answer:Not yet determined with any certainty, but evidence suggests that the quantity of greenhouse gases (gases that retain the heat of the sun instead of reflecting it back to space, as a greenhouse does) may be a contributing factor. Quite a bit of the increased production of greenhouse gases has to do with human industrial action, in particular burning of hydrocarbons. This isn’t proven beyond a reasonable doubt, to borrow a term from law, but (at least from where I sit) it’s proven by a preponderance of the evidence.