The greenhouse effect: Why do increased greenhouse gas emissions cause climate change?
One of the most important climatic variables, the temperature at the Earth's surface, is determined by the balance of radiation entering and leaving the Earth's atmosphere. An increase in the concentration of greenhouse gases alters that balance, and hence alters the temperature at the Earth's surface.
The situation is summarised in the following diagram:
Explaining the greenhouse effect in terms of incoming and outgoing radiation
The earth's atmosphere (the air that we breathe) contains a number of so called greenhouse gases. The ones most closely associated with global warming are carbon dioxide (CO2) and methane (CH4). These gases behave like the glass panes in a greenhouse.
Incoming shortwave radiation from the sun
- Sunlight (short wave solar radiation) enters the earth's atmosphere.
- Most of this solar radiation is absorbed by the earth's surface (land and sea) and warms it.
- Some of it is reflected by the earth back into the atmosphere.
Outgoing longwave (infrared) radiation from the earth
- In return the earth admits long wave energy back into the atmosphere.
- Because it is longwave energy (not shortwave like the energy carried by the rays from the sun), some of it gets trapped by the greenhouse gases.
- This causes the earth to be warmer than it would without the greenhouse gases.
- The thicker the blanket of greenhouse gases, the more the outgoing energy gets trapped and the greater the warming effect.
The greenhouse analogy
Have you ever seen a greenhouse? Most greenhouses look like a small glass house. Greenhouses are used to grow plants, especially in the winter. Greenhouses work by trapping heat from the sun. The glass panels of the greenhouse let in light but keep heat from escaping. This causes the greenhouse to heat up, much like the inside of a car parked in sunlight, and keeps the plants warm enough to live in the winter.
For a more scientific understanding of the greenhouse effect, you need to read the next section.
Energy balance equation
This is the basic energy balance:
Let S denote the average solar radiation (342 W m-2) entering the earth's atmosphere.
Let A denote the percentage of solar radiation (30%) which is sent directly back to space (A stands for albedo).
Then the incoming shortwave radiation is given by S(1-A).
Let E be the radiation emitted from the Earth's surface (W m-2). Then E is given by the Stefan-Boltzmann law as:
E = σT4
Where T denotes the temperature at the surface of the Earth and σ is the Stefan-Boltzmann constant (with a value of 5.67 x 10-8 W m-2 K-4).
The longwave radiation leaving the Earth's atmosphere is just E minus the amount of radiation trapped by the greenhouse gases, call this G (W m-2).
So the outgoing longwave radiation is given by σT4 - G.
Over a short timeframe the Earth's atmosphere is in equilibrium. Hence the incoming shortwave radiation must equal the outgoing longwave radiation:
S(1-A) = σT4 - G
Rearranging, we get:
S(1-A) = σT4 = S(1-A) + G
From this it is apparent that T, the temperature at the Earth's surface will increase if G increases.
It is also apparent that the temperature at the Earth's surface will decrease if A, the albedo, increases. From this it is clear why it has recently been suggested that the greenhouse effect could be combated in part by painting all the roofs in the world white. Why? White roofs would increase the percentage of solar radiation reflected back out of the atmosphere (being shortwave radiation very little would be trapped by the atmosphere - the greenhouse effect is about longwave radiation, not shortwave radiation). In effect painting the roofs white would increase the albedo, thus decreasing the temperature at the Earth's surface.