Causes

By itself, the climate system experiences various cycles which can last for years (such as the El Niño–Southern Oscillation) to decades^[53] or centuries.^[54] Other changes are caused by external forcings. These forcings are "external" to the climate system, but not always external to the Earth.^[55] Examples of external forcings include changes in the composition of the atmosphere (e.g. increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in the Earth's orbit around the Sun.^[56]

Attributing detected temperature changes and extreme events to human-caused increases in greenhouse gases requires scientists to rule out known internal climate variability and natural external forcings. Therefore, a key approach is to use physically or statistically based computer modelling of the climate system to determine unique fingerprints for all potential causes. By comparing these fingerprints with observed patterns and evolution of climate change, and the observed evolution of the forcings, the causes of the observed changes can be determined.^[57] Scientists have determined that the major factors causing the current climate change are greenhouse gases, land use changes, and aerosols and soot.^[58]