( ( When ( ( fossil fuels ) ( are burned ) ) ) ( , ( ( their ( carbon atoms ) ) ( ( ( combine ( with ( oxygen ( in ( the air ) ) ) ) ) ( to ( make CO2 ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (JJ fossil) (NNS fuels)) (VP (VBP are) (VP (VBN burned))))) (, ,) (NP (PRP$ their) (NN carbon) (NNS atoms)) (VP (VBP combine) (PP (IN with) (NP (NP (NN oxygen)) (PP (IN in) (NP (DT the) (NN air))))) (S (VP (TO to) (VP (VB make) (NP (NNP CO2)))))) (. .)))
When fossil fuels are burned, their carbon atoms combine with oxygen in the air to make CO2.
(ROOT (S (NP (NNP Air)) (VP (VBZ is) (VP (VBN made) (PP (IN of) (NP (NNS atoms))))) (. .)))