( ( The ( chief ( hothouse gas ) ) ) ( ( is ( ( ( water ( vapor-not ( carbon dioxide ) ) ) or ) methane ) ) . ) )
(ROOT (S (NP (DT The) (JJ chief) (NN hothouse) (NN gas)) (VP (VBZ is) (NP (NP (NN water) (NN vapor-not) (NN carbon) (NN dioxide)) (CC or) (NP (NN methane)))) (. .)))
The chief hothouse gas is water vapor-not carbon dioxide or methane.
(ROOT (S (NP (NP (DT The) (NN combustion)) (PP (IN of) (NP (NN propane) (NN gas)))) (VP (VBZ produces) (NP (NP (NN carbon) (NN dioxide)) (CC and) (NP (NN water) (NN vapor)))) (. .)))