( ( ( Higher Levels ) ( of ( Carbon Dioxide ) ) ) - )
(ROOT (NP (NP (JJR Higher) (NNS Levels)) (PP (IN of) (NP (NNP Carbon) (NNP Dioxide))) (: -)))
Higher Levels of Carbon Dioxide -
(ROOT (S (NP (NN Oxygen) (NN gas)) (VP (VBZ is) (VP (VBN given) (PRT (RP off)) (PP (IN by) (NP (NNS plants))))) (. .)))