The Lost Cavern and Other Stories of the Fantastic (Vanguard Press, 1948) by H.F. Heard (a.k.a. Gerald Heard) is a collection of four short stories: "The Lost Cavern", "The Cup", "The Thaw Plan", and "The Chapel of Ease". (Synopses with spoilers follow...)
My initial interest was in "The Thaw Plan" because it briefly mentions a tree octopus, but the story's setting and nascent-Cold-War perspective are interesting in their own right: induced global warming is used by both super powers as a strategic weapon, resulting in a future Earth where humanity has split into two separate species living at either pole, separated from any interaction by an impenetrable, primeval, equatorial jungle -- home to tree octopuses, naturally.
The first part of the story sets the world-building in motion. The year is 1975, 30 years into W.W.2.A. (World War Second Armistice). The world is divided into two super powers, the US and the USSR. The seat of power in the Soviet Union has moved East, to the city of Karakorum. The Chinese have taken control, turning Moscow into a Holy City and elevating the Russian people to the highest (and most powerless) level of "Ritual Rank".
Many in the West see this as a good thing since "a Chinaman never likes war". But the new leader of the USSR, Supreme Commissar Yang, has a devious plot to diminish -- figuratively and literally -- the West (including the Russians): they will use atomic power to melt the tundra, causing sea levels to rise 100 feet, flooding most of the world but leaving the tablelands of Tibet and China uninundated, from which the USSR will rule as the lone superpower.