Eh, "must" is a strong word. "Should" might be better.
I'd imagine studying climate differences might be like studying sports injuries. Some metrics would seem global ("rate of concussions") but would be pretty regional (e.g., because US plays more American Football). Some would seem regional ("rate of childhood knee sprains"), but would be pretty global (bc kids play soccer almost everywhere).
Similar thing with climate: some metrics would seem global (distance from nearest pole), but are pretty regional (bc jet streams, etc.).
In both cases, you don't HAVE to take a regional view, but it's certainly easy to draw the wrong conclusions looking globally.
I'd imagine studying climate differences might be like studying sports injuries. Some metrics would seem global ("rate of concussions") but would be pretty regional (e.g., because US plays more American Football). Some would seem regional ("rate of childhood knee sprains"), but would be pretty global (bc kids play soccer almost everywhere).
Similar thing with climate: some metrics would seem global (distance from nearest pole), but are pretty regional (bc jet streams, etc.).
In both cases, you don't HAVE to take a regional view, but it's certainly easy to draw the wrong conclusions looking globally.