It doesn’t? There’s literally no other way to explain the observations of warming than it being man made, natural forcing dictates a cooling or stable climate and warming has continued unabated. As for it not providing a specific reason, when the climate warms, the entire distribution of temperatures for a given region shifts along with it, which also means the probability of below normal temperatures decreases substantially while above average temperatures increase in both frequency and intensity. December is not the only month this has been happening lately, it’s just the most extreme example