Passive voice question

Passive voice question
 

And in Politicians believe that the global warning is dangerous.

The global warning is believed to be dangerous by politicians or
the global warning is believed by politicians to be dangerous.

global warming

Global warming is believed to be dangerous by politicians.
Global warming is believed by politicians to be dangerous.

Both sentences mean the exact same thing.

but where should be place the prepositional phrase by politicians?

Prepositional phrases can be placed in various places.
By politicians, it is believed that global warming is dangerous.
It is believed by politicians that global warming is dangerous.
It is believed that global warming is dangerous by politicians.

The third example is somewhat awkward sounding in this case, but attaching the prepositional phrase at the end of the sentence is common.