1) The temperature effect doesn't really matter. Trees remove CO2 from the atmosphere which raises the pH of the oceans.
2) You're confusing the end of the Medieval Warm Period with the beginning of the Little Ice Age. The Medieval Warm Period ended in approximately 1250 AD; the Little Ice Age began in approximately 1550 AD. While it is possible that the two events are related, there is no reason to assume they are.
2) You're confusing the end of the Medieval Warm Period with the beginning of the Little Ice Age. The Medieval Warm Period ended in approximately 1250 AD; the Little Ice Age began in approximately 1550 AD. While it is possible that the two events are related, there is no reason to assume they are.