Climate 201: What on Earth are Greenhouse Gases?
Author’s note: In 2020 I started publishing a series of episodes under the title of “Climate 201”. The idea was to introduce topics in climate change, climate science, and climate policy — which I’ve studied for some years as a student- and explore them — and their implications — in much more detail than the simplistic framings you often see on the news. Ideally in the process I wanted to answer any questions that my audience had about climate change.
What on Earth are Greenhouse Gases?
Welcome to Climate 201. In this show, we’re going to attempt to explain some issues surrounding climate change in greater depth. This is my area of expertise, if I have one, and I’ve noticed that the debate surrounding climate change — especially on climate Twitter — can get extremely complicated. Before too long, we’re using obscure acronyms — ECS, BECCS, RCPs, GCMs, ESMs, IAMs — and the whole procedure, the whole debate, becomes extremely technical and specialist. Yet I know that millions of people around the world would be fascinated to learn more about climate change, so that they can evaluate these claims and decide what to do in their own personal lives, as well as to better understand one of great trends and historical forces that are going to dominate this coming century and affect all of our lives. Sadly, for years, a lot of conventional media coverage of climate change has been overly simplistic — it was just a few years ago that the media finally decided to move on from the long-ago solved question of “is it happening or not?” to the far more fascinating and complex question of “what can, and should, we do about it?” So — Climate 201, where I will tackle each issue at a time, be they scientific, political, social, or economic — and try to explain enough of the thinking about it so that you get up to speed with the real climate debates that shape our world.
First off, we’ll start with: what’s a greenhouse gas?
Greenhouse gases are essentially any gas that can undergo the greenhouse effect. The idea, in physics terms, is pretty simple. Every object with a temperature is emitting radiation of some kind. You can imagine temperature as expressing the energy available in molecules in terms of their vibrations or movement — in fact, the average energy available to a molecule is proportional to the Boltzmann constant multiplied by its temperature (although some molecules will be vibrating much more quickly). Very approximately speaking, when objects emit radiation, it’s in the form of what’s called a “blackbody spectrum”.
This blackbody spectrum emits across a range of different wavelengths of light or electromagnetic radiation, which depend on the energy that the object has. You will all be familiar with a classic example of this, which is heating an object — say a poker. At first it will go red hot; if you heat it higher, it will go “white hot”.
You can understand this by imagining the spectrum — the shape of which is called the Planck spectrum if you want to look it up — shifting towards higher and higher frequencies, and shorter wavelengths, as the object is heated up and becomes more energetic. At lower temperatures, the peak of the spectrum is at the low-energy red part of the visible spectrum — the object will also be emitting radiation at frequencies that we can’t see, such as infrared. As you increase the temperature, the peak of the spectrum shifts to cover more and more of the visible spectrum, until it’s strongly emitting every colour at once — that’s when you get to “white hot”! So this procedure is familiar to all of us.
So now consider a really simple system of the Earth and the Sun. Remember a key thing about physics — simple models are wrong because they miss out details, but by capturing the spirit of what’s going on they can help us to understand it. The Sun’s surface temperature is about 5800K or 5500 degrees celsius, and it irradiates the Earth with higher-frequency light in the visible and UV spectrum mostly. Because the Earth is millions of miles away from the Sun, only a fraction of that solar energy is absorbed by the Earth — you can imagine it spreading over a sphere, for example.
The laws of thermodynamics illustrate that, over time, if there’s no external interference or changing energy supply, things tend towards a thermodynamic equilibrium — an unchanging, constant temperature.
Now the Earth as a whole is going to be in thermodynamic equilibrium. And because the Earth as a whole is surrounded by space, where there’s virtually no matter at all, it can’t transmit heat through conduction or convection — unlike when you touch a hot object and its vibrating molecules pass heat into your hand. The only choice for the Earth as a whole is to absorb and emit radiation to come into thermodynamic equilibrium.
