Though it was not provided, I have the reason behind this trend:
Pirates used to cause a fair bit of destruction, chaos, mayhem... lots of small battles, fights and skirmishes; many people left lifeless.
Now for the equation:
Fewer pirates = fewer battles, fights and skirmishes
Fewer battles, fights and skirmishes = fewer people left lifeless
Fewer people left lifeless = more people left alive
More people left alive = more total body heat
More total body heat = higher global average temperature
Dammit! "Too many people" is the cause of global warming!!
Also, the graph should, therefore, have the axes switched since, clearly, the reduction in number of pirates is the control variable here.