Why is there controversy to the opinion that human contribution of carbon dioxide to the atmosphere is causing global warming?
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Because the Earth goes through periodic cycles of cooling and warming.
Because the levels of carbon dioxide being released by humans has not increased since the past.
Because climate is so complex that the two cannot be definitively related.
Because the statistics are not clear about how much additional carbon dioxide input would be required to raise global temperatures.

Global warming can be define as the increase in temperature worldwide due to increase in abundance of greenhouse gases in the atmosphere. These gases are responsible for trapping the heat energy from sunlight.

Carbon dioxide is also a greenhouse gas which is chiefly responsible for global warming. There is a controversy that human beings are responsible for global warming as release of carbon dioxide into the atmosphere is due to burning of fossil fuels and deforestation on the large scale. But volcanic eruption and other natural factors are also responsible for such increase in abundance. Therefore, a proper statistical account for release of carbon dioxide in the environment has not been framed.