( ( ( Major effect ) ( of ( burning ( fossil fuels ) ) ) ) ( ( is ( addition ( of ( CO2 ( to atmosphere ) ) ) ) ) . ) )
(ROOT (S (NP (NP (JJ Major) (NN effect)) (PP (IN of) (NP (VBG burning) (JJ fossil) (NNS fuels)))) (VP (VBZ is) (NP (NP (NN addition)) (PP (IN of) (NP (NP (NNP CO2)) (PP (TO to) (NP (NN atmosphere))))))) (. .)))
Major effect of burning fossil fuels is addition of CO2 to atmosphere.
(ROOT (S (NP (JJ Co2) (NN gas)) (VP (VBZ is) (VP (VBN released) (PP (IN into) (NP (NP (DT the) (NN atmosphere)) (SBAR (WHADVP (WRB when)) (S (NP (JJ fossil) (NNS fuels)) (VP (VBP are) (VP (VBN burned))))))))) (. .)))