The main reason is because there is very little heat capacity to air. When snow melts in the spring, air temperature has very little effect, while radiation does most of the melting. On a cloudy day, little snow melts, even when air is warm, because air has very low heat capacity. What this means is that ice accumulates during ice ages not because the air gets cold but because overcast skies block out the sun's radiation. The best explanation I can imagine is the oceans heating up.
Deep wells produce warm water, because there is a lot of heat coming from the core of the earth.
Oceans are deeper than deep water wells. So the oceans are picking up a lot of heat from the earth's core. Any change in the temperature of the earth's core will have major effects upon the weather. Physicists do not know why the earth's core is hot. Regardless, there should be some radioactive material in the earth's core.