Polar Sea Ice provides the Set Points for Earth Thermostats
The sun melts a considerable amount of ice and snow every summer. This happens in the middle of a Medieval Warm Period, it happens in the middle of a Little Ice Age, it happens in the middle of a Major Ice Age. That is what a summer sun does; it melts ice and snow every summer.
When earth is cold and polar waters are frozen year round or when they freeze early in the fall or winter, there is little source of surface water for moisture and it don't snow much and the ice and snow that melted is not replaced by new snowfall, Albedo decreases and the equilibrium temperature of earth increases.
When earth is warm, when the oceans are warm, when polar waters are not frozen, there is a huge source of surface water to supply moisture to the atmosphere and it snows more than enough to replace the ice and snow that melted in the summer, Albedo increases and the equilibrium temperature of earth decreases.
When the Polar Oceans are frozen, it doesn't snow much and earth warms. When the Polar Oceans are not frozen, it does snow much and earth cools.
The ocean temperature that melts Polar Sea Ice is the Set Point for Earth Temperature. When the Polar Sea Ice is melted the Ocean Effect Snow Monster is turned on. When The Polar Oceans are frozen, the Ocean Effect Snow Monster is turned off.
Polar Sea Ice is the Thermostat that turns the cooling for earth on and off.
The Sun supplies the warming and it is always turned on.