No laughing matter

What about the other greenhouse gases? (9 minute read)

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One of the science experiments I can still remember from school involved Condy’s crystals, or, more scientifically, potassium permanganate. A few tiny crystals, so dark they appear almost black, immediately begin to dissolve when dropped into a jar of water. At first, you see tendrils of intense magenta where the crystals fall through the water. Then, you see blobs of magenta surrounding the crystals as they sit on the bottom. The blobs expand, spreading to cover the bottom of the jar. Eventually, all the water in the jar will be pink, although it takes time. If you want the experiment to go more quickly, you can use hot water, or you can cheat and stir the water. (If you need to remind yourself of what this experiment looks like, I’ve linked to an example here.)

Experiments using Condy’s crystals are used to illustrate various principles – they can be used to show diffusion, dilution or how an apparently solid crystal is made up of tiny particles. But the lesson that has stayed with me is how a tiny quantity of the crystals can change the colour of a large volume of water. It takes very few grains of Condy’s crystals to turn everything pink.

The experiment with Condy’s crystals has been on my mind as I’ve been thinking about greenhouse gases. Carbon dioxide makes up only 0.04% of our atmosphere, a tiny amount for something with such a big impact. It is measured in parts per million – currently, out of every million molecules in the atmosphere, 416 are carbon dioxide. The quantity of methane makes carbon dioxide seem huge, with just under 2 molecules of methane for every million molecules in the atmosphere. In fact, methane is usually measured in parts per billion instead of parts per million, so its concentration is usually given as 1896 parts per billion.

The quantities of carbon dioxide and methane are huge when we compare them to most other greenhouse gases. Nitrous oxide (also known as laughing gas) is currently around 335 parts per billion. Chlorofluorocarbons and hydrofluorocarbons are so rare that they are measured in parts per trillion. But there’s one greenhouse gas that is more abundant – water vapour, which can vary from 0-4%. So why do gases like carbon dioxide, methane and nitrous oxide matter when there is so much more water vapour in the atmosphere? Why isn’t it water vapour which is driving the world’s climate?

This turns out to be a complicated question. Water vapour is important to the world’s climate – around half of the greenhouse effect is attributed to water vapour in the atmosphere. But just because it is responsible for around half of the greenhouse effect doesn’t mean it is driving global warming.

Now, that may sound like a confusing statement, so I will take a step back and explain what I mean.

At first glance, terms like global warming, greenhouse effect and climate change almost sound as if they mean the same thing. But they each have a slightly different meaning.

The greenhouse effect is not something caused by human activity. It is the name given to the way certain gases in our atmosphere trap some of the heat from the sun, in the same way that the glass panes of a greenhouse trap heat inside and allow us to grow plants which might not survive outside. There’s a simple video here from NASA which explains it well. Without the greenhouse effect, the average temperature on the surface of the earth would be cooler by around 33° Celsius (or 59° Fahrenheit for my American friends).

We didn’t cause the greenhouse effect, but we are adding to it, by putting more greenhouse gases into the atmosphere. The result is an increase in the average temperature on the surface of the earth, which is known as global warming. Climate change is the long-term change in average weather patterns which we are now seeing, as a result of global warming. Climate change includes warming temperatures, but it’s more than that. It also encompasses changes in rainfall, wind and extreme weather events.

So, water vapour in the atmosphere is an important part of the greenhouse effect. But water vapour doesn’t behave like the other greenhouse gases. When we pump carbon dioxide into the atmosphere, it stays as a gas in the atmosphere. So does methane, and so does nitrous oxide. These gases can be condensed into liquid or solid forms, but that doesn’t happen at the pressures and temperatures they encounter in the atmosphere.

Water vapour is different. It is constantly evaporating into the atmosphere and then condensing out again as rain (and snow, dew, etc). I’ve linked to a video with a basic explanation of the process here. The cycle of evaporation and condensation is driven by temperature, because the warmer the temperature, the more water vapour the air can hold.

Desert after storm, Hanksville, Utah (image credit: Melanie Newfield)

We haven’t been pumping water vapour directly into the atmosphere in the same way we’ve been pumping out carbon dioxide and methane. When you look at greenhouse gas emissions, such as this summary of New Zealand’s contribution, you don’t see water vapour listed. But we are changing the amount of water vapour in the atmosphere, indirectly.

