Climate Change Basics
Greenhouse Effect, Global Warming Potential Trace Gases
Global Warming Potential
A global warming potential (GWP) of a gas is the atmosphere’s absorptivity of the gas multiplied by its lifetime then divided by the atmosphere’s absorptivity of carbon dioxide multiplied by the lifetime of carbon dioxide. Basically, this is a calculation of how much heat the gas will trap in the atmosphere compared with carbon dioxide. It is used for global policies though it does have some flaws for that purpose.
Trace Gases
Large quantities of relatively stable, steady, short-lifetime gases won’t do much to change the atmosphere if they don’t absorb, react or stick around for a long time. Small, trace gases that stick around and do a lot of damage greatly impact the atmosphere. For example, one chlorine atom can destroy up to 10,000 ozone molecules in its lifetime. For a while, human activity was pumping a relatively small amount into the atmosphere but due its properties we were able to impact the ozone layer much more than previously thought possible. The same idea is for adding greenhouse gases to the atmosphere. Small amounts have big impacts!
The Greenhouse Effect
The Greenhouse Effect is a natural phenomenon. The overall effect is that the surface of the Earth gets warmer due to extra heat from the atmosphere. The process arises because the Earth doesn’t absorb all the Sun’s energy- just short-wave radiation. Some of this energy is also reflected by clouds due to the Earth’s albedo. Most of the rest of the energy gets absorbed by the surface. The Earth warms up and then radiates long-wave radiation back to the atmosphere. This long-wave radiation is absorbed by certain gases. These gases don’t absorb the incoming short-wave radiation, just the outgoing long-wave energy. Then, half this absorbed energy is radiated back to warming the Earth and the rest up to space. The Greenhouse Effect is the heat absorbed and radiated back to Earth.
The Earth is warmed by 30 degrees Celsius because of the Greenhouse House effect, versus just 17 degrees warmer because of the atmosphere. The gases responsible for the Greenhouse Effect are present in trace concentrations in the atmosphere, which is why even a small portion of anthropogenic sources can affect this otherwise natural phenomenon. These specific gases absorption rates are directly related to the concentrations present and are also highly specific as far as absorption. These particular gases absorb mainly in a specific window region of IR radiation and an increase in absorption here is critical for upsetting the balance of the Greenhouse Effect.
Stratosphere Cooling
The stratosphere cools as the atmosphere warms due to radiative transfer. The incoming short-wave radiation heats up the surface which then emits long-wave radiation back to the atmosphere. The amount of heat the Earth absorbs and the amount it reflects are fairly constant, even during global warming (which is caused by an increase in absorption, not input). Since the heat gets trapped here by greenhouse gases that absorb it, less heat travels outward to the stratosphere. It’s as if radiated heat on the Earth is a dense collection of balloons—if there are more balloons near the surface, there will be fewer balloons for higher up.
