Radiation Heats and Cools Earth: The Greenhouse Effect

Remember from Grade 5

In Grade 5 you learned that greenhouse gases act a bit like a blanket around Earth, that we need some of them to stay warm, and that burning fossil fuels adds carbon dioxide and is changing Earth's climate. Now you will use what you know about radiation to explain how this blanket actually works.

A frosty mystery

It is October in Ontario. Two nights in a row, the evening temperature is the same. The first night is cloudy, and in the morning the grass is just wet. The second night is clear and starry, and in the morning the grass is white with frost. Why did the clear night get colder?

Radiation in, radiation out

Daytime: sunlight passes through the atmosphere. About 30% of it is reflected straight back to space by clouds, ice, snow and other bright surfaces. Most of the rest is absorbed by the land and oceans, which warm up.

All the time: Earth's warm surface gives off infrared radiation. Some of it escapes to space. This is how Earth cools.

At night: there is no incoming sunlight, but Earth keeps radiating infrared. So the ground and air cool down. The longer the night, the more they cool, which is one reason winter nights in Canada get so cold.

If Earth absorbs as much energy as it gives off, its average temperature stays steady. This is Earth's energy balance.

The greenhouse effect

Some gases in the atmosphere, called greenhouse gases, let most incoming sunlight pass through, but they absorb much of the infrared radiation coming up from Earth's surface. Then they give off that energy in all directions, so some goes back down toward the surface.

The main greenhouse gases are water vapour, carbon dioxide (CO₂), methane and nitrous oxide.

This natural greenhouse effect is essential. Without it, Earth's average surface temperature would be about −18 °C, and most of the planet would be frozen. With it, the average is about 15 °C.

Back to the frost mystery: clouds are made of water droplets, and the air around them holds water vapour. They absorb infrared from the ground and send some back down, like a blanket. On a clear night, more infrared escapes straight to space, so the ground cools faster and frost can form.

The enhanced greenhouse effect

Since the 1700s, people have burned huge amounts of coal, oil and natural gas, and cleared forests. This has added carbon dioxide and other greenhouse gases to the atmosphere. Scientists have measured that carbon dioxide has risen from about 280 parts per million before then to more than 420 parts per million today, an increase of about 50%.

With more greenhouse gases, more infrared is absorbed and sent back down, and less escapes to space. Earth's energy balance is upset: more energy comes in than goes out, so Earth warms. This extra warming is the enhanced greenhouse effect, and it is the main cause of today's climate change.

Why "greenhouse"?

The name compares Earth to a glass greenhouse, but the comparison is not perfect. A real greenhouse stays warm mostly because the glass stops the warm air from rising and blowing away (it stops convection). The atmosphere has no glass roof; greenhouse gases warm Earth by absorbing and re-emitting infrared radiation. Scientists still use the name because everyone knows it.