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Greenhouse gases - what’s the big story?

Greenhouse gases, or GhGs, were much discussed at the recent UN Climate Change Conference in Glasgow. The benefits of their warming properties are accepted, indeed without GhGs the temperature on Earth would be catastrophically lower. But they become problematic as their proportion in the atmosphere increases. We analyse the main constituent gases and how they have earned their reputation, both good and bad. 

The main components of the Earth’s atmosphere are nitrogen (78%) and oxygen (21%). Among the remaining 1% are gases that can absorb and reflect the sun’s heat as it bounces off the Earth’s surface, thereby warming the lower atmosphere. Hence the ‘greenhouse’ label. When in balance, this benefits the Earth. With no GhGs the average temperature would be minus 18C, not the 15C at which we currently thrive. But as the proportion of GhGs rises, more and more heat is reflected back to Earth, forcing temperatures up. And that is what we know as global warming.

Carbon, mainly in the form carbon dioxide molecules, is viewed as the arch villain of the piece. It is powerful in absorbing and reflecting the sun’s heat. The stock of carbon dioxide in the atmosphere has jumped by almost 50% since the Industrial Revolution. And that’s due to human activities, such as burning fossil fuels and deforestation. Carbon levels are now out of balance, yet slow to reverse, remaining in the atmosphere for over a hundred years.

Methane is an even more powerful GhG than carbon. As such, it has the potential for a greater warming effect on Earth, even though it remains in the atmosphere for only a dozen years. Furthermore, methane molecules can react with ozone, producing more CO2 and water droplets. But by doing this, the number of ozone molecules in the stratosphere is depleted, creating holes in our protective shield against the sun’s rays.

Perhaps surprisingly, the greatest percentage of the greenhouse effect is created by water vapour, which has evaporated from the Earth’s surface. This is often in the form of clouds, which act as a very effective insulator, forcing temperatures below them to rise. As they do so more water evaporates, establishing a ‘positive feedback loop’. That’s not as good as it might sound, as the water vapour amplifies the effects of the other GhGs, pushing temperatures ever higher.

So how will the world deal with this rise in GhGs? Carbon is the main target of climate activists, corporates and governments. Their aim is to reduce carbon emissions as swiftly as possible, achieving net zero by mid-century and limiting global warming to 1.5C above pre-industrial levels. The Glasgow Climate Pact, phasing down coal as a power source, moves towards this end. The development of electric vehicles and renewable energy sources, as well as a commitment to the world’s forests, will be crucial to limiting the negative impact of GhGs.