Greenhouse Effect

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?

• World wide Coral Bleaching
• Warming of the oceans in specific areas ( here's some global data )
• Rapid Retreat of tropical glaciers
• All the data on greenhouse gas concentrations!

The energy balance in the atmosphere is shown here:

The main components in this diagram are the following:

• Short wavelength (optical wavelengths) radiation from the Sun reaches the top of the atmosphere.

• Clouds reflect 17% back into space. If the earth gets more cloudy, as some climate models predict, more radiation will be reflected back and less will reach the surface

• 8% is scattered backwards by air molecules:

• 6% is actually directly reflected off the surface back into space

• So the total reflectivity of the earth is 31%. This is technically known as an Albedo . Note that during Ice Ages, the Albedo of the earth increases as more of its surface is reflective. This, of course, exacerbates the problem.

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

• 19% gets absorbed directly by dust, ozone and water vapor in the upper atmosphere. This region is called the stratosphere and its heated by this absorbed radiation. Loss of stratospheric ozone is causing the stratosphere to cool with time, which, of course, greatly confuses the issue of global warming.

• 4% gets absorbed by clouds located in the troposphere. This is the lower part of the earth's atmosphere where weather happens.

• The remaining 47% of the sunlight that is incident on top of the earth's atmosphere reaches the surface. This is not a real significant energy loss.

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?

• If it all went directly back into space, the earth would be a significantly colder place than it is.

• 15% is directly radiated back by the cloud-free land surface: 6% of that is absorbed by the atmosphere and 9% goes directly back into space

• 60% is re-radiated back into space by the net emission of the atmosphere and the clouds. The total radiated back into space is 69% meaning that 31% is temporarily stored as energy and emitted back later.

• Of this 31%, 24% is used to facilitate evaporation. This heat is later released through condensation. This process is called latent heat.

• 7% is stored by the earth's crust and then re-radiated at later times through a complicated heat exchange network of convection and conduction. - Note, this is why heat pumps work. At a few meters below the surface of the earth, the temperature is nearly constant over most of the year because of this low heat flux

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:

• Carbon Dioxide: 280 ppm 360 ppm (29%)
• Nitrous Oxides: 280 ppb 360 ppb (11%)
• Methane: 0.70 ppm - 1.70 ppm (143%)

• In principle, CFCS are very bad and will dominate the greenhouse gases in our atmosphere if their terrestrial usage remains high. CFC worldwide production, however, has been significantly reduced because of concern about The Ozone Layer

• Methane is more directly related to food production and population growth so it could also dominate in the near future

• Frozen methane is also found in the Arctic Ice Caps and will be released due to global warming thus exacerbating the problem. This is far more serious that people realize or has been reported.

Data showing the increasing concentration of Greenhouse Gases:

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

NO_x is leveling off due to pollution abatement on vehicles

CH₄ is only recently been measured. Rate of increase is about 10 ppb per year. However, since 1 CH₄ molecule is worth 25 CO₂ molecules, the equivalent growth rate in terms of CO₂ is 2.5 ppm. Thus methane will eventually be the dominant enhanced greenhouse gas in our atmosphere. What can we do about this?

Proceed on to Global Warming

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