Nib asked
How does the greenhouse effect cause global warming?
The greenhouse effect is natural and keeps Earth livable; global warming is what happens when we thicken it by adding carbon dioxide, so less heat escapes to space.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the twist
Earth's average surface temperature is about 15 degrees Celsius. If the air had no greenhouse gases at all, the planet would settle near minus 18, cold enough to freeze the oceans. That gap of about 33 degrees is the natural greenhouse effect, and life depends on it.
So the effect itself is not what we are worried about. Global warming is what happens when we strengthen it.
The natural greenhouse effect makes Earth about 33 degrees warmer, and we need it.
a common mix-upThe greenhouse effect is not the problem. It is natural and essential. Global warming is the extra warming from adding more greenhouse gases to the air. (A glass greenhouse mostly works by stopping warm air from rising, a different trick…
2the budget
Sunlight is the only big energy source. After the clouds and ice reflect some of it away, Earth absorbs about 240 watts for every square meter of its surface, averaged over the whole planet. To stay at a steady temperature it has to radiate the same 240 watts back to space as invisible infrared heat.
A planet with no heat-absorbing gases would need to be at about minus 18 degrees to radiate that much. Our real surface is much warmer, so something must be slowing the heat's escape.
In steady state, energy in equals energy out: about 240 watts per square meter.
If Earth absorbed 240 watts per square meter, how much heat must it send out to stay at the same temperature?
Show the answer
The same, 240 watts per square meter.
3light gets in
About half of the sunlight that reaches Earth makes it all the way to the surface. It arrives mostly as visible light, which clear air lets through almost untouched. The ground soaks it up and warms.
Sunlight passes through the air and warms the ground.
4gas catches heat
The warm ground cannot shed its energy as visible light. It radiates infrared, a form of light we feel as heat. Molecules of carbon dioxide, water vapor and methane have just the right shape to absorb infrared, and they then re-emit it in random directions. Some goes up and escapes, but a large share heads back down and warms the ground again.
Greenhouse gases absorb infrared heat and send much of it back down.
a common mix-upGreenhouse gases are not a lid that bottles heat up. They slow its escape by repeatedly absorbing and re-radiating it.
5the numbers
Nitrogen and oxygen make up about 99 percent of dry air, but they do not absorb infrared, so they play almost no part. The trace gases do the work. A surface at 15 degrees radiates about 390 watts per square meter, yet only about 240 reach space. The difference, around 150, is held back and returned to the ground. That is the natural greenhouse effect in numbers.
The ground sends up about 390 watts per square meter, but only about 240 escape.
About how much heat per square meter is sent back to the ground?
Show the answer
Roughly 150 watts, the 390 sent up minus the 240 that escape.
6more CO₂
Before the Industrial Revolution the air held about 280 parts per million of carbon dioxide. Burning coal, oil and gas, and clearing forests, has pushed it past 420, a rise of about 50 percent, and it is still climbing. Measurements of air trapped in ice show this is far above anything in at least the last 800,000 years.
Carbon dioxide has risen from about 280 to over 420 parts per million.
7the warming
With more carbon dioxide, the air absorbs and returns more of the ground's heat, so less escapes to space. For a while Earth takes in more energy than it sends out, and the extra energy warms the surface, oceans and air. Warming continues until the planet radiates 240 watts again, at a higher temperature. So far the world is a little over 1 degree Celsius warmer (about 1.2 by recent decade averages) than before industry, and the more carbon dioxide we add, the further this goes.
More CO₂ means less heat escapes, so Earth warms until in equals out again.
Why does adding carbon dioxide warm the planet if sunlight hasn't changed?
Show the answer
Less of the ground's heat escapes to space, so energy builds up until the surface is warm enough to radiate it all.
8the whole idea
So, the whole picture: sunlight in, a gas blanket that sends heat back, heat out. Thicken the blanket and the planet settles warmer.
The greenhouse effect is Earth's warm blanket, and we are making it thicker.