Question: External forcings consist of what?
About the article "Global warming", section "Initial causes of temperature changes (external forcings)".

Candidate passages:
Initial causes of temperature changes (external forcings).
The climate system can respond to changes in "external forcings."
External forcings can "push" the climate in the direction of warming or cooling.
Examples of external forcings include changes in atmospheric composition (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.
Orbital cycles vary slowly over tens of thousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.
Spontaneous fluctuations in global climate might explain past variability.
It is very difficult to know what the true level of variability from only internal causes might be, since other forcings, as noted above, exist; and their magnitude may not be known either.
One approach to evaluating internal variability is to use long integrations of coupled ocean-atmosphere global climate models.
These have the advantage that the external forcing is known to be zero; but the disadvantage is that they may not fully reflect reality.
These variations may result from chaos-driven changes in the oceans, the atmosphere, or interactions between the two.
Two studies have concluded that the demonstrated inherent variability is not great enough to account for the Little Ice Age. 
In the mathematical field of set theory, the proper forcing axiom ("PFA") is a significant strengthening of Martin's axiom, where forcings with the countable chain condition (ccc) are replaced by proper forcings. 
Climate "forcings" may push a climate system in the direction of warming or cooling, for example, increased atmospheric concentrations of greenhouse gases cause warming at the surface.
Forcings are external to the climate system and feedbacks are internal processes of the system.
Some feedback mechanisms act in relative isolation to the rest of the climate system while others are tightly coupled.
Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes.
The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming.
The main negative feedback comes from the Stefan–Boltzmann law, the amount of heat radiated from the Earth into space is proportional to the fourth power of the temperature of Earth's surface and atmosphere.
Other examples of positive feedback subsystems in climatology include:  A warmer atmosphere will melt ice and this changes the albedo which further warms the atmosphere.
Methane hydrates can be unstable so that a warming ocean could release more methane, which is also a greenhouse gas.
The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."
A paleoclimate indicator, also known as a paleoclimate proxy, can reveal important information about what global climate may have been like in the past.
Paleoclimate studies on tree growth rings, deep sea cores, ice cores and paleosols are just a few of the many common proxies used to evaluate the major forcings on paleoclimates.