Initial causes of temperature changes (external forcings)
Greenhouse effect schematic showing energy flows between space, the atmosphere, and Earth's surface. Energy exchanges are expressed in watts per square meter (W/m2).
The increase of atmospheric carbon dioxide (CO2) concentrations from 1958–2013. Monthly CO2 measurements display seasonal oscillations in an upward trend; each year's maximum occurs during the Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO2.
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 a glacial period within the current ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.