I was struck by the timing of this delightfully brief and slightly meta exchange about meaning, which follows immediately after the discussion of the apocalyptic state of the natural world outside. Thinking way back to "Greening the Absurd," I can't really seem to decide whether the characters think of nature within the environmental or ecological framework. I know you've said that Hamm and Clove's circumstance can be read as ecological: which makes sense, they are so deeply defined and subject to their natural surroundings (or subversion thereof). Their environment certainly does seem to serve as a metaphor—a reflection of Hamm + Clov's interior landscape. This reflection is a causal one: at least that's what's explicitly referenced by Hamm and implicitly understood by an audience with our own troubled reality.
The connection to nature and meaning is a pretty direct cue to look closer, because it's an association poets and artists and writers have drawn for like...ever, not necessarily in the character's reality but certainly in our own. What these few lines (and indeed, the play as a whole) achieve is to invert our own erroneous understanding of the relationship between meaning and nature, an inherently unsustainable one that returns relentlessly (either out of instinct or arrogance) to nature as the metaphor that never stops giving. Think of whole "Pastoral" genre we learned about a while ago in whatever survey class. Or the "Transcendentalist" painting movement in early America. We milk it, reduce it to an image and load up that image with all of the emotion we're incapable of just expressing outright. We exploit it (much the same way we do for resources) for meaning, reframing and reframing to our convenience (dare I say, even laziness?). Like so many screenshots of the same jpeg image, a clear and truthful image becomes corrupt.* Imagination becomes reality the same way meaning becomes causality. All of this to say we don't think of it as OUR ecosystem anymore but a tabula rasa for us to manipulate into the perfect reflection of ourselves, until we stop existing as a part of it, and it starts (or ceases) existing as a part of us. O, inadequate poetic imagination!
I just realized now I haven't gotten around to your question, bear with me. I was initially interested in this passage because Hamm does seem to share or at least be aware of our poorly-balanced causal understanding of nature. Perhaps he is so emotional because of this reversal, wherein nature gives meaning to him , and an ugly one, too.
*I realize how ironic it is that I'm using a technological metaphor here, I just didn't want to use a natural one.