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The temperature on the Moon can range from -175°C in the shade to 125°C in the sun. That’s a difference of about 300°C. The average temperature on Earth is about 14°C and daily highs and lows rarely span more than 30°C. So what accounts for this difference? Greenhouse gases.

The Earth’s atmosphere is mostly nitrogen and oxygen, but the remaining gases play an important role in regulating planet’s temperature.

The Earth’s atmosphere is a thin band of air that encircles the planet. It is mostly composed of nitrogen (78%) and oxygen (21%) but some of the remaining gases (1%) play a very important role in our climate. Among them are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons and a few others—the greenhouse gases. Such gases do exist naturally in the environment, but through industrialization we have produced and released trillions of tons of them over the past couple of hundreds of years. It should not be a surprise that they are having the impact that they are.

Aside: Water vapour also acts like greenhouse gas, and in terms of its immediate effect it is the most powerful. However, water vapour in the atmosphere is self-limiting (i.e. when there is too much of it, clouds form and the water falls as precipitation). The atmospheric lifetime of water vapour is about 9 days. For carbon dioxide, it’s from 30-95 years. That is a long time for it to continually warm up the planet.

So, how can such small amounts (1%) of gases in our atmosphere affect the temperature and climate of the Earth? It has to do with the science of how they absorb infrared radiation.

This image shows the intensity of infrared (heat) radiation coming from Earth.

Let’s back up a bit: With the exception of heat from deep underground, the Earth receives all of its energy from the Sun. When the sun’s light reaches the Earth, 30% of it is immediately reflected back into space by clouds, the atmosphere and reflective surfaces on Earth (e.g. ice, water). But the rest of the light is absorbed and converted into heat energy—also known as infrared radiation. This is the type of energy that thermal or infrared imaging devices can measure.

As the heat energy is radiated off of the Earth’s surface, some of it “hits” the greenhouse gas molecules and gets reflected back to Earth (in fact, 90% of it gets caught in this way). The way this works has to do with chemistry: greenhouse gas molecules are configured in such a way that they are able to absorb and re-emit infrared radiation. The details are complex and involve a little bit of physics. But the effect is clear enough: greenhouse gases act like a “shield” that helps keep the heat in.

The upside of this is that we have a nice, warm planet to live on. The potential downside is that if we add too much greenhouse gas to the atmosphere the Earth may heat up enough to trigger runaway global warming.

Video: Greenhouse Gases (NASA’s Earth Minute, 1:42)

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