carbon molecule joined to two oxygen molecules) we can deduce that there must have been a greater mass of CO2 than the amount of oxygen that was made from it. In fact it must have been a lot more since much of the oxygen that was first produced was taken up in reactions with metallic minerals such as iron.
From this we can interpret that the proportion of CO2 in the Archean Earth must have been in the order of at least 25%. It is worth noting that that quantity of CO2 did not devastate life on Earth, in fact it encouraged it. Despite the massive release of oxygen, there was still a substantial amount of CO2 in the atmosphere throughout what was called "the Cambrian Explosion" of life - much more than we see today. How do we know? Because of the existence of coal and oil.
As we have noted, plants and animals take carbon dioxide from the air and use the carbon to build their bodies. However all plants and animals, in time, die. When they die, sometimes their bodies decay and the carbon is released back into the air, but often their bodies remain in the earth or on the ocean floor.
The Carboniferous Period - a time when the Earth was warmer and wetter than today - was characterised by vast forests, where huge insects flew through a thicker, more buoyant CO2 rich atmosphere. As its huge trees and ferns died, they formed huge peat bogs that eventually turned to coal. That coal was eventually buried under rocks and sediment until humans began to dig it out. At the same time, uncountable trillions of tiny organisms, some encased in microscopic shells of calcium carbonate died and sank slowly to the bottom, there to be covered in silt and later turn into what we now call “crude oil”. Other tiny marine organisms, the corals, built enormous coral reefs in the sea which slowly compressed and sank under their own weight. This mass of coral exoskeletons, made of calcium carbonate, eventually turned into limestone, which in turn, after more heat and pressure turned into marble.
What this story tells us is that, from the time that life evolved on Earth, carbon in the bodies of plants and animals, has been continually buried in the earth as coal, as oil and as carbonate rocks, so there has been a continuous lessening of the amount of carbon in the atmosphere. Because millions of tonnes of coal were formed in the Carboniferous, we know that there was more carbon before the Carboniferous Period than after it. And in fact that has been the case in all ages, throughout geological time.
As recently as the Miocene, a warm period about ten million
years ago, extensive deposits of brown coal were laid down in
In other words, living creatures have been sequestering carbon for at least a billion years. The amount of CO2 in the atmosphere has been decreasing steadily ever since the first cyanobacterium split a CO2 atom into its component parts.
Now, as we know, the quantity of CO2 in the atmosphere has a direct bearing on the amount of solar heat retained in the atmosphere and so, as the fraction of CO2 has decreased, so has the temperature. However, while the decrease in temperature has, with a few bumpy exceptions, been a fairly steady process, its effect on biological systems has not been. As temperatures have dropped there have been significant “tipping points” in the Earth’s ecology.
One of the most dramatic tipping points came at the end of the Mesozoic Era when CO2 levels and temperatures finally dropped to a level where many of the flora and fauna of that era could no longer survive. The most visible example was the extinction of the dinosaurs. In a (geologically) short period of a few million years, the entire ecology of the planet changed. Plants which had been happily evergreen started losing their leaves in winter to conserve energy and the cold-blooded reptiles that had relied on a warm climate and abundant plant food to survive, gave way to warm-blooded animals that could conserve heat with fur and feathers and live more frugally.
Many theories have been advanced to explain the disappearance of four whole orders of reptilian species, including theories of meteorite impact, however none of these are necessary. The simple answer was that the dinosaurs belonged to a high carbon world and could not genetically make the leap to a lower carbon world.
But the end of the Mesozoic was not the end of the story. The loss of carbon has continued to the present day. The the world’s oceans are a carbon sink. The surface of the ocean dissolves carbon dioxide from the atmosphere, forming a dilute carbonic acid. Marine organisms, mostly microscopic or at least, very small, like the krill, extract this carbon to make their shells and their bodies. When they die their tiny bodies drift like a steady rain to the bottom of seas and lakes. Even though the dilute acid of the ocean can dissolve them and re-absorb the carbon, a small but steady proportion of that carbon ends up sequestered in the sludge at the bottom of the lakes and oceans we call “ooze” - the precursor to the formation of oil.
