( ( ( ( ( Scientists ( are ( ( becoming ( even ( more certain ) ) ) ( that ( humans ( are ( ( a ( key contributor ) ) ( to ( global warming ) ) ) ) ) ) ) ) ) , ) and ) ( some ( ( are even ) ( contending ( that ( ( human causes ) ( have ( been ( ( the ( ( most significant ) factor ) ) ( leading ( to ( warming ( in ( recent decades ) ) ) ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NNS Scientists)) (VP (VBP are) (VP (VBG becoming) (ADJP (RB even) (RBR more) (JJ certain)) (SBAR (IN that) (S (NP (NNS humans)) (VP (VBP are) (NP (NP (DT a) (JJ key) (NN contributor)) (PP (TO to) (NP (JJ global) (NN warming)))))))))) (, ,) (CC and) (S (NP (DT some)) (VP (VBP are) (ADVP (RB even)) (VP (VBG contending) (SBAR (IN that) (S (NP (JJ human) (NNS causes)) (VP (VBP have) (VP (VBN been) (NP (NP (DT the) (ADJP (RBS most) (JJ significant)) (NN factor)) (VP (VBG leading) (PP (TO to) (NP (NP (NN warming)) (PP (IN in) (NP (JJ recent) (NNS decades)))))))))))))) (. .)))
Scientists are becoming even more certain that humans are a key contributor to global warming, and some are even contending that human causes have been the most significant factor leading to warming in recent decades.
(ROOT (S (NP (NNP Human) (NNS beings)) (VP (VBP is/are) (NP (NP (DT the) (JJ main) (NN cause)) (PP (IN of) (NP (JJ recent) (JJ global) (NN warming))))) (. .)))
Human beings is/are the main cause of recent global warming.