Earth's millennial climate cycle has been studied by scientists for decades, but its cause has been lacking. It warms the Earth for several hundred years and melts some of the ice sheets in Greenland and Antarctica. My analysis discovered that it is caused by the astronomical conjunction of the planets Jupiter, Saturn and Neptune. This paper describes the observations that led to its discovery and explains how its 1,470-year spin-orbit resonance cycle warms the Earth with geothermal heat. The current cycle started warming the Earth in the year 1791 and will probably reach peak temperature in year 2060 at the next Jupiter, Saturn and Neptune conjunction. Then it will cause cooling oscillations for the rest of the cycle.