Greenhouse effect is considered to be due to the increased levels of
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Major greenhouse gases include
\[
\boxed{
\mathrm{CO_2},\;
\mathrm{CH_4},\;
\mathrm{N_2O},\;
\mathrm{H_2O},\;
\mathrm{O_3}
}
\]
Among these, \(\mathrm{CO_2}\) is the most commonly cited contributor to global warming.
Step 1: Recall how the greenhouse effect works. Certain gases in the atmosphere let sunlight in but absorb the long wave infra-red radiation the Earth radiates back out, trapping heat near the surface. Step 2: Compare the four gases listed. Carbon monoxide is toxic but not a significant heat trapping gas, sulphur dioxide mainly causes acid rain, and chlorofluorocarbons are potent absorbers but present only in trace amounts. Step 3: Identify the dominant one. Carbon dioxide is released in enormous quantities from fuel combustion and deforestation and is the gas whose rising atmospheric level is most directly linked to global warming. \[ \boxed{\text{Carbon Dioxide}} \]