Warmer air can hold more water vapour. But, also, more water vapour can hold more heat. That means as the atmosphere warms due to increasing carbon dioxide, it also holds more water vapour, which holds more heat, which warms the atmosphere further… I’m sure you can see where this is going. There’s a feedback loop with water vapour in the atmosphere, which intensifies the effect of the carbon dioxide and other gases we are emitting. But it’s not that simple, because clouds can help cool the atmosphere too. The effects of atmospheric temperatures on water vapour, and water vapour on atmospheric temperatures, are complex, and scientists are still working out the implications.

But what about the other greenhouse gases, the ones that are so rare that they are measured in parts per billion? What is their role in global warming? Are they really important when they are present in such tiny amounts?

Before I began researching the article on methane that I published a couple of weeks ago, I hadn’t appreciated how important nitrous oxide was as a greenhouse gas, especially for New Zealand. Globally, nitrous oxide contributes about 5% of humanity’s total contribution to climate change. For New Zealand, that figure is higher, at around 11% in 2020. The figure is higher for New Zealand because nitrous oxide is emitted as a result of agricultural activities, and agriculture makes up a much higher proportion of New Zealand’s emissions compared with other countries.

The figures for the contribution of nitrous oxide to climate change are high in proportion to the actual quantities of the gas emitted.  As I mentioned earlier, the total atmospheric concentration is only 335 parts per billion – if we translate the figure for carbon dioxide into the same scale, it would be 416,000 parts per billion. How does such a rare gas have such a big effect?

I’m afraid I can give you only part of the answer to that question, but I will tell you as much as I’ve learned and explain what I haven’t been able to find.

The reason why greenhouse gases like carbon dioxide, water vapour and nitrous oxide trap heat is that the chemical bonds inside the individual gas molecules vibrate when they are struck by infrared rays (heat). Nitrogen and oxygen, present in the atmosphere as N₂ and O₂, have symmetrical molecules, and the vibration of the chemical bonds doesn’t alter the symmetry. But more complex molecules, such as carbon dioxide, water vapour and methane, are lopsided, so when they are struck by infrared rays the bonds vibrate in ways that absorb heat. The process is explained in this video here.

The reason that the tiny amounts of nitrous oxide in the atmosphere are important is that different gases absorb different amounts of heat, that is, they have different Global Warming Potentials. Methane is worse than carbon dioxide, but nitrous oxide is worse again, having a Global Warming Potential of about 310 times that of carbon dioxide, over 100 years. Because it stays in the atmosphere for about 100 years, the Global Warming Potential doesn’t change as much for nitrous oxide as it does for the short-lived methane. Over a 20 year period, the Global Warming Potential of nitrous oxide is around 280.

What I haven’t been able to find out is why nitrous oxide should absorb so much more heat than a molecule of carbon dioxide. If you happen to know, let me know in the comments.

So, nitrous oxide is a highly potent greenhouse gas. But where does it come from? I touched on nitrous oxide emissions briefly, when I wrote about the impacts of nitrogen fertiliser, but at the time I didn’t appreciate how much of a contribution it made to climate change. Briefly, nitrous oxide is emitted by soil microbes when more nitrogen ends up in the soil than plants can use. Some of that comes from fertiliser, but much of it comes from animal urine and, to a lesser extent, dung.

This is bad news, once again, for pastoral farming. But is there anything that can be done? The answer is yes, but there are some problems.

It is possible to apply chemicals to the soil which inhibit the nitrous oxide-producing microbes. As well as reducing nitrous oxide emissions, these chemicals also reduce nitrate runoff into waterways, and allow farmers to apply less fertiliser without reducing yield, saving money. Overseas, a number of these chemicals are available to farmers, and one of them, with the name dicyandiamide (more commonly known as DCD) has been used in New Zealand.

The problem is that, in 2013, small amounts of DCD were found in New Zealand milk. Although experts argued that the small amounts were not a health risk, the detection prompted concern about the effect on trade. DCD was withdrawn from the New Zealand market and is no longer used on New Zealand farms. Our Agricultural Greenhouse Gas Research Centre is looking at alternatives, as well as forage plants that could lead to reduced emissions, but, right now, we don’t have an answer. It’s a reminder that when we try to solve one problem, we often create another, and finding a balance between the two can be difficult indeed.

There is one more group of greenhouse gases that I haven’t looked at yet, but they are a story in themselves and so I will leave them for another time.

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