I'm not even someone who is a big global warming guy but...is it surprising that premier US military minds are worried about it? Think about the chaos and angst stirred by migration right now - waves of people fleeing military conflict or terrorism in their home countries. Now imagine that turned up to 11 because people are fleeing climate related disasters. Crops failing, drought, flooding. What does the Middle East look like in 10-20 years? What does Europe do in a similar timeline, facing mass migration and the right wing lurch that tends to cause? Hell look at South American migration. What's the long term impact of Brazil's government fucking up the Amazon.
Imagine the US military response that will be "required" by much of this. Given how pedestrian Chinese weapons systems are compared to the US, I don't see this being some far fetched opinion.
That being said, giving a speech to active duty military and telling them global warming is the biggest threat seems pretty laughable but hey....Biden being Biden has gotten him this far and it's working so uh...maybe internet hand wringling do not matter, as the kids say.