In addition, there is the ice melting in the world, and especially in the arctic.    The arctic ice cap has been in existence for at least 3 million years, and likely 11 million years or more, according to the information I've learned over the past many years.   But we may have a blue ocean even this year.   Here is a nice graph showing how much the ice has diminished since 1979.   

I don't think the medieval warm period is relevant here, based on climate science today.