Last lesson left a gap. Without air the ground would sit near −18°C. With air it sits near 15°C. The air is doing about 33 degrees of work. Yet air is mostly nitrogen and oxygen, and those two let heat straight through. Something else in the air is doing it.
Nitrogen78%
Oxygen21%
Everything else, including the gases that catch heat1%
Sunlight passes through the air and warms the ground. The ground glows heat back up. A few gases catch that heat and glow it out in every direction, some of it back down. They are under one part in a hundred of the air. People call them greenhouse gases: a greenhouse also lets light in and keeps warmth from leaving.
Sunlight, on its way in
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Heat, on its way out
Which way do these gases work?
Sunlight passes in. Heat on its way out is partly caught.
Right. That is the whole trick: the gases are clear to light and cloudy to heat. Light gets to the ground; the heat the ground gives off has a harder time leaving.
They block sunlight. That is why it is cooler in the shade.
Not yet. Shade comes from clouds, trees and roofs, which are things. These gases are invisible to sunlight; it goes through them as if they were not there.
Both pass straight through. The ground just holds heat well.
Not yet. If both passed straight through, the ground would settle at −18°C like bare rock. The 33 degrees have to come from something slowing the heat out.
Sunlight comes through the air→The ground warms and glows heat up→Gases catch some and glow it back down→The ground must be warmer to push it all out
So the ground gets heat twice: from the Sun, and again from the air above sending some back down. To get rid of all of it, the ground has to glow harder, and glowing harder means being warmer. Heat is not trapped forever. It leaves more slowly, from a warmer ground.
Four things in the air share the blanket's work today. Tap the one that does the most.
Tap the part of the picture that answers it.
Think about what is in a cloud, and where it came from.
Water vapour50%
Clouds25%
Carbon dioxide20%
Methane and other gases5%
Rough shares of the warming the blanket gives.
Water vapour
Right. Most people tap carbon dioxide. Water vapour does about half. But water falls out as rain within days, so how much is up there is set by how warm the air already is. It follows the temperature; it does not lead it.
Clouds
Not yet. Clouds do two jobs at once: they bounce sunlight away and they hold heat in. What is left after both is about a quarter of the blanket.
Carbon dioxide
Not yet. About a fifth, so not the biggest. But it stays up for centuries, and its amount is set by what we burn, not by the temperature. That is why it leads and water vapour follows.
Methane and other gases
Not yet. Methane catches heat far more strongly than carbon dioxide, molecule for molecule. There is very little of it, so its share of the whole is small.
Carbon dioxide in 1750280 per million
Carbon dioxide today420 per million
Carbon dioxide is the one we have changed most. For every million bits of air, about 280 were carbon dioxide in 1750. Today about 420 are. That is half again as much. The blanket got a little thicker, and the ground has to glow a little harder to get its heat through it.
Move the control to set how much carbon dioxide is in the air. The warming follows the usual rule of thumb: each doubling adds about 3 degrees once the oceans have caught up.
Warmer than 1750, once the oceans catch up0°C
Warming measured so far1.2°C
280 per millionThe level in 1750. Nothing added yet, so this is the starting point.
Warmer than 1750, once the oceans catch up1.8°C
Warming measured so far1.2°C
420 per millionAbout today's level: half again as much as 1750. Once the oceans have caught up, that is about 1.8 degrees warmer.
Warmer than 1750, once the oceans catch up3°C
Warming measured so far1.2°C
560 per millionDouble the 1750 level. About 3.0 degrees warmer once caught up.
Warmer than 1750, once the oceans catch up4.8°C
Warming measured so far1.2°C
840 per millionThree times 1750. About 4.8 degrees — the second doubling is not finished yet.
Warmer than 1750, once the oceans catch up6°C
Warming measured so far1.2°C
1120 per millionFour times 1750, which is two doublings. About 6.0 degrees: each doubling added about the same 3.
Going from 280 to 560 adds about 3 degrees. How much does going from 560 to 1,120 add?
Almost nothing. The blanket is already full.
Not yet. A thicker blanket keeps working; nothing about it fills up. Venus, with a blanket hundreds of times thicker than ours, is over 400 degrees warmer than the sums for bare rock give.
About 6 more. Twice as much gas, twice the warming.
Not yet. It is twice the gas, but the warming grows by doublings, not by amounts. 560 to 1,120 is one doubling, the same as 280 to 560, and it adds about the same 3 degrees.
About 3 more. Each doubling adds about the same, however much is already there.
Right. The first bits of a gas do the most work and each extra bit does a little less. So doubling from a high level adds the same as doubling from a low one, not more.
Move the control.
How much warmer is the Earth's surface today than in the late 1800s?
Pick one. Nothing is scored.
0°C0.67°C1.34°C
19002020
Warming since the late 1800s, by decade, rounded.
About 1.2°C.Guesses run high because the news is about record heat, or low because one degree sounds like nothing. About 1.2 degrees, averaged over the whole surface, land and sea, day and night. The next chapter shows why one degree in the average is a big change at the edges.
Two levers: how much sunlight gets in, and how easily heat gets out. Which lever does each of these pull?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Carbon dioxide
Fresh snow
Methane
Haze from a big volcano, high in the air
Water vapour
A bright desert
Carbon dioxide → Slows heat getting out
Not yet. Carbon dioxide slows heat getting out. It lets sunlight straight through and catches the heat on its way back up.
Fresh snow → Bounces sunlight away
Not yet. Snow bounces sunlight away. It is white, so the light goes back to space before it ever becomes heat.
Methane → Slows heat getting out
Not yet. Methane slows heat getting out. Like carbon dioxide it is clear to sunlight and catches heat, only more strongly.
Haze from a big volcano, high in the air → Bounces sunlight away
Not yet. Haze bounces sunlight away. High in the air it turns light back before it reaches the ground.
Water vapour → Slows heat getting out
Not yet. Water vapour slows heat getting out. It is the biggest heat-catcher of all, and sunlight passes through it.
A bright desert → Bounces sunlight away
Not yet. A bright desert bounces sunlight away. Pale sand sends much of the light back up, so less is soaked up as heat.
Sunlight bouncedless in
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Heat caughtharder out
Right. The gases work only on the second lever: they are clear to sunlight and catch heat. Snow, haze and pale ground work only on the first: they send sunlight away before it becomes heat at all.
167°CMercury, nearest the Sun
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464°CVenus, thick carbon dioxide air
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15°CEarth
Average ground temperature.
Venus has air almost entirely of carbon dioxide, ninety times thicker than ours. Its ground is about 464°C. Mercury is nearer the Sun and averages about 167°C. What explains Venus?
Volcanoes heat it from below.
Not yet. Heat from inside a planet is tiny next to sunlight. On Earth it is about one ten-thousandth of what the Sun delivers. Venus is hot from the top down, not the bottom up.
It is closer to the Sun than the Earth.
Not yet. It is, but Mercury is closer still and averages 300 degrees cooler. If distance were the whole story, Mercury would be the hottest, and it is not.
A very thick blanket. Its ground must be very hot to push heat out through it.
Right. Venus gets less sunlight than Mercury and is far hotter. Distance cannot explain that. A blanket of almost pure carbon dioxide, ninety times thicker than ours, can.
Lesson complete
A few gases let sunlight in and slow heat on its way out; more of them means a warmer ground.