C60, Buckminsterfullerene
(Click on the link above to see a delightful YouTube introduction to Carbon.)
As Professor Tyson points out, carbon is the stuff of life. It is a magical pixie dust capable of combining in so many ways that life itself becomes possible. And to say that is almost the same as saying that carbon is alive! The introductory sentence of the venerable text, Organic Chemistry by Morrison and Boyd, makes a similar point: “Organic chemistry is the chemistry of the compounds of carbon.” That is to say, everything related to organisms is dependent upon carbon.
It’s important to note that carbon did not exist at all immediately after the Big Bang. Instead stars had to form and age and die. In their ageing carbon was created during nucleosynthesis. And when red giants collapsed and exploded in supernovae, outer space became inseminated with carbon-rich dust that could coalesce into new stars and new planets . . . with the potential for life. Joni Mitchell had it right when she sang, “We are stardust.”
But the form in which carbon is most most nutritious, most beneficial to life, is that currently reviled atmospheric “pollutant” -- carbon dioxide. Almost all life (say 98% or so) depends directly or indirectly on CO2 for its existence. Photosynthesis uses CO2 + H2O + light energy to create sugars and oxygen. Animals devour plants and thus derive energy and materials for further construction of life. Only a few forms of bacteria thrive without access to CO2 (either in the air or dissolved in water).
At present CO2 occurs in our atmosphere at an abundance of about 400 parts per million. The word “abundance” is perhaps a misnomer. Our atmosphere is 78% nitrogen, 21% oxygen, and 1% argon, with only trace amounts of other gases. CO2 is one of these trace gases, composing less than 0.04% of Earth’s atmosphere.
So nearly all life on Earth depends on this gas that exists in limited quantity.
While the current focus is on the recent increases in atmospheric CO2, a somewhat expanded view of history shows a dramatic drop in CO2 levels from more than 2000 ppm 60 million years ago to approximately 400 ppm today:
“Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth's history is important for a reconstruction of the links between climate and radiative forcing of the Earth's surface temperatures. Although atmospheric carbon dioxide concentrations in the early Cenozoic era (about 60 Myr ago) are widely believed to have been higher than at present, there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most important for the control of CO2concentrations over geological timescales. Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations. We estimate CO2 concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline between 55 and 40 Myr ago that may have been caused by reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial. Since the early Miocene (about 24 Myr ago), atmospheric CO2 concentrations appear to have remained below 500 p.p.m. and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 Myr ago.”
Source: Nature 406, 695-699 (17 August 2000) | doi:10.1038/35021000; Received 2 November 1999; Accepted 22 June 2000
A graph of CO2 levels in the past 500 million years (the period of multicellular life on Earth) shows a fairly precipitous decline from roughly 0.5% of the atmosphere to today’s 0.04% -- a greater than 10-fold reduction!
These declines in CO2 seem to be caused both by the gradual decline in volcanic outgassing and the gradual increase in buried carbon (i.e., coal and carboniferous rocks as well as oil and natural gas encased in rock). That buried carbon is the residue of past life that decays, flows into the sea, and is compressed into carboniferous rocks (or becomes oil and natural gas encased in rocks).
Thus, carbon that is now coal or oil was once readily available for formation of life as CO2. And a case can be made that the real threat to life on Earth has been the dramatic depletion of atmospheric CO2 in the past 500 million years.
Measures of atmospheric CO2 in the distant past are obviously indirect and imprecise. After all, no humans were alive to take measurements and keep records. We depend on inferential evidence from the geologic record. Such evidence may well be incomplete and inaccurate. Similarly, evidence about Earth’s average temperature in the distant past is very likely imprecise. Still, here is a graph of the information we have:
The following graph from the same wiki page shows another view of Earth’s temperature on a linear time scale that emphasizes more recent periods of time:
And here is a graph that overlays CO2 and temperature with the time scales going in the same direction:
If there is any correlation between Earth’s average temperature and atmospheric CO2, it is not immediately obvious. One could, perhaps, argue that in recent times (the past 100 million years) CO2 has dropped from 2000 ppm to less than 400 ppm while Earth’s average temperature has fallen by about 14 degrees Celsius. In very recent times (the past 20,000 years) Earth has begun emerging from one of its deepest Ice ages.
For me, the key facts are these: CO2 is the basis of advanced life on Earth. Its precipitous decline over the past 500 million years should be a cause for concern. Burning of fossil fuels has returned some small percentage of buried carbon back into the atmosphere, but CO2 levels are still alarmingly low. Planet Earth is emerging from one of its deepest Ice ages. This began before humans started burning fossil fuels and is continuing. Increased atmospheric CO2 is leading to more robust plant growth. It is so far unclear whether the changing climate will benefit human life as well as plant life. If the percentage of CO2 in the atmosphere continues to increase, the potential warming caused by the greenhouse effect is unlikely to be inimical to life unless it far exceeds the levels found 200 million years ago. Having claimed that, however, I cannot claim to have any idea at all of the potential effect of global warming on human life! But significant warming is inevitable unless everything we know about the temperature history of Planet Earth is wrong. Human beings will have to adapt or die. But life itself will thrive in conditions of increased atmospheric CO2, and since humans are clever and have proven themselves capable of surviving under the sea and in outer space, I suspect we can find a way to thrive on a somewhat warmer planet. We'd better!
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