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OUT OF THIS WORLD | What's Up In Climate Change - a glance at the most important news about our warming world

Everything you need to know on CO2 (but are afraid to ask)


Scott Sutherland
Meteorologist/Science Writer

Thursday, June 22, 2017, 1:51 PM - There seems to be a lot of confusion going around about carbon dioxide and climate change, so here's how we know that CO2 not only causes climate change, but is the primary driver of the current warming trend of our planet.

WHAT IS CARBON DIOXIDE?

Carbon dioxide, or CO2, is a naturally-occurring molecule, consisting of two atoms of oxygen bonded to one atom of carbon.

It is considered a 'trace gas', so it represents a very small portion (about 0.04 per cent) of the total composition of Earth's atmosphere.

Throughout most of the normal range of temperatures on Earth - from -78.5oC, when it freezes into dry ice, to +58oC, which is the highest recorded temperature on the planet - carbon dioxide is a gas. This is a very important part of why carbon dioxide is so significant for climate change, even though it makes up a fairly small part of Earth's atmosphere.

WHY IS CARBON DIOXIDE IMPORTANT?

Carbon dioxide is what's known as a greenhouse gas, due to its ability to absorb and retain heat energy.


An idealized carbon dioxide molecule.
Credit: Wikimedia/Scott Sutherland

As Earth basks in the light from the Sun, much of that direct sunlight is able to pass through the atmosphere unaffected. X-rays are absorbed in the upper levels of the atmosphere, the more potent wavelengths of ultraviolet light are absorbed by ozone (O3) molecules in the stratosphere, but the air is effectively transparent to the rest, which is absorbed or reflected when it reaches the ground. The absorbed light is stored as energy, which causes the ground, water and objects affected to heat up, and they radiate most of that excess energy away as infrared light.

Here's where carbon dioxide's 'greenhouse effect' comes into play.

Human civilization has enjoyed fairly moderate temperatures throughout its history, but with Earth's size and distance from the Sun, the planet's "effective temperature" is actually -21oC (252 K). That is the average temperature of the globe if the atmosphere was completely transparent to all wavelengths of light - direct sunlight comes in, heats the surface and infrared radiation is emitted directly into space. This is what Earth would be like if its atmosphere had no greenhouse gases, and was only a mixture of nitrogen, oxygen and argon (the three primary gases in the air).

With carbon dioxide and other greenhouse gases, however, and the average temperature of Earth goes up to around 15oC (288 K). So, just the tiny amount of greenhouse gases (0.04 per cent of the atmosphere) is able to raise Earth's temperature by 36oC!

How does this happen?

All atoms and molecules vibrate (as long as they are at a temperature above absolute zero), and it just so happens that the vibrations of carbon dioxide molecules match certain wavelengths of the infrared light emitted by objects and the ground. This doesn't require any special circumstances. Simply by being present in the atmosphere, carbon dioxide absorbs infrared light. It also radiates infrared light, but that is then absorbed by the ground, objects or different CO2 molecules, and the process repeats.

This has been likened to what happens when you put a blanket over you. The fibers of the blanket, and the air trapped between those fibers, absorbs and traps the heat from your body, preventing it from quickly escaping into the larger volume of air in the room. So, by absorbing the infrared radiation emitted by the Earth, carbon dioxide keeps that heat in the atmosphere longer, and the more CO2 added to the air, the 'thicker' the 'blanket' is.

The same thing happens with molecules of methane (one carbon atom and 4 hydrogen atoms, denoted CH4), and with water vapour (H2O), but there's a very specific reason why carbon dioxide is more important to the planet, and to the issues of global warming and climate change.

Methane is generally shorter-lived in the atmosphere than carbon dioxide (12 years, compared to 5-200* years for CO2), but its four carbon-hydrogen bonds have the potential to trap more infrared radiation, making it far more potent - molecule for molecule - on short time scales. However, there is over 200 times more carbon dioxide in the atmosphere than methane. So, while increasing concentrations of methane in the atmosphere are troubling, carbon dioxide still plays the larger role, right now.

As for water vapour, there is about 10 times more of it in the atmosphere than carbon dioxide (estimated, on average), but there is a major limiting factor for the contribution of water vapour to the greenhouse effect - its presence as a greenhouse gas in the atmosphere is highly dependent on temperature..

Remember, from above, how Earth's temperature without greenhouse gases would be around -21oC? That is well below the temperatures at which water vapour will first condense out of the air as liquid, fall out of the sky, and then freeze solid on the ground.

Carbon dioxide doesn't actually have a liquid phase on Earth (the atmospheric pressure needs to be much higher). Instead, it remains a gas all the way down to -78.5oC, at which time it solidifies directly into ice. Temperatures on Earth can briefly get down below that point, but they don't stay that cold long enough to keep carbon dioxide solid. So, since it stays in gaseous form, carbon dioxide keeps the environment warm enough (at least for a significant portion of the Earth) so that water can remain in vapour form, and thus contribute to the greenhouse effect.

