temp
Explanation
This comic uses the "Monkey's Paw" premise -- a cursed artifact that grants wishes in the most destructive way possible.
The top panel shows a news anchor reporting apocalyptic news: "And, for unknown reasons, the entire planet is negative forty degrees everywhere. All agriculture, indeed all ecology, will soon be gone. Death is imminent. Goodbye." A graph behind him shows a catastrophic temperature drop.
The bottom panel, labeled "Earlier...", reveals the wish that caused this: a woman says to a Monkey's Paw, "Monkey's Paw, I wish Fahrenheit and Celsius users could just agree about temperature."
The joke is that -40 is the one temperature where Fahrenheit and Celsius are exactly equal (-40F = -40C). The Monkey's Paw granted the wish in the most literal and catastrophic way possible: it made the entire planet's temperature the one value where both systems agree, which happens to be a temperature incompatible with life.
This is a clever intersection of the classic "be careful what you wish for" trope with a well-known mathematical/scientific fact. The humor comes from the absurd escalation -- a mild annoyance about unit conversion disagreements leads to an extinction-level event -- and from the Monkey's Paw finding the most technically correct yet devastating interpretation of a seemingly harmless wish. Anyone who has encountered the -40 degree trivia fact will appreciate the precision of the joke.