Temperature variations are caused by a combination of physical and human factors.
Physical causes of global warming and cooling include:
These physical causes of global temperature change have always existed and have been responsible for the alternating heating and cooling cycles of the Earth's temperature.
The human causes of global warming have been in the news a lot in recent years, you can probably think of a few examples. Human factors are the result of growing population and economic developments.
The main greenhouse gases in the atmosphere are carbon dioxide, methane and nitrous oxide. They are naturally occurring in the atmosphere, but are believed to have increased through burning more oil, petrol, and coal, while forests have been removed.
Carbon dioxide, methane and nitrous oxide have caused an enhanced greenhouse effect. The build-up of greenhouse gases such as carbon dioxide (CO2) in the atmosphere, is believed to result in more heat being retained in the atmosphere than would usually be the case. Naturally, more of the heat from the Sun would be reflected back into space.
Collectively, these physical and human causes have produced the pattern seen in the graph above.