We know that the total energy that an object radiates is roughly proportional to temperature to the power of 4 — this is an approximation, but it works pretty well for most objects. Consequently, if the Earth is exposed to solar radiation, it’s going to heat up until its temperature is high enough that it’s radiating away the same amount of energy that it’s absorbing from the Sun. You can use this, plus the distance from the Sun to the Earth, to figure out an approximate temperature.
If you do this, crudely taking into account the reflectivity of the Earth’s surface to determine the energy that would be absorbed, you find that Earth’s temperature would be around -18C on average — but it’s actually around 15C. [Both of these figures are from NASA’s Goddard Institute for Space Sciences.]
Part of the reason for the difference is the greenhouse effect. Effectively, greenhouse gases in the Earth’s atmosphere have molecules that can interact with radiation. The Earth is heated by solar radiation from space, and it balances this by emitting infrared radiation from its surface. That radiation is absorbed by certain gases — those which have energy levels that correspond to the spectrum of energy emitted by the Earth. When they re emit, they emit the radiation in randomized directions, which means that some of it effectively cannot escape, and returns to Earth, which absorbs it and heats the Earth more.
In other words, greenhouse gases in the Earth’s atmosphere mean that it is less effective at emitting radiation to space. To compensate for this, the Earth has to heat up so that it’s emitting more energy, making up for the presence of greenhouse gases in the atmosphere.
You can tell that this effect is going on by looking at the radiation that the Earth emits from space, which is called the outgoing longwave radiation. When you do that, and plot it as a function of frequency, you see something that looks very similar to the Earth’s blackbody radiation spectrum — but with big bites taken out of it, where radiation of certain frequencies can’t escape nearly as much. You can then look at these individual notches and you will see that these exact notches correspond to the known energy levels in given molecules, which we can study in the lab. In other words, by examining molecules of CO2, or ozone, or water vapour in the lab — observing which frequencies of light they can absorb and emit — we can determine exactly what we’d expect them to do in the Earth’s atmosphere, and then we can actually observe this effect through satellite data. Not only is it extremely well established that greenhouse gases are acting to heat the Earth beyond what incoming solar radiation would do alone — if this didn’t happen, the Earth would be 33C colder and likely totally inhospitable for human life.
The dominant greenhouse gas in the Earth’s atmosphere is water vapour — this is what contributes the vast majority of that 33C temperature increase from the Earth’s “natural” radiative temperature to the actual temperature of the Earth. Now, human activities don’t directly do a huge amount to increase the amount of water vapour in the atmosphere, and water vapour cycles through the atmosphere very quickly — we call this “rain”. However, water vapour’s greenhouse effect does contribute to climate change, because it acts as an amplification effect for what we do. As we increase the concentrations of CO2, which increase the temperature of the atmosphere, the atmosphere holds more water vapour, which leads to a feedback effect. This feedback effect amplifies the direct effect of warming by greenhouse gases by between 2 or 3 times, depending on what you include…
So anthropogenic climate change arises due to the additional greenhouse gases that we are adding to the atmosphere. Perhaps too often, this is synonymous with carbon dioxide and carbon emissions, but it’s worth pointing out that there are a great many other greenhouse gases that humans either emit or have some influence on emitting that absorb at different parts of the spectrum.
There are two main reasons we predominantly focus on carbon dioxide. The first is that, in terms of the impact that our emissions have, it’s the largest contributor. For example, going back to 2011 and the IPCC’s AR5 report, the report quantifies the effects of different pollutants emitted by humans in terms of their impact on the “radiative forcing” of the climate. In other words, having these greenhouse gases and other pollutants is expressed as an equivalent additional heating of the atmosphere — you can think of it as the amount of energy that the greenhouse gases are preventing from escaping, per unit area. For this reason, the radiative forcing is expressed in Watts per square m.
In terms of radiative forcing, then, the radiative forcing due to CO2 back in 2011 was 1.7 W/m². The next highest contributor was methane, at 0.97W/m². Then halocarbons, at 0.18W/m². Then nitrogen dioxide at 0.17W/m².
There are also some negative contributions from aerosols and the cloud adjustment to aerosols, but since we’re just focusing on the positive contributions from greenhouse gases today, we’ll leave these for now, pausing only to note that they partially compensate for the warming due to greenhouse gases.
