( ( ( The combustion ) ( of ( natural gas ) ) ) ( ( ( releases ( less ( of ( ( the ( greenhouse ( gas ( carbon dioxide ) ) ) ) ( per ( unit energy ) ) ) ) ) ) ( than ( does ( ( petroleum or ) coal ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN combustion)) (PP (IN of) (NP (JJ natural) (NN gas)))) (VP (VBZ releases) (NP (NP (JJR less)) (PP (IN of) (NP (NP (DT the) (NN greenhouse) (NN gas) (NN carbon) (NN dioxide)) (PP (IN per) (NP (NN unit) (NN energy)))))) (SBAR (IN than) (S (VP (VBZ does) (NP (NN petroleum) (CC or) (NN coal)))))) (. .)))
The combustion of natural gas releases less of the greenhouse gas carbon dioxide per unit energy than does petroleum or coal.
(ROOT (S (NP (JJ Geothermal) (NN energy) (NN energy)) (VP (VP (VBZ is) (ADJP (JJ clean))) (CC and) (VP (VBZ does) (RB not) (VP (VB release) (NP (NN greenhouse) (NNS gases))))) (. .)))