Pine trees are bursting into flames. Boggy peatlands are tinderbox dry. And towns in northern Russia are sweltering under conditions more typical of the tropics.

Reports of record-breaking Arctic heat – registered at more than 38 Celsius in the Siberian town of Verkhoyansk on June 20 – are still being verified by the World Meteorological Organization. But even without that confirmation, experts at the global weather agency are worried by satellite images showing that much of the Russian Arctic is in the red...