Forcings in the climate sense are external boundary conditions or inputs to a climate model. Obviously changes to the sun’s radiation are external, and so that is always a forcing. The same is true for changes to the Earth’s orbit (“Milankovitch cycles”). Things get a little more ambigous as you get closer to the surface. In models that do not contain a carbon cycle (and that is most of them), the level of CO2 is set externally, and so that can be considered a forcing too. However, in models that contain a carbon cycle, changes in CO2 concentrations will occur as a function of the climate itself and in changes in emissions from industrial activity. In that case, CO2 levels will be a feedback, and not a forcing. Almost all of the elements that make up the atmosphere can be considered feedbacks on some timescale, and so defining the forcing is really a function of what feedbacks you allow in the model and for what purpose you are using it. A good discussion of recent forcings can be found in Hansen et al (2002) and in Schmidt et al (2004).
Typically refers to a three-dimensional model of the global atmosphere used in climate modeling (often erroneously called “Global Climate Model”). This term often requires additional qualification (e.g., as to whether or not the atmosphere is fully coupled to an ocean–see ‘Atmosphere-Ocean General Circulation Model’).
The length scales that are resolved in these models is typically on the order of 100s of kilometers (i.e. features that size or smaller are not directly resolved). The timestep for the models (how often the fields are updated) is usually 20 minutes to an hour. Thus in any day there would be 24 to 72 loops of the main calculations.
The basic variables are the temperature, humidity, liquid/ice water content and atmospheric mass. The physics usually consists of advection, radiation calculations, surface fluxes (latent, sensible heat etc.), convection, turbulence and clouds. More elaborate Earth System models often contain tracers related to atmospheric chemistry and aerosols (including dust and sea salt).
Greenhouse Gases (GHGs) refer to any atmospheric gases that absorb long wave radiation (emitted from the surface), thereby causing the planet’s surface to be warmer than it would be otherwise. These gases include water vapour, CO2, CH4, N2O, many CFCs (chloro-fluro-carbons). Ozone (O3) as well as being a shortwave absorber (in the ultra-violet range) also has a small longwave greenhouse effect. Other components of the atmosphere also absorb longwave radition (specifically aerosols and clouds) and hence have a greenhouse effect while not being gases themselves.
Oxygen (O2) and nitrogen (N2) while being the dominant gases in the atmosphere do not have significant absorption lines in the relevant longwave range and so are not greenhouse gases.
Instrumental data describing large-scale surface temperature changes are only available for roughly the past 150 years. Estimates of surface temperature changes further back in time must therefore make use of the few long available instrumental records and natural archives or ‘climate proxy’ indicators, such as tree rings, corals, ice cores and lake sediments, and historical documents, to reconstruct patterns of past surface temperature change. Due to the paucity of data in the Southern Hemisphere, recent studies have emphasized the reconstruction of Northern Hemisphere (NH) mean, rather than global mean temperatures over roughly the past 1000 years.
Isotopes can be thought of as different ‘flavours’ of a particular element (such as oxygen or carbon), that are distinguished by the number of neutrons in their nucleus (and hence their atomic mass). Carbon for instance most commonly has a mass of 12 (written as 12C), but there are also a small fraction of carbon atoms with mass 13 and 14 (13C and 14C), similarly oxygen is normally 16O, but with small amounts of 17O and 18O. All of the isotopes of an element behave in similar way chemically. However, because the mass of each isotope is slightly different there are certain physical processes that will discriminate (or ‘fractionate’) between them. For instance, during evaporation of water, it is slightly easier for the lighter isotopes to escape from the liquid, and so water vapour generally has less 18O than the liquid water from which it came. Because of these physical effects, looking at the ratio of one isotope to another can often be very useful in tracing where these atoms came from.