outside of common sense, here is the reason heat (higher temperatures) is bad for comic books:
First, understand that paper's strength comes from the length of the fibers used to construct the paper (from pulp processing).
The shortest fibers are found in newsprint papers (also comic books) made from groundwood pulps; this pulp is made by the mechanical grinding of wood that is then made into paper without first purifying it chemically.
When matter gets hotter, its molecules get more energy and move faster. Excess heat can not only remove the natural moisture content of pulp (used to make paper), that can result in the drying out of paper, thus becoming less flexible, a term we call in comicbookdome as "brittleness", it can also "agitate" (make molecules move faster) the chemicals and inks used in comic book manufacturing, thus accelerating the natural chemical breakdown process in the book. This acceleration of the chemical breakdown in paper results in acids forming within the paper.
Research also shows that these acids continue to accumulate within paper as they attach themselves to paper through strong intermolecular bonds.
Fibers made of cellulose chains degrade when exposed to an acidic environment. This degrading of the chains results in weakening of the paper. (ie. it is bad!). and so on an so forth...
So, there you have it, in a nutshell (thumbs u