The paper is oxidizing, one of several destructive processes that affect paper.
Newsprint is high in lignin, one of the main structural components of wood. When lignin is in contact with oxygen, a highly reactive chemical, the reaction changes the way the fibers reflect light, discoloring the paper. Oxidation also produces more acid, a major culprit in the breakdown of many modern papers.
Older paper was usually made from linen or cotton fibers, which required little treatment to offer a good writing surface. Wood is much cheaper, but manufacturers found they had to use a thin chemical layer called sizing on the paper surface to make it less absorbent and prevent blurring when ink is applied.
A commonly used sizing is a compound of alum, or aluminum sulfate, and when exposed to warmth and high humidity, alum molecules split up and form an acidic solution.
Cellulose fibers, which are made of molecular chains of carbon atoms, are easily split by even weak acids. The strength of the fibers is destroyed, and the paper becomes fragile.
The acid problem affects nearly half the books published since 1850, conservators estimate, and it was not until the 1950s that publishers began to abandon alum sizing for alternatives to make some paper acid-free.
Conservators are also working on treatments to deacidify paper.