Global Warming

Greenhouse Effect

Shown above is an Infrared Map of the Earth. Red areas represent regions of high heat retention in the atmosphere. This is an equatorial band because that is where the atmosphere has the most water vapor.

The earth has a natural greenhouse effect due to trace amounts of H20 and CO2 that naturally occur. The enhanced greenhouse effect refers to the augmentation of these natural gases by human activities.

What's the evidence?

The energy balance in the atmosphere is shown here:

The main components in this diagram are the following:

What Happens to the 69% of the incoming radiation that doesn't get reflected back:

Achieving Thermal Equilibrium:

This short wavelength radiation is absorbed by the earth which heats the earth to a finite temperature. Since the earth wants to stay in thermal equilibrium, it must re-radiate this energy.

The earth has an equilibrium temperature of about 300 Kelvins . At this temperature, the wavelength of the emitted radiation is in the infrared.

What happens to the outgoing infrared radiation?

So clearly, if human activities increase the ability for the earth's atmosphere to absorb IR radiation, this produces a net warming of the atmosphere over time. This is the Enhanced Greenhouse effect:

Concentration increases since 1750:

Data showing the increasing concentration of Greenhouse Gases:


CO2 is steadily increasing at the rate of about 1 ppm per year


NOx is leveling off due to pollution abatement on vehicles


CH4 is only recently been measured. Rate of increase was about 10 ppb per year in the late 80's and early to mid 90's. Since 1 CH4 molecule is worth 25 CO2 molecules, the equivalent growth rate in terms of CO2 is 250 ppb (or 0.25 ppm)

However, in the late 90's the rate of methane addition to the atmosphere had leveled off. The exact physical reasons for this are unclear at the moment and how long this situation will remain is also unclear.

If methane concentrations begin to rise again, then it will likely dominate the enhanced greenhouse effect.

Proceed on to Global Warming

Previous Lecture Next Lecture Course Page