I wonder how the Vikings kept warm during winter. It's mentioned in the article that there almost weren't any trees in Greenland and it must have been extremely cold there in winter.
There are other things you can burn: peat (probably not plentiful in Greenland), dried cow manure. Also, you wear furs and sleep under furs. Even my grandparents, living in a place with far more plentiful wood at a much later time in history, had stories of breaking the ice on the wash basin in the morning.