They're known as bang ye, or "exposing grandfathers"(despite their age range) who pull up their shirts to stay cool during the summertime heat. Considered uncouth in China, these men of a variety of ages, social standing, and (unfortunately) stomach sizes, have fallen victim to the fashion faux pas that's the Chinese equivalent of knee-high black socks with shorts. 

There have been efforts to try and quell shirtlessness in general, with government-officiated "civilization" initiatives, and newspapers printing images of shirtless men with bulging stomaches in order to shame them into compliance.  However the "civilization" initiative appears to have been a failure. In the age old question of comfort vs style - its pretty obvious who the clear winner is.