Arable land of course. Expanding the world's deserts while pushing cereal grain production to higher latitudes is a very bad idea. Those soils will not support grasses without what is probably an impossible amount of soil amending.how so? You talking navigation or airable land?
The good news is that the process will be mostly a slow motion train wreck for Man. Excepting the occasional failed worldwide harvest, and the mass migrations and wars.
; )