And if you go back a few hundred years from that from around 1200AD-1850 you had the mini Ice Age. But that was after the Mediaeval Warm period from around 800AD-1400AD when Greenland was being farmed by Vikings was warmer and lush that the Vikings were planting crops, farming, etc., hence the name Greenland. Ocean temps were obviously warmer. So it waxes and wanes up and down.
P.s. I meant to add, with that graph starting around 1850, we were at the end of the mini Ice Age. Obviously, the oceans would be at their coldest.