So you can see that CO2 is around twice as large in impact as its next-biggest competitor, methane. But, as we’ll discuss later, directly comparing these two gases according to how much radiative forcing they are exerting on the planet right now can be misleading.
The other reason that CO2 is focused on is that it lasts in the atmosphere for the longest — some of CO2 emissions will remain in the atmosphere for thousands of years. Again, according to the IPCC’s Fourth Assessment report:
About 50% of a CO2 increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years.
Let’s look at some of these other climate pollutants. Methane has a natural cycle, just like carbon, but in recent years methane emissions have increasingly been dominated by processes influenced by humans. Cows and other grazing animals emit methane — both directly from their digestive processes, where the microbes that help them break down grasses produce methane as a by-product, and also in their manure, where methane is emitted by the microbes that feast on it. That accounts for perhaps 40% of the human-influenced methane. But there are also methane emissions from rice paddies — when they flood, microbes break down the rice and also emit methane. And fugitive emissions from the gas and oil sector, where methane leaks from oil wells or natural gas pipelines, probably account for at least 25% of the human-caused emissions of methane — although there are indications that this figure is rising as the world increasingly shifts from using coal to using natural gas, where leaky pipelines are responsible for substantial methane emissions. As the demand for meat / beef increases, and the use of natural gas also increases, anthropogenic methane emissions generally rise year on year. Crucially, methane only has a lifetime in the atmosphere of around 12 years, before typically decaying into CO2 and H2O by interacting with OH- in the atmosphere. We’ll come onto this more a little later.
Thinking of some of the others, we have nitrous oxide, N20. This is one of the more long-lived greenhouse gases, with a lifetime in the atmosphere of around 120 years on average; it’s emitted primarily due to the application of nitrogen-heavy fertilisers in agriculture, and also in the procedures that make those fertilisers. In the US around 82% of nitrous oxide emissions come from agriculture or manure, while the rest comes from various different fuels being burned in transportation and various industrial processes. In the US, the trend for these emissions is pretty flat. But the use of nitrogen fertilisers is a huge part of what has allowed us to produce as much food as we do today, as part of the green revolution, which means that these emissions are difficult to eliminate entirely. However, there are approaches to agriculture and soil management that involve using nitrogen much more sparingly and efficiently which can minimise these emissions.
There are additional greenhouse gases to consider, including so called F-gases or flourinated gases. These are gases with flourine in them, which include CFCs and HFCs, which are used in refrigeration and typically last in the atmosphere for centuries. CFCs are of course more famous for helping to deplete the ozone layer, which we covered back in the episode: Climate Change, Lessons from Montreal — which attempted to explain why it was that humanity has done a relatively decent job at fixing the hole in the ozone layer, once it was discovered, and why climate change is a different — and frankly much more difficult — problem. Under the Montreal protocol, CFCs are banned, but their replacements — HFCs, which do not deplete the ozone layer — are not yet subject to a global ban.
The most common F-gases today are hydrofluorocarbons (HFCs), which contain hydrogen, fluorine, and carbon. They are used in a multitude of applications including commercial refrigeration, industrial refrigeration, air-conditioning systems, heat pump equipment, and as blowing agents for foams, fire extinguishants, aerosol propellants, and solvents. Perfluorocarbons (PFCs) are the compounds consisting of fluorine and carbon. They are widely used in the electronics, cosmetics, and pharmaceutical industries, as well as in refrigeration when combined with other gases. PFCs were commonly used as fire extinguishants in the past and are still found in older fire protection systems. They are also a by-product of the aluminium smelting process.
Sulphur hexafluoride (SF6) is used primarily as an insulation gas. It can be found in high-voltage switchgear and is used in the production of magnesium.
