1946: Hal Clement's novelette "Cold Front" (first published in Astounding Science Fiction, July 1946, available at the Internet Archive) is set some time after the (intentionally) human-caused greenhouse effect has warmed the earth considerably with significant melting of the polar ice caps. An Earthman wants to sell the denizens of a cold planet on a similar project:
"There is one fact that I think will help you greatly," he said. "This planet is in an ice age — we could tell that from space. In this hemisphere, where it is now two Earth years past midsummer, the ice cap extends more than thirty degrees from the pole. In the other, the large island and continental masses possess glacial sheets scores of feet in thickness to within forty degrees of the equator; and heavy snow fields reach to less than twenty degrees south latitude in spots. On smaller islands, whose temperatures
should be fairly well stabilized by the ocean, there appears to be much snow at very low latitudes.
"I suppose, though that's outside my line, that these people developed their civilization as a result of the period of glaciation, just as the races of Earth, Thanno, and a lot of the other Federation planets seem to have. Now, however, they have the situation of a growing race cramped into the equatorial regions of a planet — admittedly a large one, but with most of its land area in the middle latitudes.
"On Earth we pushed the isotherms fifteen degrees further from the equator, and benefited greatly thereby. How about selling the same idea to the Heklans, if you
really want a convincing example of what we can do for them?"
"Two questions, please," returned Vickers. "First, what's this about changing the Earth's weather? I don't recall ever having heard of such a thing. In the second place,
I'm afraid we'll have to sell the Heklans a little more than possible advantages. Our working theory, remember, is that I inadvertently got them leery of the combative and
competitive elements of Federation culture. How would curbing their ice age, if you can do it, help that? Also, and most important, how does it help us to get a corner on the
metal trade here before a real Federation agent steps in and opens the place up? Once that happens, every company from Regulus to Vega will have trading ships on Hekla; and
we want Belt Metals to be solidly established here by that time. How about that?"
"To answer your first point, we didn’t change Earth's weather, but it's climate. There'd be no point in trying to explain the difference to you, I guess. They stepped up the CO2 content of the atmosphere, producing an increased blanketing effect. At first the equatorial regions were uncomfortably hot as a result; but when the thing stabilized again a lot of the polar caps had melted, and a lot of formerly desert land in
the torrid zones, which had been canalized for the purpose, had flooded in consequence. The net result was an increased evaporation surface and, through a lot of steps a little too technical for the present discussion, a shallower temperature drop toward the poles. The general public has forgotten it, I know, but I thought it was still taught in history. Surely you heard of it sometime during your formative years.”