A bracing is one of the struts of wood which support the guitar top.
If the problem isn't there all the time, then it is probably and most certainly will not be this.
Climate and humidity can certainly affect a guitar. Contraction and expansion can occur at certain conditions. My guitar has a low action and when I played it last night, it sounded thin and unresponsive.
This might account for Matt's problem of buzzing on the low E for no apparent reason.
A high action produces a fuller sound, although more effort is required to fret the strings, but it is less likely to be affected by climate.
Thinking about it more, plucking the 1st or 2nd string may cause one or more of the other strings to resonate harmonically. Again, the problem might be climatic.