Quite a lot of times on the internet, there is a vast amount of confusion on exactly what something means; even things like “global warming” can be misunderstood as “temperature of the Earth,” or that the “greenhouse effect” is bad. Here, I will try to clear some up, or at least how I will use the terms in my posts. Note that there are generally authoritative definitions on the subject, such as those used in the IPCC glossary at http://www.grida.no/climate/ipcc_tar/wg1/518.htm , or by the National Academies, etc.
Greenhouse Gases (GHG’s)- Greenhouse Gases include Carbon Dioxide (CO2), water vapor (H2O), methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons (CFCs including CFC-12 and CFC-11 ). Where appropriate, clouds can also behave as a greenhouse substance. To behave as a greenhouse gas, it must have a presence in the atmosphere, and possess the ability absorb terrestrial infrared radiation. For those more chemistry inclined, the molecule must possess a dipole moment or some of its vibrational motions must generate a temporary dipole moment, and so homonuclear diatomic molecules (including oxygen and nitrogen which are abundant in our atmosphere) are NOT greenhouse gases.
Greenhouse effect- The GHG effect is naturally occurring, and rises from the fact that greenhouse gases are in the atmosphere absorbing Earth’s radiation. At the surface, there is an outward longwave radiation (OLR) flux of approximately σT4 (from Stefan-Boltzmann law)(~390 W/m2). At the top -of-atmosphere (TOA), it is about 240 W/m2. This is equivalent to the net solar radiation coming in (~240 W/m2). The ~150 W/m2 difference comes from the greenhouse effect. Because of this, the temperature on Earth is about 33 K warmer than it would be with no atmosphere.
Global Warming- Global Warming will be defined as the “change in” temperature from initial to final conditions. Though “initial” can be somewhat arbitrary, the pre-industrial temperature will be the “zero point” on this blog. If it should deviate, it should be made clear (e.g. there was a global warming during the Eocene). This has nothing to do with the cause of global warming, only the observation that average temperatures have increases since 1750 (the IPCC defines this as pre-industrial, and so will be adopted here).
Anthropogenic (Global Warming)(AGW)- This will specifically refer to the human contribution to the change in temperature.
Climate Change- A “climate change” in this blog will be used to mean the change, or transition of one climatic state to another. It would be best to say “global warming causes climate change.” On the globe, trends do not have to be similar in time or space, so it quite possible for global warming to be happening, and have a cooling trend in your hometown, which may or may not be a result of the changing climate.
Climate- Climate will be defined as “average weather.” 30 year averages is conventional in climatology, though this probably depends on the discussion (e.g. the climate during the glacial period was colder than today). Climate differs from weather (below) in that it is based on long-term averaging and so is well representative of the target (e.g. Arizona’s climate is “hot and dry” whereas its weather will vary day-to-day.)
Weather- Weather is what we experience day-to-day, day-to-night, and on seasonal and interannual scales, and is much more variable than climate.
Radiative Forcing- Radiative forcing is the change in the *net* irradiance (longwave and shortwave), in Watts per square meter (W/m-2 ) at the Top-Of-Atmosphere due to an internal or external forcing of the climate system. In other words, we measure how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. A “balance” would indicate a eqilibrium between solar radiation coming in and longwave going out, and an imbalance is indicative of the climate moving in one direction, depending on the sign of the imbalance (although radiative forcing and radiative imbalance aren’t the same thing). Something which has a positive radiative forcing (ex. increase of CO2, increasing Total Solar Irradiance) causes warming, something with a negative forcing (e.g. decreasing solar irradiance, putting more aerosols up) causes cooling. The net total forcing is the sum of all forcings: F= f1 + f2 + f3…fn
Forcing vs. Feedback- To further clear this up, a forcing is a perturbation of the system which moves it from state 1 to state 2. It is cause in this change. A feedback is a response to this change, which will either amplify the initial forcing and move the system more toward the direction it was headed (positive feedback), have no effect on the original forcing (neutral), or offset the forcing and move the system back closer to initial conditions (negative feedback).