The climate system can respond to changes in "external forcings."[61]
[62]
External forcings can "push" the climate in the direction of warming or cooling.[63]
Examples of external forcings include changes in atmospheric composition (e.g., increased concentrations of greenhouse gases
), solar luminosity
, volcanic
eruptions, andvariations in Earth's orbit
around the Sun.[64]
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.[65]