Just because it's not humid, doesn't mean that you don't have microbe growth. Microbes are air borne, regardless of climate, though they tend to grow faster in humid climates. The elements of peanut butter - regardless of location - also provide an almost perfect nutrition source for microbes. Like I stated previously, the peanut butter may not taste, look or smell different, but it is harboring relatively large numbers of microbes.

The two months is a maximum recommended for unrefrigerated, regardless.