Also, as carbon dioxide concentrations increase, this causes more warming, and because warmer air holds more water vapour, the atmosphere takes up more water vapour, and this feeds into a positive feedback loop.

So, while water vapour is abundant, and an important part of the overall greenhouse effect, without carbon dioxide and other greenhouse gases, which can remain gaseous below 0oC, another feedback loop would start - as water vapour condensed in cooler weather and climates, the lower concentration of water vapour would allow more infrared radiation to directly escape the atmosphere, thus the atmosphere would cool, causing more condensation, etc. Eventually temperatures would drop below zero, the water would freeze and you'd end up with a snowball Earth.

*Note: Each carbon dioxide molecule stays in the atmosphere for a period of between 5 and 200 years, before it is taken up by one of the processes of the carbon cycle (see below). Due to other processes in the cycle, however, they often simply exchange places with a carbon dioxide molecule that is released. Therefore there is a substantial amount of the excess carbon dioxide (over and above the average of ~240 ppm) that remains in the atmosphere. Studies have shown that this is somewhere around 20 per cent, and this excess can actually persist for a few thousand years before it is processed into mineral forms, such as calcium carbonate.

Bottom line: Either due to its relative abundance, or because it remains a gas throughout the most of the range of Earth's temperatures, carbon dioxide is the primary driver of human-caused climate change at this time.

IF CO2 IS NATURAL, WHY IS IT CONSIDERED A POLLUTANT?

No doubt, you've heard the phrase "carbon dioxide is plant food", used as an attempt to counter the idea that carbon dioxide is a pollutant that needs to be regulated and controlled.

It's true that plants use carbon dioxide in their respiration, just as humans use oxygen. However, over the past 800,000 years or so, plants have adapted to conditions with relatively stable concentrations of carbon dioxide in the atmosphere, of about 240 parts per million, give or take about 20 per cent (that is, in a million molecules of air, between 200-280 of those molecules would have been CO2). As the graph below shows, any major changes in CO2 levels during that time took place over thousands, to tens of thousands of years.


The Keeling Curve, showing the past 800,000 years of carbon dioxide concentrations in the atmosphere. Credit: Scripps Institution of Oceanography/UC San Diego

Over the past 150 years, in a relative blink of the eye on the above chart, the burning of fossil fuels - coal, oil and gas - has increased the concentration of carbon dioxide in the atmosphere to around 410 parts per million. That's an increase of nearly 50 per cent over the pre-industrial concentration (280 ppm) and it's a roughly 70 per cent increase over the long-term average set during the past 800,000 years.

If you increased oxygen levels in the atmosphere by those amounts, some things would become easier, but it would not be better for us, overall. Certain types of athletes would find their chosen sport or athletic activity easier, for example. However, the human body is accustomed to a specific range of oxygen levels. Too low and you suffer from hypoxia - too little oxygen is reaching your tissues, especially your brain, and you can die. Too high and your system becomes overloaded, leading to oxygen toxicity, which can cause long-term health issues. There would also be an increase in oxidation (metal objects would rust and corrode faster) and more frequent wildfires (and possibly more fires in general), as it would become easier to spark a flame.

Also, even though plants do benefit from high concentrations of carbon dioxide, they don't tend to benefit in ways that are beneficial to us. For example, plants that we characterize as weeds tend to do the best with higher carbon dioxide concentrations, and while those plants we grow as food may grow larger, the parts of them that we eat have been found to contain fewer nutrients and vitamins. So, while we may get more or larger food plants out of this, we'd have to increase the amount we consume in order to get the same benefits. So it evens out, at best, and certainly does not compensate for all the other negative effects of climate change.

There is also the issue of ocean acidification, which is when the ocean water absorbs carbon dioxide, and it is converted to carbonic acid. This changes the pH of the ocean water, from slightly basic towards neutral, which makes it more difficult for certain basic species to survive.

Bottom line: More of something is not necessarily a good thing in nature, and nature has been "used to" a certain amount of carbon dioxide for close to a million years. Since the excess can and does cause problems in nature (and thus for humans and human civilization), this excess can rightly be considered a pollutant that needs to be limited and cleaned up.

WHAT ABOUT THE CARBON CYCLE?

The carbon dioxide in the air during that 800,000 years was caught up in a cycle. Plants take it in, use it for respiration, splitting the bonds between the oxygen and the carbon, taking up the carbon and expelling the oxygen. Animals inhale the oxygen, which bonds to their blood, carrying energy through their body, picking up a carbon atom along the way, and is expelled as carbon dioxide. Carbon dioxide is taken up by the soil, and also released by the decay of organic matter. Carbon dioxide dissolves into the waters of the oceans, and outgasses from those same waters, in a fairly even cycle, as well.

Scientists have identified this whole system as The Carbon Cycle.


