Theme 9 – The Greenhouse Effect

ASTR 101Prof. Dave Hanes

The Greenhouse Effect

A First Important Point

A large fraction of the sun’s energy reaches us as

visible light. The Earth’s atmosphere is transparent

to visible light. (We can see the stars!)

On Reflection…Q: If visible light passes through the atmosphere on the way in, what will it do if it reflects from a snow-covered surface? (Here, England and Ireland.)

Answer: it will pass back through the atmosphereand go out into space! (That’s how astronautscan see the continents, oceans, and so on.) Theatmosphere is transparent in both directions.

In other words, reflected light will not heat the surface.

But Not All the Sunlight is Reflected!

A dark roadway looksblack because it absorbs the light – and thus the energy, which heats it.

Similarly other surfaces and

objects.

Trapping the Energy

The warmed surface glows in the infrared… but the infrared light cannot flow through!

It is absorbed by the so-called greenhouse gases in the atmosphere, heating it from below.

The Usual Suspects

The famous greenhouse gases are H2O, CO2, CH4 and O3 (water vapour, carbon dioxide, methane and ozone.)

Because of their abundance, water vapour (distributed in the air, not just in clouds) and carbon dioxide are the most important.

A New Balance

The trapping of energy does not go on without cease!

An eventual equilibrium is reached, with an atmosphere that is warmer than expected. The upper part of the troposphere emits infrared radiation out into space, and all is in balance.

WITHOUT greenhouse gases, our planet would be colder by 30-50 degrees at the surface… so some greenhouse effect is welcome and indeed essential!

The Earth, as Seen by ‘Infrared Eyes’

we see only the warm blanket of air, glowing in the IR

But Why Greenhouse Effect?

A greenhouse works in an analogous way: the glass prevents the infrared radiation from escaping.

So: the visible light warms allthe plants, earth and pots. But the re-emitted IR cannot get out through the glass! So the greenhouse stays warm.

(Note that in this case the atmospheric gases are not responsible.)

Glass is Opaque to Infrared

Although visible light is streaming out through these windows on a cold winter night, very little heat is lost.

Danger! – The Effect Can be Deadly

Conversely: The Desert at Night – Why is it So Cold?

The soil heats up in the day, but there is no water vapour in the air (deserts are dry!) to prevent the heat radiating away during the night.

Although some greenhouse effect is welcome,

We Want to Avoid a Runaway!