Module 2704


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Does carbon dioxide affect Earth’s energy balance?

What is the effect of carbon dioxide in our atmosphere?
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What would be our average temperature if there were no carbon dioxide?

Is carbon dioxide good stuff or bad?

In this module, we focus on the effect on temperature due to the presence of carbon dioxide in our atmosphere, and the magnitude of the effect. Postpone for now questions of how carbon dioxide exerts an effect (Module 2705 The "greenhouse effect"), and why it does its thing (Module 2706 Why do carbon dioxide molecules absorb radiations emitted from Earth?).
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Here is how it works (Level 1 of understanding)

Carbon dioxide molecules "capture" some of the radiation emitted from Earth's surface, and which would otherwise go off on a long journey into deep space.
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Imagine the Earth in a state of energy balance (See Module 2701 Is Earth in energy balance?), the temperature staying constant. And then we inject some carbon dioxide into the atmosphere. So not as much outgoing radiation (heat energy) escapes from Earth, and so the rate of incoming radiation is more than the rate of emitted radiation. Temperature will increase.
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Let's look at another way - the reverse. Suppose that Earth is in a state of energy balance, and we can somehow instantaneously remove all of the carbon dioxide that is in the atmosphere. Since there are now no carbon dioxide molecules to "capture" the outgoing heat energy, the rate of emission of energy into space will increase. There is no longer energy balance: the rate of outgoing radiation exceeds the rate of incoming radiation. The temperature will decrease.

So carbon dioxide is keeping us warmer than would otherwise be the case.

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By how much is Earth warmer?
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According to climate scientists’ calculations, Earth is warmer by 33 °C than it would be without carbon dioxide in the atmosphere. Huge!
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The average temperature of the surface of Earth is 15 °C. So without carbon dioxide in the atmosphere, the average temperature would be – 18 °C

Who would prefer that the average temperature is -18 °C rather than + 15 °C? Not many of us, other than perhaps polar bears?

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So, is carbon dioxide a good stuff, or a bad stuff?

Here is an approximate representation of a carbon dioxide molecule - not that anyone has ever seen one of them.
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In a carbon dioxide molecule, two oxygen atoms are bonded to a carbon atom. The relative sizes shown are representative of best estimates. The colours shown are used just to distinguish the atoms: carbon atoms are not grey, and oxygen atoms are not red. Are they?

But carbon dioxide is getting a bad wrap. It represents gloom and doom ....

Is carbon dioxide evil? The daily press and the popular science press have nothing good to say about it.

So I am beginning to wonder if a molecule of carbon dioxide really looks different from that above .......

A carbon dioxide molecule?

But then, if there were no carbon dioxide in the atmosphere, our average temperature would be -18 °C.  That sounds like a pretty good substance to me! Revised (again) image of carbon dioxide molecules .....

A carbon dioxide molecule - all sweetness and goodness?

Maybe carbon dioxide is both good and bad - depending on context. The "dual nature" of carbon dioxide?

Its good side is discussed above and in Module 2705 The "greenhouse effect". It's not-so-good side will become apparent in Module 2707 The "enhanced greenhouse effect".

LEARNING CHEMISTRY WITH UNDERSTANDING

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