Question on Modifiers

Hi All,

I was reading the Manhattan GMAT guide on Sentence Correction and came across the below sentence.

Citizens of may countries are expressing concerns about the environmental damage caused by wide spread release of greenhouse gases, DAMAGE THAT MAY BE IMPOSSIBLE TO REVERSE.

I understand that in the underlined section - "That may be impossible to reverse" modifies DAMAGE.
However, I am not clear on what portion of the sentence does the noun modifier - "damage that may .....reverse" modifies.

In the above sentence does "damage that may .....reverse" not modify greenhouse gases, while it is intended to refer to environmental damage?
Some how the "Damage that...reverse" seems to be a fragment in the sentence.

What am I overlooking here?
Can some one please help!

Thanks,
Chechaxo