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
• 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:

 

Table 1

Characteristics of some major (climate changing) greenhouse gases

Greenhouse gas	Sources	Sinks	Importance for climate
Carbon Dioxide (CO2)	1) Burning of fossil fuel 2) Land-use change (deforestation)	1) Ocean Uptake 2) Plants!& photosynthesis	Absorbs infrared radiation; affects stratospheric O3
Methane (CH4)	1) Biomass burning 2) Enteric fermentation 3)Rice paddies	1) Reactions with OH 2) Microorganisms uptake by soils	Absorbs infrared radiation; affects tropospheric O3 and OH; affects stratospheric O3 and H2O; produces CO2
Nitrous Oxide (N2O)	1) Biomass burning 2) Fossil-fuel combustion 3) Fertilizers	1) Removal by soils 2) Stratospheric photolysis and reaction with O	Absorbs infrared radiation; affects stratospheric O3
Ozone (O3)	Photochemical reactions involving O2	Catalytic chemical reactions involving NOx, ClOx and HOx species.	Absorbs ultraviolet and infrared radiation
Carbon Monoxide (CO)	1) Plant emissions 2) Man-made release (transport, industrial)	1) Soil uptake 2) Reactions with OH	Affects stratospheric O3 and OH cycles; produces CO2
Chlorofluorocarbons (CFCs)	Industrial production	Insignificant in troposphere, dissociated in stratosphere (photolysis and reaction with O)	Absorbs infrared radiation; affects stratospheric O3
Sulphur Dioxide (SO2)	1) Volcanoes 2) Coal and Biomass burning	1) Dry and wet deposition 2) Reactions with OH	Forms aerosols, which scatter solar radiation

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