Earth & climate
The greenhouse effect
Why Earth is comfortably warm: greenhouse gases trap heat, and too many of them add extra warming.
What you need first
Have you noticed this? A car sits in the sun, and after half an hour it is much hotter inside than outside. That is a good first impression, but be careful: a car heats up mainly because the warm air cannot escape. With the Earth a different mechanism is at work, and that is what we will look at now.
How the greenhouse effect works
Sunlight passes through the atmosphere almost unhindered and warms the ground. The warm ground releases the energy again as invisible . And here comes the trick: certain gases in the air, the greenhouse gases, let sunlight through but absorb part of the heat radiation and re-radiate it in all directions, including back down to the ground. So more warmth stays near the surface, even though the air can circulate freely. The most important greenhouse gases are carbon dioxide (CO₂), methane and water vapour.
The greenhouse effect is natural: without it, Earth would be about 33 °C colder.
Too much CO₂ strengthens the effect and Earth warms up.
A natural effect
The greenhouse effect is a good thing at first, because it is completely natural and ancient. Without it, Earth's average temperature would be around −18 °C, the oceans would be frozen and life as we know it would not exist. Thanks to the natural greenhouse gases, the average is about 15 °C instead. That difference of roughly 33 degrees is the gift of this thin blanket of gas.
When the blanket gets thicker
What matters is the amount of greenhouse gases. If extra CO₂ or methane enters the air, the blanket gets thicker: even more heat radiation stays in the atmosphere, and the Earth warms beyond its natural level. This is exactly what researchers have been measuring for many decades. So the effect itself is neither good nor bad, it all depends on the balance. How humans are changing this balance and what we can do about it is what you will explore in the next topic.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What do greenhouse gases do in the atmosphere?
Which of these gases is a greenhouse gas?
Without the greenhouse effect, Earth's average temperature would be −18 °C, with it 15 °C. By how many degrees does the natural greenhouse effect make the Earth warmer?
Put the steps of the greenhouse effect in the correct order.
- 1Sunlight passes through the atmosphere and reaches the ground.
- 2The gases re-radiate the heat in all directions, including back to the ground.
- 3The warm ground gives off energy as invisible heat radiation.
- 4Greenhouse gases absorb part of this heat radiation.
Inside a greenhouse it is 8 °C warmer than outside. Outside the thermometer shows 13 °C. How many degrees Celsius is it inside the greenhouse?
Without the natural greenhouse effect, Earth's average temperature would be about … °C instead of a pleasant 15 °C.
Where this leads