The Carbon Cycle. Credit: US Department of Energy/NASA Earth Observatory

However, many millions of years ago, several global disaster killed and buried an overwhelming number of plant and animal species, which have slowly decayed, and the carbon that was locked up in those forms of life was converted into a soup of organics that we now use as fossil fuels - coal, oil and gas. In the process, though, the carbon contained in those fossil fuels had been completely removed from the natural carbon cycle for those millions of years.

So, the world adapted to having a much lower amount of carbon in the carbon cycle, and subsequently, a much lower amount of carbon dioxide in the oceans and atmosphere. Simply based on the relative lengths of time that we're talking about here, the level of carbon dioxide that has been present in the atmosphere over the past 800,000 years can be considered the "normal" amount, especially since that's the level that has kept the climate fairly stable during the development of human civilization.

Now, over the past 150 years, humanity has been digging up these deposits, and that carbon, which was locked away for so long, has been released back into the environment. Thus, the fairly careful balance the world had struck - with carbon exchanged between plants and animals, ground and water, with just enough lingering in the atmosphere to keep the planet warm - is being tipped out of balance.

Bottom line: While there is certainly a natural component of carbon that gets circulated in the carbon cycle, this is a carefully balanced system that requires a lot of time to adapt to a new equilibrium. Humans are throwing the system out of balance by disrupting the equilibrium on a time scale that is far too short for the system to effectively adapt to.

HOW DO WE KNOW THAT IT'S THE CARBON FROM FOSSIL FUELS THAT'S THE PROBLEM?

We know that it's the emitted carbon dioxide from fossil fuels that's causing the imbalance due to three things:

1) Scientists have verified in the laboratory, a long time ago, that if you increase the concentration of carbon dioxide in the air, that air will retain more heat. They have described it like throwing an insulating blanket over the atmosphere.

2) As nations around the world have been burning more coal, oil and gas, carbon dioxide emissions from these sources have increased to around 20 billion tonnes per year. Not even volcanoes can match that amount.

3) The carbon atoms in carbon dioxide come in different 'isotopes' - the standard carbon-12, the rare carbon-13 and the extremely rare carbon-14 - and scientists have found that different sources of carbon dioxide have different ratios of these isotopes. One thing they've noticed over several decades of observations is that while the total amount of carbon dioxide in the atmosphere is increasing, the relative ratio of carbon-13 to carbon-12 atoms represented in global carbon dioxide has been decreasing. So the source of the extra carbon dioxide being added to our atmosphere must have a lower number of carbon-13 atoms than the carbon dioxide that has been in the carbon cycle for the past 800,000 years. It just so happens that the carbon dioxide released by burning fossil fuels has a ratio of carbon-13 to carbon-12 that's roughly 2 per cent lower than the rest. Thus, the excess carbon dioxide that is being added to the atmosphere is most certainly what we're releasing from the burning of fossil fuels.

Bottom line: The work of scientists over several decades points the finger of blame for the excess carbon dioxide in our atmosphere and oceans squarely at the burning of fossil fuels.


WHAT ABOUT THE TIMES WHEN WARMING OF THE PLANET CAUSED CARBON DIOXIDE TO INCREASE?

Yes, in the past, long before humans started digging up and burning fossil fuels, there were times when the increase in carbon dioxide in the atmosphere came after temperatures began rising. The initial temperature rises, in these past cases, were usually due to geological events (volcanoes, asteroid impacts, etc) or changes in Earth's axis of rotation or orbit around the Sun.

However, while carbon dioxide does increase due to rising temperatures, this is due to a positive feedback loop - temperatures rise, carbon dioxide outgasses from deposits frozen in permafrost and ice, or from under the oceans, which causes even more warming, etc.

Thus, these past events confirm that raising carbon dioxide levels in the atmosphere causes the atmosphere to warm. It's just that there was a different, natural trigger for the initial increase in temperature for those past events.

For this current warming episode, the initial trigger for the warming is human activity dumping millions to billions of tonnes of carbon dioxide back into the atmosphere - carbon dioxide that was sequestered away from nature and the natural carbon cycle for millions of years. This trigger will, in turn, cause even more carbon dioxide (and methane) to be released into the atmosphere, which will add to the human-caused temperature rise.

Bottom line: It is greenhouse gases, emitted by human activity, and carbon dioxide in particular , that is responsible for the current warming period.

Editor's note: A previous version of this article stated that methane has a longer lifetime in the atmosphere than carbon dioxide. It takes around 12 years for a molecule of methane to be removed from the atmosphere, and while an individual molecule of carbon dioxide can be removed from the atmosphere in as little as 5 years (making the previous statement true), some CO2 molecules can remain for as long as 200 years. The article has been changed to reflect this fact, and a note of explanation has also been added to explain the persistence of carbon dioxide in the atmosphere, due to exchange rates in the carbon cycle, which results in a new equilibrium of CO2 that can last for a few thousand years. We apologize for any confusion.

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