All of these gases are relatively small contributors to global warming, and are produced in relatively small quantities compared to CO2 or methane. But it should be pointed out that, molecule for molecule, non-CO2 greenhouse gases result in much more warming than CO2 does per molecule. There are a couple of reasons for this. Firstly, we have to consider the molecular structure of the greenhouse gases. Generally, more complex molecules have more energy levels, and the energy levels will be at different parts of the Earth’s infrared spectrum. So typically you need to consider which gases are going to take a nice big chunk out of the Earth’s blackbody infrared emissions to space, based on where the molecule’s energy levels are and how they line up with what the Earth would naturally emit. Secondly, and more importantly, you need to consider how much of the gas already exists in the Earth’s atmosphere.
As explained, these molecules can only absorb part of the Earth’s outgoing radiation, depending on their energy levels. This means that as the atmosphere contains more and more of a particular greenhouse gas, it becomes more and more opaque to given frequencies of infrared radiation being emitted from the Earth’s surface. But this in turn means that each new molecule you add is going to be less effective at trapping radiation.
One way to imagine why is by considering the atmosphere as “layers” of these molecules. Let’s say a layer of a certain thickness, t, can cut transmission by 50%. [This is all a massive oversimplification, of course, and not what physically happens in the atmosphere, but it’s really just an analogy.]
If we inject enough greenhouse gas to form a layer that has thickness t, then we know that we’ll cut the infrared radiation transmitted through the layer by 50%. If we add another layer of thickness t, then we’ll cut the transmission by a further 50%. But since only half of the light made it through the first layer, cutting that in half means that 25% of the original light has made it through both layers. In other words, adding the second layer has just cut down on the original light by ~25%. If you added another layer, it would only reduce the original light by 12.5% of its initial energy, and so on. This is very similar to what actually happens in the atmosphere; the radiative forcing depends on the logarithm (inverse exponential) of the concentration of the gas. Each doubling of the concentration will increase the radiative forcing by the same amount. You can imagine this as the atmosphere becoming more and more opaque to certain frequencies as the concentration of gas increases.
So molecule for molecule, injecting a molecule of CO2 into the atmosphere has much less impact on warming than a single molecule of HFC12, which is a much much rarer gas in the atmosphere. By some metrics, each molecule of HFC12 is worth between 1,000–3,000 molecules of CO2 — but, since we emit billions of tonnes of CO2 a year and far less of HFC12, it’s much less important component of anthropogenic climate change, even though pound for pound it’s huge.
So now we have enough background to start to explain some different terminology and introduce some of the active debates in the climate change space. Because the problem that you’ll likely be identifying here is that the greenhouse gases all have different properties! They warm the climate by different amounts, and they have different lifetimes in the atmosphere, and there are different properties for mitigating them.
The issue is that you’ll face endless problems in setting climate policy, both directly and indirectly, where you will try and compare the impacts of the different greenhouse gases. And the issue here is fungibility — can you set an exchange rate and say that 1 tonne of methane is worth 3 of carbon dioxide? Can you happily get a single number that will compare the emissions of different gases to each other in a meaningful way?
There are plenty of places where you’re going to want to do this. Take switching from coal to natural gas. This is going to decrease your CO2 emissions — typically natural gas, when burned, produces half the CO2 of goal, which (along with the expense) is why the world is increasingly shifting from coal to natural gas as a fossil fuel. But it’s also going to increase your methane emissions from leaks in the pipelines that transport that natural gas around.
So how do you account for this?
The trouble is that there are a couple of different things to account for — both the different lifetimes of the gases in the atmosphere, and the different warming potential that they have. A single molecule of methane is much more effective at trapping heat than CO2 — but it decays much more quickly in the atmosphere. So if we want to compare the different greenhouse gases, we need to find some way to take that into account.
Typically the approach from science has been to try and create different “metrics” that allow you to compare the different greenhouse gases. And these metrics have to depend on the timescale over which you’re concerned about the warming.
For example, methane clearly has much more short-term impact on warming — for the first 12 years much of it will still exist in the atmosphere as methane, and so the fact that it’s more effective at heating is more important. Over the longer term, though, more of the methane will have decayed into CO2 — so, a hundred years after you emit a tonne of methane into the atmosphere, the warming impact that it will have then will be quite close to that of a tonne of CO2 that was emitted. And this is further complicated because technically this is all time-dependent — by the time your methane decays into CO2, the CO2 and methane concentrations will be different, so the changes to radiative forcing you’ll get will also be different, although that is a second order effect.
