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Climate.gov--
"Over at least the past million years, glacial and interglacial cycles have been triggered by variations in how much sunlight reaches the Northern Hemisphere in the summer, which are driven by small variations in the geometry of Earth’s orbit around the Sun. But these fluctuations in sunlight aren’t enough on their own to bring about full-blown ice ages and interglacials. They trigger several feedback loops that amplify the original warming or cooling. During an interglacial,
sea ice and snow retreat, reducing the amount of sunlight the Earth reflects;
warming increases atmospheric water vapor, which is a powerful greenhouse gas;
permafrost thaws and decomposes, releasing more methane and carbon dioxide; and
the ocean warms and releases dissolved carbon dioxide, which traps even more heat.

These feedbacks amplify the initial warming until the Earth’s orbit goes through a phase during which the amount of Northern Hemisphere summer sunlight is minimized. Then these feedbacks operate in reverse, reinforcing the cooling trend."