Not to be heretical but have you considered the alternative case of the correlation? It is quite possible that global warming has led to the dearth of pirates.
Stronger squalls slamming into the boat before all hands can get up into the rigging and shorten sail. More frequent and stronger hurricanes and the increased difficulty of ordinary pirate activities such as clawing off a lee shore.
I think the chart possibly clinches the case for the deleterious effects of anthropogenic global warming and would not be surprised to see it featured in the next AlGore documentary.