Even if the overall trend is not toward warming, the prophecy may nevertheless be fulfilled if temperature rises in areas of higher population density. It wouldn't be necessary or feasible that every inch of globe is warmed but especially those where humanity has located its civilization.
That it is referring to vials poured on the Sun; yes, but it makes clear that the Sun does radiate to a degree such that the inhabitants of the earth are burned by it. I spent some time one Summer in San Antonio and if you are exposed long in that region you will burn. If temperatures rise in the way both science and the Bible expect them to, very few of us will escape its heat.
The Bible might be construed in one spot as indicating the seasonal extremes will grow more harsh; that is, Winter will become more harshly cold, while Summer will be historical in its merciless onslaught of furnace-like heat.