To untangle this, scientists typically look at a time-frame. We need to integrate the effects of the different molecules over time so that we can take into account how they’re changing, and how much warming they contributed in that time frame.
This is done using Global Warming Potential or GWP. Typically, when GWP is specified, it’s GWP over a timeframe of the next 100 years — but it can also sometimes be calculated as GWP 100, or global warming contribution over the next hundred years.
So for example, in terms of warming over the next 20 years, taking into account the decay of methane into CO2, we see that a single molecule of methane contributes around 85x the warming of a molecule of CO2. Over the next 100 years, it will contribute around 32x the warming of a molecule of CO2.
So these global warming potentials provide a kind of exchange rate where we can say, integrated over a hundred years, one molecule of methane warms 32x more than CO2, one molecule of nitrous oxide warms 280x more than CO2, one molecule of CF4 warms 7000x more than CO2, and so on.
Some people then go further and try to use these global warming potentials to multiply the tonnes of different gases that are being emitted.
So for example, they might say that if you’re emitting 100 tonnes of CO2, and 1 tonne of methane, because methane has a GWP100 of 32, then you’ve effectively emitted 132 tonnes of CO2-equivalent.
But you can see now why this has the makings of a lot of debate and controversy. When you choose to integrate the emissions over time, you lose a lot of detail. Who decides that 100 years is the important timeframe? The warming due to methane is massively concentrated in the next decade or so, and tails off to nearly nothing towards the end of that hundred years. But this is buried if you use the CO2-equivalent metric, which equates emissions from CO2 to emissions from methane.
On the other hand, some of those flourinated gases can persist in the atmosphere for hundreds or even thousands of years, as there are very few chemical processes that can naturally remove them. In this case, GWP100 is ignoring the contribution to warming in subsequent centuries — do we care about that, or not, and how should we value it if we do?
So if our goal is a certain temperature target by the end of the century, for example, then the methane we emit today is much less important than the CO2 we emit today. There’s an argument — that if the temperature in 2100 is what we really care about — that we should focus on cutting back on the CO2 quickly, while allowing methane to take a back seat, because the methane we emit today won’t be affecting climate then, even though the CO2 still will.
This might all sound a little bit esoteric — does it really matter if we get these exchange rates correct when we should be focusing on cutting emissions as quickly as possible anyway? — but it is important because of the conflicts that it sets up between different sectors in your economy, and between different countries. The agricultural sector mainly emits methane. If we choose an exchange rate that emphasises the importance of methane much more than CO2, then the pressure increases on this sector to reduce its emissions. Maybe a lot of government funding and research effort goes into projects that can reduce or eliminate methane. Maybe in the case of a carbon tax — which is still one of the most common policy techniques proposed to reduce greenhouse gas emissions — farmers are being taxed more extensively to reduce their methane emissions. Maybe when you do the calculating, it would be more important for people to campaign to reduce meat intake in their diets, rather than driving more efficient cars or reducing the number of flights they take, and so on.
And this is also a problem when it comes to international agreements like the Paris Agreement. Under the Paris Agreement, countries are agreeing to limit their own emissions, ratcheting up ambition as we go along. But how can we judge different countries’ measures of success? For a country like New Zealand where a lot of cattle are raised and relatively little heavy industry shows up, methane is around 45% of their greenhouse gas emissions in terms of global warming potential — in the US, it’s just 10%. So a metric that emphasises methane unfairly compared to CO2 would put more pressure on NZ relative to the US to reduce its emissions.
Ultimately, policy-makers have to make decisions about what to focus on, what to prioritise, where to allocate funding, what to tax, and so forth if they want to meet their greenhouse gas emissions goals. You can see, then, that the metric that you choose to use to exchange between these gases can end up being extremely important to how climate policy is actually shaped, even though these fiddly debates about precisely how and for how long different gases contribute to global warming can seem quite esoteric.
