My Earth, or is it Eaarth?

Earthrise, NASA Apollo 8 mission

The earth that we knew -- the only earth that we ever knew -- is now gone. (p. 27)

This might be for me the most heartbreaking sentence in the first chapter.  I don't want to accept it.

There is so much beauty on our world, in all things great and small, from wildflowers to mountains. 

How could it be that our earth is "gone"?  And what does that mean?
My years meeting regularly with climate change experts allows me to consider, difficult as it may be, that McKibben might, alas, alas, may not be overstating the case.  (Our class will research this.  That will be our first task. We need to find out!)

Climate change is supposed to be for the future, for the grandchildren, that I don't yet have.  Everyone knows it is for the grandchildren. (I just googled "climate change and grandchildren" and got over 1.7 million hits.)  McKibbon asks,

So how did it happen that the threat to our fairly far-off descendants, which required that we heed an alarm and adopt precautionary principles and begin to take measured action lest we have a crisis for future generations, et cetera -- how did that suddenly turn into the Arctic melting away, the tropics expanding, the ocean turning acid? How did time dilate, and "100 or 200 years from now" become yesterday? (p. 13)

If this has happened, what the hell?  Why hasn't everyone been screaming about it?  Why aren't they screaming now?