Scientists, even the untenured con-men pretending to be scientists, agree that we experienced a warming period around the earth about 1,200 years ago.
This is about the time Vikings employed themselves as plunderers, stealing treasure, kidnapping maidens and burning villages.
Since nobody knows what caused the earth to warm, we are left to speculate. And because everyone and their grandmother speculates on global warming, we can, too.
The reason the earth warmed is because the Vikings burned so many villages. The burning villages heated the clouds which caused rain to fall instead of snow, and soon the Vikings were growing grapes in Greenland.
With the warm weather, the Vikings found they could employ their slaves to grow grapes and other food. This left time for the Vikings to pursue other activities like counting their loot. Laying about while overseeing slaves, counting loot and ogling maidens was certainly a less dangerous line of work and more pleasurable.
The Vikings pursued this occupation for about 200 years. And since they weren’t burning villages anymore, the clouds didn’t heat up. So the earth stopped warming, the snow returned, the Earth got hit with a 300 year mini-ice age and the marauding Vikings sailed off into history.
See, now, I'd imagine more grapes leads to more wine, more wine leads to more pillaging. And it does. You should see me hitting the yard sales on weekends after a drink or two. It's not pretty.