'
A gradual increase in carbon monoxide concentrations wouldn't be noticed,

...and that would lead to death, (as it often does).  

Similarly, increasing radiation wouldn't be detected, even at dangerous or fatal levels.  

 Numerous other toxic substances can have the same unobservable impacts, 

….as can disease organisms.
.
Those are things that can kill a human who isn't aware of his surroundings.
.

In another scenario, humans can be all too aware of their surroundings,

…and choose to ignore evident reality.
.

Our world will continue to heat up,

… and some people, maybe too many people, will continue to prefer ignoring the threat.

As in the other examples, death is the likely outcome.

Too bad they'll take so many others with them when they go.
.
.
Silence Is Not an Option: Resisting the Right in the Age of Trump | The Smirking Chimp

http://www.smirkingchimp.com/thread/bill-berkowitz/70491/silence-is-not-an-option-resisting-the-right-in-the-age-of-trump