For the purposes of determining the difficulty of the Subsist downtime activity, is the Spine of the World considered a desert (Master difficulty) or a barren wasteland (Legendary difficulty)?
If it's the one that matches the Forgotten Realms, it looks more like mountains leading to arctic. I don't remember a Spine of the World in Golarion.
Now, the CROWN of the world I'd call barren wasteland for the most part. It's definitely not a 'typical' desert; what works in the Sahara or Mojave won't cut it in Antarctica.
Yes, I may have had the names mixed up. I'm referring to the glacial regions to the far north of Golarion.
Looking at 1st Edition's People of the Wastes, it appears as though the only wastelands are the Mana Wastes, the Sodden Lands, the Southern Fangwood, Southern Sargava, Tanglebriar, and the Worldwound.
This leads me to believe that it may qualify as a frozen desert.
Pathfinder Adventure Path #51 Jade Regent: The Hungry Storm has a chapter on The Crown of the World by Jason Nelson. It separates the ecosystems of that northern continent into tundra, taiga, and highlands, grouped together as the "Outer Rim," and the polar ice cap called "High Ice." In the Ecology section, Jason Nelson wrote:
Ecology of the Crown
The subpolar tundra, taiga, and highlands of the Outer Rim of the Crown of the World are not radically different in climate and inhabitants from cold lands farther south, such as Irrisen and Mendev. The transition from thickly forested mountains and hillsides to the vast empty expanse of grassy permafrost plains is a gradual one. Herds of caribou, musk oxen, and even woolly rhinoceroses and mammoths roam widely, and nomadic herders follow these creatures in their migrations, as do wolves, bears, worgs, and more exotic predators. Rivers and lakes are thick with salmon, trout, pike, and sturgeon, especially during spawning season, and migratory birds number in the millions during the warmer months, particularly along the coasts. Seals, walruses, and marine crustaceans abound, usually following the outer edges of the polar pack ice as it expands and contracts with the turning seasons.
...
While the mile-thick High Ice icepack, the bitter cold, and the extreme diurnal shifts of the Boreal Expanse are inhospitable to normal animals, they are hardly devoid of life. Besides the ever-present lichen, the blue polar ice is rich with tiny organisms that thrive in the cold and provide food in turn for tiny colorless bore-worms. The snowdrifts overlying the icepack are home to novosis—a flat-bodied, smooth-skinned amphibian that burrows through the soft snow, grinding the surface ice with bony mandibles and sucking bore-worms into its gullet. Erutaki sometimes dig shallow pits in the snow and release a handful of bore-worms into them, watching for the novosis to come close to the surface so hunters can spear them as they feed.
...
The random encounter table on page 83 listed draugr, bunyin, hoarfrost spirit, ice troll, yeti, orca, selkie, winter wolf, glacier toad, wolly rhinoceres, frost drake, quallupilluk, mastodon, frost giant, witchfire, frostfallen mammth, saumen kar, white pudding, frost worm, and taiga giant for the Outer Rim. The High Ice lacked a random enounter table.
I would put the taiga and highlands at Trained level (DC 15), the tundra at Expert level (DC 20), and the ice cap at Master level (DC 30) for Subsist.
The 7th-level PCs in The Hungry Storm had to lead a caravan across the Crown of the World from the Land of the Linnorm Kings in Avastian to Hongal in Tian Xia. We had more fun to sending the martial characters out hunting winter wolfs and cold-immune goats that grazed on the cold-immune grass of the Alabastrine Peaks than to claim that they hauled enough food to feed themselves for the month on the High Ice.