The World Wildlife Fund has declared that global warming, a

[MdAbuAsad]

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.

[MdAbuAsad]

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.

What's the core of this sentence?
global warming 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.
Is it? But it does not make any sense to me!

[dmitryknowsbest]

No, the entire part between the two commas is a modifier. The core is "The WWF has declared that global warming will create havoc." "A phenomenon" creates a noun modifier for "global warming." Then "that" creates a modifier for phenomenon, like this:

The WWF has declared
that global warming
- a phenomenon
-that scientists agree is caused by burning of fuels
will create havoc
-among birds
-by . . .

[MdAbuAsad]

No, the entire part between the two commas is a modifier. The core is "The WWF has declared that global warming will create havoc." "A phenomenon" creates a noun modifier for "global warming." Then "that" creates a modifier for phenomenon, like this:

The WWF has declared
that global warming
- a phenomenon
-that scientists agree is caused by burning of fuels
will create havoc
-among birds
-by . . .

Thanks for the kind reply!

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.

So, what if the is is removed from the sentence? It seems that would still make sense! Isn't it?
^^ Can't we write like the below?
a phenomenon caused by human beings' burning of fossil fuels? Do we need to put IS before ''caused''?

[dmitryknowsbest]

We can say "a phenomenon caused." However, once we create a modifier with THAT, we need the verb IS. It's "a phenomenon caused by X" or "a phenomenon THAT IS caused by x." "A pretty song" or "a song THAT most people agree IS pretty."

However, this varies depending on the verb used. Technically, "agree" is already a verb, but we can't say "agree caused" or "agree pretty." However, some other verbs can go straight into a modifier. For instance, I could say "a song that most people find pretty" or "a problem that most people solve easily."