Is this the correct way of using “agree”?

Question

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?

Answer 1

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.

To me, this seems absolutely grammatical. The scientists agree that it (global warming) is caused by human beings burning of fossil fuels.

Answer 2

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...

I think we do best to understand most scientists agree as a parenthetic within the apposition. It might be clearer if we use which instead of that.

The World Wildlife Fund has declared that global warming, a phenomenon which, most scientists agree, is caused by human beings’ burning of fossil fuels, will create havoc among migratory birds...

With that instead of which, and with different verbs (conclude, think, suspect, believe, understand. agree, concur) the situation gets dicier, since not all verbs will accept both to be caused and is caused equally as complement.