The amount of carbon dioxide (CO2) in Earth's atmosphere has changed over time. In the distant past, natural phenomena (such as volcanic eruptions) altered the amount of CO2 in our planet's air. During the past couple of centuries, humans have influenced CO2 levels by burning large amounts of fossil fuels.
Carbon dioxide is a greenhouse gas. When there is more CO2 in the atmosphere, global temperatures tend to rise. When there is less CO2, temperatures tend to be lower. By carefully studying temperature and CO2 levels from the past, scientists have been able to formulate a mathematical relationship between atmospheric CO2 concentration and average global temperatures. Stated simply, whenever the carbon dioxide concentration in our atmosphere doubles, global temperatures rise by about 3° C (5.4° F).
The simple calculator below allows you to play around with this relationship. Drag the slider to raise or lower the CO2 concentration. See how the temperature changes as a result.
For reference, prior to the rapid increase of human-generated CO2 emissions in the early to mid 1800's, the concentration of carbon dioxide had hovered around 270 parts per million (ppmv) for several centuries. During that time, the average global temperature had been around 13.7° C (56.7° F). Currently, atmospheric CO2 concentration is just shy of 400 ppmv (and rising).