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Negative and Positive Feedbacks

Geologist, Alasdair Skelton, explains how negative feedbacks stabilize climate and how positive feedbacks destabilize climate.
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Over geological timescales, Earth’s climate is controlled by three factors. The amount of the heat the Earth receives from the Sun. The amount of that heat which is reflected back into space, the albedo effect. And the greenhouse effect, meaning the amount of heat that is trapped by the atmosphere. Now, those three factors vary naturally over geological times. And remember, geological times are millions of years.
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Those variations explain why Earth was warmer when dinosaurs lived here.
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Such variations also explain why Earth was cooler during the last glaciation. But those changes are incredibly slow. And that’s a good thing. But why are those changes so slow? It’s because Earth has a built-in thermostat- -that regulates its temperature over geological timescales. So, what is that thermostat? Well, it’s the very, very slow weathering of rocks. What do I mean by the weathering of rocks? I mean that rocks dissolve. They dissolve very, very slowly when they react with carbon dioxide- -from the atmosphere and water. But what’s very, very special, is it is not just the rocks that dissolve. The carbon dioxide dissolves too, and is removed from the atmosphere- -as rocks slowly dissolve.
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You might think now: “Well, carbon dioxide is a greenhouse gas, and if it is being removed”- -“it will reduce the greenhouse effect and the Earth will become cooler.” That all makes sense.
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Then, you’ll be justified in asking the question: “How is that a thermostat?” “Doesn’t that just make the Earth cooler?” But it’s cleverer than that. You see, the reaction whereby rocks dissolve goes faster when it’s hotter. So, let’s think what that means. If the Earth gets warmer, rocks dissolve faster. That means carbon dioxide is removed from the atmosphere faster.
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That makes the greenhouse effect weaker, and Earth becomes cooler. But what happens if the Earth becomes cooler? Well, rocks dissolve more slowly.
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That means they cannot take CO2, carbon dioxide, out of the atmosphere as efficiently anymore.
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Which means that carbon dioxide from volcanoes starts to accumulate in the atmosphere- -the greenhouse effect becomes stronger and the Earth becomes warmer. And this is a feedback, because as soon as Earth becomes warmer, rocks dissolve faster again. This is called a negative feedback. This negative feedback holds Earth’s climate in balance- -and has done so for billions of years. We know that Earth’s climate has been stable, not just for millions, but billions of years. How do we know that? We find fossils like these. These fossils are called stromatolites. They are structures built by cyanobacteria colonies.
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We can see that they are made up of layers, when we look down in this crack.
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I run my finger along one of those layers now.
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And another one, a bit lower down.
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So why does that matter? Well, these are living organisms, cyanobacteria. For them to live requires a stable climate. These are not the oldest stromatolites in the world. The ones I’m sitting on here are approaching one billion years old. The oldest are three billions. That means there were living organisms on Earth for three billions years. What can we learn from that? That Earth’s climate has been exceptionally stable. Indeed, when we look back through geological history, there are just a few exceptions. A number of them ended in mass extinctions. One of those exceptions is here and now, and we are the cause.
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By burning fossil fuels, we are releasing almost a hundred times as much carbon to the atmosphere- -as the amount that comes from volcanoes naturally. But rocks do no dissolve any faster.
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That means much of that carbon remains in the atmosphere- -some of it for thousands of years. So, what does that do? Carbon dioxide is a greenhouse gas. The Earth becomes warmer. So what does that do? That causes the polar ice caps to melt. What effect does that have? The albedo effect becomes weaker. So, Earth is not as good a mirror anymore. It doesn’t reflect heat from the Sun as effectively. Why does that happen? Because the bright polar ice caps are replaced by dark oceans, which absorb heat.
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So, the temperature rises. More of the ice melts. The Earth becomes even darker. The Earth absorbs more heat. It becomes warmer. This is what we call a positive feedback. It makes things go faster and faster and faster. It makes them runaway. A positive feedback in the climate system is a bad thing.

Geologist, Alasdair Skelton, explains the 3 factors which control Earth’s climate on geological timescales (heat from the Sun, the albedo effect and the greenhouse effect) vary gradually, explaining why Earth was warmer when dinosaurs lived here and why it was cooler during the last glaciation.

That natural climate variations are slow arises because of Earth’s built-in thermostat. This is the very slow weathering of rocks, a chemical process which removes carbon dioxide from the atmosphere. Because this process happens faster when the Earth is warmer and because it cools the Earth by removing the greenhouse gas, carbon dioxide, weathering of rocks stabilizes Earth’s climate. This stabilizing mechanism is called a negative feedback.

Standing on dome-shaped fossils of ancient life called stromatolites, Alasdair Skelton explains that evidence of life on Earth going back 3 billion years confirms that this mechanism has stabilized climate for a very long time.

He also warns of a few exceptions, when this stabilizing mechanism was overwhelmed by other processes, and Earth’s climate destabilized ending in mass extinctions. He points out that one of these exceptions is here and now and we are the cause. By burning fossil fuels, we are releasing far more carbon dioxide to the atmosphere than can be removed by weathering of rocks. This is destabilizing the climate making it warmer.

Warming melts the polar ice caps, making the Earth darker which weakens the albedo effect. This makes the Earth even warmer causing the ice caps to melt faster. This destabilizing mechanism is called a positive feedback.

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