ological ▁mechan isms ▁active ▁on ▁the ▁Earth ' s ▁surface . ▁ ▁Organ isms ▁effect ▁the ▁amount ▁of ▁o xygen ▁vs . ▁carbon ▁dio x ide ▁through ▁resp iration ▁and ▁photos yn thesis . ▁They ▁also ▁affect ▁the ▁levels ▁of ▁nit ro gen ▁through ▁fix ation , ▁nit r ification , ▁and ▁den it r ification . ▁The ▁ocean ▁is ▁capable ▁of ▁absor bing ▁carbon ▁dio x ide ▁from ▁the ▁atmosphere , ▁but ▁this ▁var ies ▁based ▁on ▁the ▁levels ▁of ▁nit ro gen ▁and ▁ph osph orus ▁present ▁in ▁the ▁water . ▁Hum ans ▁have ▁also ▁played ▁a ▁role ▁in ▁changing ▁the ▁atmos pher ic ▁composition ▁of ▁the ▁Earth ▁through ▁industrial ▁by products , ▁def or est ation , ▁and ▁motor ▁vehicles . ▁ ▁The ▁lum inos ity ▁of ▁the ▁Sun ▁increases ▁as ▁it ▁progress es ▁through ▁its ▁life ▁cycle ▁and ▁are ▁visible ▁over ▁the ▁course ▁of ▁millions ▁of ▁years . ▁Sun sp ots ▁can ▁form ▁on ▁the ▁Sun ' s ▁surface , ▁which ▁can ▁cause ▁greater ▁vari ability ▁in ▁the ▁em issions ▁that ▁Earth ▁receives . ▁ ▁Vol cano es ▁form ▁when ▁two ▁pl ates ▁meet ▁and ▁one ▁sub duct s ▁under ne ath ▁the ▁other . ▁They ▁thus ▁form ▁along ▁most ▁plate ▁boundaries ; ▁the ▁Ring ▁of ▁Fire ▁is ▁an ▁example ▁of ▁this . ▁The ▁study ▁of ▁vol cano es ▁along ▁plate ▁boundaries ▁has ▁shown ▁a ▁correlation ▁between ▁er u ptions ▁and ▁climate . ▁Alan ▁Rob ock ▁the or izes ▁that ▁vol can ic ▁activity ▁can ▁influence ▁climate ▁and ▁can