What is carbon dioxide? Greenhouse gases

Carbon dioxide, or CO2, is an odorless and colourless gas that is vital for life as we know it. It is a naturally-occurring compound composed of one carbon atom covalently double-bonded (sharing electron pairs) to two oxygen atoms. It is also a greenhouse gas.

CO2 exists in our atmosphere as a trace gas at a concentration of approximately 400 ppm (parts per million) or 0.04%. Natural sources include geysers, hot springs and volcanoes – it is freed from carbonate rocks by dissolution in acids and water.

Carbon dioxide is soluble in water – it dissolves in water – and occurs naturally in lakes, rivers, seawater, groundwater, ice caps and glaciers. It is also present in deposits of natural gas and petroleum.

Carbon DioxideA carbon dioxide molecule consists of one carbon atom and two oxygen atoms.

There is CO2 on Earth and in space

CO2 is one of the commonest and simplest molecules in the Universe. There are carbon dioxide molecules in outer space, where they formed in nebulae after supernova explosions.

As space is extremely cold, the CO2 out there is in the form of ice – CO2 freezes at -56.4°C (-69.5°F). When stars formed out of these nebulae, some of the CO2 ice became part of the planets. That is why there is carbon dioxide not just on Earth, but also on Mars, Venus and other planets.

Earth’s surface is considerably warmer than space, so our carbon dioxide melted and became a gas – the CO2 molecules float in the air. Even though a tiny percentage of our atmosphere consists of carbon dioxide, if it did not exist there would be no life on Earth.

Plants make their cells principally out of carbon. They get their carbon by breathing in CO2 and breaking off the oxygen, which they breathe out (and keep the carbon). So, plants are made of the carbon in carbon dioxide molecules, while the oxygen becomes the oxygen we and other animals breathe.

CO2 and the carbon cycleCO2 is a major part of the carbon cycle. According to NASA: “The carbon cycle traces carbon’s path from the atmosphere, into living organisms, then turning into dead organic matter, going into the oceans, and back into the atmosphere. Scientists describe the cycle in terms of sources (parts of the cycle that add carbon to the atmosphere) and sinks (parts of the cycle that remove carbon from the atmosphere).” (Image: climatekids.nasa.gov)

The carbon returns to the atmosphere when a plant dies and decays, or burns – it mixes with oxygen to form carbon dioxide molecules again.

Greenhouse gases and the greenhouse effect

Carbon dioxide, along with methane, water vapour, nitrous oxide and ozone are greenhouse gases – they are responsible for the greenhouse effect.

The greenhouse effect is a natural process in which a planet’s – such as Earth’s – atmosphere traps some of its parent star’s energy, warming it enough to support life. Our parent star is the Sun.

It is thanks to the greenhouse effect that we, other animals, plants and all life forms on Earth exist.

Without methane, CO2 and other greenhouse gases, Earth’s surface temperature would average -18°C (0°F), rather than the current 15°C (59°F) – our days would reach 123°C (253°F), while at night the temperature would plunge to -153°C (-243°F), like on the surface of the Moon.

CO2 concentrations since Industrial RevolutionSince the middle of the last century, CO2 concentrations have been increasing at a much faster rate. The graph ends in 2010 – the figure for today is much higher. (Image: gsu.edu)

CO2 concentrations rising

Carbon dioxide concentrations have been ideal for life to thrive most of the time on Earth for hundreds of millions of years, as have concentrations of the other greenhouse gases. Sometimes they have fluctuated for various reasons, especially before and during some of the mass extinctions.

At the moment, scientists and policymakers across the world are becoming increasingly concerned about CO2 concentrations, which have been rising since 1750 – the beginning of the Industrial Revolution. Carbon dioxide atmospheric concentrations have increased by over 40% since 1750, and will probably continue going up.

Levels of other greenhouse gases have also risen. Earth’s temperature has increased since the industrial revolution. Experts predict that our planet’s surface and atmosphere will get warmer and warmer. There is a risk that the polar ice caps will melt, which would cause sea levels to rise.

Sources of carbon dioxide USAThese estimates are just for the United States for the period 1990-2014. (Image: epa.gov)

Millions of people across the world who live in cities and towns on the coast could end up with nowhere to live if their homes become submerged.

CO2 is a heat-trapping greenhouse gas – the more of it there is in Earth’s atmosphere, the warmer it gets. Whenever we burn fossil fuels such as natural gas, oil and coal – whether it is to make electricity, manufacture products, warm our homes, or drive our cars – we are producing CO2.

Carbon is not only found in the atmosphere. Our oceans store huge quantities of carbon, and so do deposits of coal, oil, natural gas deep underground, and plants. Carbon moves naturally from one part of our planet to another through the carbon cycle.

At the moment, we are adding carbon (in the form of CO2) to the atmosphere faster than the natural processes can remove it. That is why carbon dioxide concentrations in our atmosphere are increasing, which is causing global warming (climate change).

Video – What is the Carbon Cycle?

This video explains in simple terms what the carbon cycle is and where CO2 fits into it.