Complex sentence: They alleged that the utilities are the five largest emitters of GHGs in the United States, collectively emitting 650 million tons of carbon dioxide annually and by contributing to global warming they are violating the federal common law of interstate nuisance.
Split into simpler sentences: They alleged that the utilities are the five largest emitters of GHGs in the United States, collectively emitting 650 million tons of carbon dioxide annually. Furthermore, by contributing to global warming they were violating the federal common law of interstate nuisance.