Recently, several scientists have been arguing for a new metric that treats methane differently. The reasons why are described in an op-ed for carbon brief by Myles Allen, one of the authors of the paper. It essentially boils down to understanding how the climate responds to changing emissions of different greenhouse gases.
If you stop emitting CO2 tomorrow, much of the CO2 you’ve emitted stays in the atmosphere for centuries and continues to affect the climate — it will take many decades or centuries for it to be removed by natural processes and for cooling to start. If you stop emitting methane tomorrow, however, the concentration of methane will fall over the next few decades and actually cool the climate compared to today. Similarly, if you emit methane at a constant rate, eventually the concentration of the gas in the atmosphere, and hence its impact on warming, over a given time period is going to stabilise as its decay into CO2 cancels out your emissions. On the other hand, if you emit CO2 into the atmosphere, it will continue to accumulate and continue to contribute to warming, on the timescales we care about as humans, its concentration won’t stabilise.
“This non-equivalence has important consequences for mitigation strategies. For example, if carbon is taxed. If all greenhouse gas emissions are taxed, then using GWP100 would unfairly penalise short-lived emissions, assuming the aim was to penalise contribution to warming.
Consider a power station and a herd of cows. A power station emits CO2 by burning fossil fuels. This CO2 is taxed. When it shuts down permanently, it emits no more CO2, so is no longer taxed. However, the CO2 already emitted continues to affect the climate for hundreds, or potentially, thousands of years. So even after closing down, that power station still contributes to holding up global temperatures because of the CO2 that remains in the atmosphere.
Now to the cows. A herd of cows emits methane, so the farmer is taxed for those emissions. If the herd remains the same size with the same methane emissions every year, it will maintain the same amount of additional methane in the atmosphere year on year. In terms of its contribution to warming, this is equivalent to the closed power station.
The power station pushed up global temperatures when it was running in the past, just as the farmer’s great-grandparent pushed up global temperatures when they were building up the herd of cattle. But neither a steady herd of cattle nor a defunct power station is pushing up global temperatures any more.
However, under almost all proposed systems for taxing emissions that attempt to include methane, the farmer would get taxed for their herd’s methane emissions every year the cows were alive, while the owner of the closed power station would not.
One way to make this fairer would be to tax greenhouse gases for every year that they remain in the atmosphere. Taxing all emissions since the start of the industrial revolution may, however, prove problematic. For example, how do we tax James Watt?
Another way would be to use GWP* to calculate equivalent emissions, as this equates changes in methane emission rates with tonnes of CO2. Thus, a stable emission of methane equates to a zero rate of CO2 emissions under GWP*, as it does not change the level of warming into the future and would, therefore, not be taxed.
The flip side of this is that any sustained increases in methane emissions would be heavily taxed, as they would contribute very substantially to future warming. Conversely, any sustained cuts would be rewarded for contributing to future cooling.”
And this type of argument also has consequences for our targets of “net zero”. Several countries, including the UK, have argued for “net zero greenhouse gas emissions” by 2050. But does this mean that each of our greenhouse gas emissions must be either zero, or compensated for by removals? Or, for example, could we allow ourselves to emit some methane if we were scrubbing extra CO2 from the atmosphere? And so on.
So, as you can see, the ways in which we should prioritise emissions from different sources, and of different greenhouse gases, remain controversial and debatable. Ultimately the reality is that you will always lose some information by trying to come up with a way to directly compare different greenhouse gases. Clearly, when they have these different behaviours, in some ways it doesn’t mean a lot to say that methane is many times more powerful as a greenhouse gas than CO2 without defining the time you care over. At the same time, you will need to be able to compare them in order to determine how you set your climate policies.
So when you pick a metric to compare emissions from greenhouse gases, be sure that you know what time-frame you’re concerned about!
Thanks for listening to this episode of Climate 201. I hope it’s given you more insight into the different greenhouse gases that are affecting the climate, the processes on earth that cause them to be released, what a greenhouse gas actually is and how it influences the Earth’s radiative balance, the issues we might have with directly comparing them and coming up with a “greenhouse gas exchange rate”, and how these can influence climate policy.