John,
I believe your chart underestimates the impact of the loss of the last remaining pirates on temperature increases. Your graph suggests, erroneously, a linear relationship. From 1860 to 1880, a loss of 25,000 pirates resulted in an increase of slightly less than .5C. From 1880 to 1920, a loss of 5,000 pirates contributed an additional .5C increase. From 1920 to 1940, a loss of 10,000 pirates contributed to an additional .5C. I believe there is a problem with the data collected during this period. With 2 world wars and the great depression, our pirate data collection efforts clearly suffered. From 1940 to 1980, a loss of 4600 pirates contributed approximately a .5C increase. However, from 1980 to 2000, a loss of a mere 383 pirates caused an additional .5C increase. With the advent of satellite pirate tracking in 1982, this data is far more reliable than data captured prior to the launch of the EPT (Earth Pirate Tracking) satellite grid.
One possible explanation of this non-linear relationship is that there is some form of pirate forcing that we do not yet understand. Additional study is needed. Alternatively, there may be a lag between pirate loss and temperature increase which would mean we have already built into the system additional warming.
It is enough to cause me to commission the construction of a frigate and set sail from New Orleans. Aaarrrrrgggghhh!