From one of the GMAT Verbal passages, it says:
The World Wildlife Fund has declared that global warming, a phenomenon that most scientists agree is caused by human beings’ burning of fossil fuels, will create havoc among migratory birds by altering the environment in ways harmful to their habitats."
The phrase
a phenomenon that most scientists agree is caused by human beings’ burning of fossil fuels
is an appositive phrase that modifies global warming. The book claims that this passage is correct. However, the use of "agree + is caused by" seems so ungrammatical to me. Is there any rules of use that I'm missing?