Carbon - element of life

Carbon is essential to all life, but its rapid release into the atmosphere in the molecular form CO2 is upsetting equilibria established over millions of years.

Carbon atoms are formed in dying stars under extreme and improbable conditions during a set of nuclear fusion reactions known as the triple-alpha process.

On collapse of the star, in a supernova, the atoms are blown out, eventually forming new star systems such as our own.

In the first 20 minutes after the Big Bang, protons, neutrons and electrons were created. It took hundreds of thousands of years of further cooling before the average energies of nuclei and electrons were low enough to form stable hydrogen and helium atoms.

A further 500 million years passed before the first stars formed. In these stars conditions were right for nucleosynthesis, the process by which elements heavier than boron are created. Carbon was the first of these elements.

Allotropes

Carbon assumes different structural forms - allotropes. Alternate forms of the same element are created by external forces, such as heat and pressure.

There are many allotropes of carbon including those that naturally occur on earth - amorphous carbon, diamond and graphite - and rarer forms such as graphene and buckminsterfullerene.

Diamond

Diamond is arranged in a three dimensional tetrahedral lattice in which every carbon atom is bound to four other carbon atoms, in the form of a tetrahedron. The proximity of atoms and the covalent bonds between them make diamond hard; breaking their covalent bonds requires a huge amount of energy.

And because there are no free electrons, pure diamond does not conduct electricity.

Graphite

In graphite atoms form in planes. Each atom is bound to its three nearest neighbours at an angle of 120°. Since only 3 of the 4 electrons are shared, the fourth is free to move; in this way graphite conducts electricity.

Each of the individual layers of atoms of which graphite is composed is called graphene. Graphene may unwittingly be produced by writing or drawing with a pencil; friction causes a layer, or more often hundreds of layers of atoms, to be removed. A single layer is graphene.

Carbon compounds

Such is carbon's propensity to form bonds that more compounds contain carbon than not, almost ten million. Organic chemistry is the study of these compounds. Those which contain only carbon and hydrogen are called hydrocarbons. Fossil fuels are hydrocarbons.

Hydrocarbons

Fossil fuels form when dead plants and animals decompose in the absence of oxygen and are submitted to intense heat and pressure over millions of years.

The amount of energy released depends on the bonds between atoms in a molecule. Methane, which has 4 single bonds, burns at around 1,950°C in air. Natural gas, tailored for domestic use, is ~90% methane and burns at about 1,100°C.

Acetylene C2H2 burns at around 3,330°C in pure oxygen (oxyacetylene process) making it suitable for welding and cutting metals. The bulk of the energy released from acetylene is derived from the triple bond that exists between the two carbon atoms in each molecule.

Polymers

Because carbon atoms form strong bonds with other carbon atoms as well as the atoms of other elements it is able to form long chains, or polymers. Many of these are critical to life, including RNA, DNA, proteins, and sugars. It is this ability to bind to itself in long chains, form stable compounds with other elements and the proximity of electrons to the nucleus, that makes carbon the best, and perhaps the only, building block of life, not only on this planet, but in the universe.

Fast times in the Anthropocene

The carbon cycle regulates the flow of carbon through the earth and the atmosphere. The distribution of carbon throughout the cycle affects temperature which alters conditions on earth to which all life must adapt.

The temperature of the seas and the land has varied widely over geologic time. The main driver for the climate is the energy reaching us from the sun, a figure which varies according to the earth's orbit and the state of the sun itself. Temperature is also affected by random events such as meteor strikes and volcanic eruptions.

When temperature and climate change slowly evolution by natural selection is possible. Sudden disruption is catastrophic, and results in the mass extinction of huge numbers of species. There have been five such events on earth, including the Permian–Triassic extinction event (the Great Dying) which accounted for 90% to 96% of all species.

Species exist when and where conditions are favourable for adaptation and evolution; when they cannot adapt to new conditions, favourable or otherwise, they go extinct.

Many of the fossil fuels we burn today were laid down when carbon dioxide levels and temperatures were higher than today. The collapse of the Carboniferous rainforest - due to climate change - provided the organic matter that would later become coal.

By burning that coal, and other fossil fuels, we have begun to change the climate, and because the change is rapid, many species have not, and will not, adapt.

Carbon emissions and reserves

Cumulative emissions of carbon dioxide to date are ~1.5 trillion tonnes.

Remaining reserves in tonnes: 900 coal, 150 oil, 105 gas, 540 shale gas, etc. = 1.695 trillion.

Carbon is also found in methane hydrates in polar regions and under the seas. Estimates vary between 500 and 3,000 gigatonnes of carbon.

Wikipedia

The isotopic signature of anthropogenic warming

Isotopes are variants of a chemical element which differ in their number of neutrons. Carbon has 3 isotopes, carbon-12, carbon-13, and carbon-14.

How do we know the extra carbon dioxide in the atmosphere is caused by burning fossil fuels?

Plants absorb less (heavy) carbon-13 from the atmosphere than (light) carbon-12, and also favour carbon-12 for photosynthesis.

Therefore when we burn fossil fuels we are releasing carbon that has relatively less carbon-13. This difference can be detected by measuring the relative ratio of carbon isotopes in the atmosphere. The isotopic signature confirms the man-made origin of current climate change.

NASA has been monitoring the natural carbon cycle using satellites for some time including Orbiting Carbon Observatory-2 and Orbiting Carbon Observatory-3. Around 2025 they will launch the Copernicus CO2 Mission, the first operational carbon dioxide monitoring satellite constellation, focused on monitoring human-produced emissions.

The sun

The energy from fossil fuels is derived from the sun; photons absorbed by chlorophyll excite electrons and thereby enable the processes by which photosynthesis produces organic compounds. Fossil fuels are a concentrated form of this energy.

Solar and wind power are renewable energy sources because the energy we obtain from them can be replaced immediately (by the sun).

The only energy source not derived from our sun is nuclear which ultimately derives from myriad distant, and now long dead stars.

The end of carbon dependency?

For a couple of hundred years we have benefited from chemical reactions irreversible in time scales appropriate to our species. The combustion of organic compounds, especially hydrocarbons, to release energy stored over unimaginably vast periods of time, is the foundation of our modern civilisations.

Will we enjoy an equally profitable but more sustainable relationship with hydrogen?

How much do your remember?

References

• Khan Academy | Carbon, the building block of life
• University of Nottingham | Periodic Videos - Carbon
• BBC In Our Time | Carbon
• BBC Frontiers | Graphene
• Dr. Edward Murphy, University of Virginia | The Origin of the Elements
• Crash Course Biology | Carbon
• Bozeman Science | The Molecules of Life
• Khan Academy | The carbon cycle (video)
• Khan Academy | The carbon cycle (article)
• U.S. Carbon Cycle Science Program | What is the Carbon Cycle?

Related content