No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature.
This is unequivocal, so how does the "Greenhouse Effect" in the atmosphere raise the temperature of the Earth's surface? The atmosphere is cooler than the surface, so the flow of heat must be from the surface to the atmosphere, and so it is. Yet a number of people claim that this is proof that the "Greenhouse Effect" does not exist; "A cooler body cannot heat a warmer body" they proclaim, while in fact no-one has even suggested that this is the case. Some even go so far as to claim the so-called "back radiation" from the atmosphere to the surface doesn't exist.
Such claims have no basis whatsoever. Atmospheric longwave infrared "back radiation" has been measured, and its spectrum plotted, routinely since the 1950s. A simple web search can reveal summaries of some of the hundreds of scientific papers on the topic. It is part of the fabric of atmospheric physics. To ignore or dispute the existence of the phenomenon can only be termed denialism. However, disputing the effect(s) of this radiation in detail is another matter entirely. Such arguments can be examined for scientific logic and therefore veracity.