About one million years ago, the Earth reached another carbon milestone. The temperatures dropped low enough for the Earth to be affected by what are called Milankovitch cycles. What that means is that global temperatures fell to a point where small regular changes in the Earths orbit and rotational tilt were able to “tip” the Earth into an Ice Age. Prior to this time there was enough CO2 in the atmosphere that these tiny variations did not matter but in the middle of the Pleistocene Period the CO2 dropped to a point where they did. The result was that, when the Earth’s position in relation to the Sun and its tilt on its axis caused a drop temperature on just a fraction of a degree, the Northern ice caps spread down to cover all of Canada and most of Europe. These ice caps were partly by a positive feedback cycle: as the ice spread, more of the sun’s heat was reflected back into space, which lowered the temperature and thus created more ice. This is proof that there are such things as cold tipping points. No example of a "warm" tipping point has ever been observed.
Despite these mechanisms, the ice caps eventually stopped spreading simply because there was not enough water, and enough precipitation to make the them any bigger (snow requires at least enough warmth to cause evaporation in the first place) and after many thousands of years, the ice caps retreated. There have been at least seven of these ice-ages in the last millions years, with what are called “interglacial periods” between them.
We are currently in an interglacial period.
What is significant about ice-ages is that they are a recent phenomenon. Although the Earth has experienced some periods of intense glaciation in the long distant past, including one suspected period of “snowball Earth”, there was no regular incidence of ice ages until the last million years or so. What this demonstrates is that the Earth has cooled so significantly that it is now in transition into a permanent ice-age. It is typical of systems that are in transition from one state to another to go through a phase of oscillating between the two states before settling into the new one. That is the behaviour demonstrated by the Earth over the last million years. The ice-ages were not the result of global temperatures rising and falling in a smooth curve like a sine wave. Rather the Earth has snapped in and out of two distinct climatic states - one warm and one cold - where temperatures vary randomly around a relatively stable average for the duration of the age.
However, as carbon continues to be deducted from the system, one day that world will stop oscillating between a glacial and interglacial phase and settle permanently into a glacial one.
The drop in temperature alone however, will not lead to the end of life on earth. What will is that, eventually, the level of carbon dioxide in the atmosphere will be too low to sustain plant life on the planet.
Humans of course have gone some way towards delaying this eventuality by digging up coal and oil and burning it, thereby returning it to the atmosphere. This has led to debate and widespread fears about “global warming”. However, while it is possible that the burning of fossil fuels has warmed the planet, what is certain is that - in the long run - regardless of this, the planet will cool. No matter how much coal and oil humans burn, they will never get it all and the combustible forms of buried carbon will eventually run out.
Humans also play a part withholding carbon from the atmosphere. Every time we build a house with timber, print a newspaper or file a document in a filing cabinet we are sequestering carbon. Our cities contain billions of tonnes of concrete which contains carbon, all our plastic containers are substantially carbon and all the cardboard packaging we throw in the bin are carbon. If these things are burnt, their carbon is returned to the sky, but because of our embargo on CO2 emissions, all these carbon forms are buried in landfill, locking it away from the biosphere.
Thus, though it may take hundred of millions of years, the carbon which living creatures have used to build the vast biomass which now covers the Earth will dwindle and living things on Earth will have to learn, yet again, to exist with less carbon. Modern plants, unlike their prosperous ancestors, must extract carbon from a tiny fraction of CO2 in the atmosphere - 400 parts per million or .0004. Could plants survive with as little as 200 parts per million, or 100 parts per million? And what sort of plants might they be? It becomes a fascinating thought exercise to imagine life on this cool, maybe frozen world where living organisms grow slowly, stoically clinging to every molecule of carbon they can acquire. At some point however, there will simply be insufficient free carbon molecules in the system to sustain life at all.
By that time, our ancestors will have long ago found another planet which has an abundance of carbon, or found a way to release carbon - by for example burning all the limestone and marble and other calcium carbonate in the world, or found a way to build their bodies and their brains out of some other element. Whatever the next planet we live on, I hope we learn to cherish this element of which are bodies and brains are made, and not revile it as many people seem to now.
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