Initial causes of temperature changes (external forcings)
What are examples of external forcings?
Global warming
Global warming Positive feedback Proper forcing axiom Attribution of recent climate change Polar forests of the Cretaceous
Global_warming-Initial_causes_of_temperature_changes_(external_forcings) Positive_feedback-In_climatology Proper_forcing_axiom-Abstract Attribution_of_recent_climate_change-Background Polar_forests_of_the_Cretaceous-Paleoclimate_proxies
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.
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."
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. 
[[Image:3 examples of internal climate variability (1950-2012), the El Niño – Southern Oscillation, the Arctic Oscillation, and the North Atlantic Oscillation (NOAA).png|thumb |right |alt=Refer to caption |This image shows three examples of internal climate variability measured between 1950 and 2012: the El Niño–Southern oscillation, the Arctic oscillation, and the North Atlantic oscillation. ]]This section introduces some concepts in climate science that are used in the following sections:Factors affecting Earth's climate can be broken down into feedbacks and forcings. A forcing is something that is imposed externally on the climate system. External forcings include natural phenomena such as volcanic eruptions and variations in the sun's output. Human activities can also impose forcings, for example, through changing the composition of the atmosphere. Radiative forcing is a measure of how various factors alter the energy balance of the Earth's atmosphere. A positive radiative forcing will tend to increase the energy of the Earth-atmosphere system, leading to a warming of the system. Between the start of the Industrial Revolution in 1750, and the year 2005, the increase in the atmospheric concentration of carbon dioxide (chemical formula: CO2) led to a positive radiative forcing, averaged over the Earth's surface area, of about 1.66 watts per square metre (abbreviated W m−2). Climate feedbacks can either amplify or dampen the response of the climate to a given forcing. There are many feedback mechanisms in the climate system that can either amplify (a positive feedback) or diminish (a negative feedback) the effects of a change in climate forcing. Aspects of the climate system will show variation in response to changes in forcings. In the absence of forcings imposed on it, the climate system will still show internal variability (see images opposite). This internal variability is a result of complex interactions between components of the climate system, such as the coupling between the atmosphere and ocean (see also the later section on Internal climate variability and global warming). An example of internal variability is the El Niño-Southern Oscillation. 
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. 