Earth requires fuel to drive plate tectonics, volcanoes and its magnetic field. Like a hybrid car, Earth taps two sources of energy to run its engine: primordial energy from assembling the planet and nuclear energy from the heat produced during natural radioactive decay. Scientists have developed numerous models to predict how much fuel remains inside Earth to drive its engines — and estimates vary widely — but the true amount remains unknown.

Prior to the advent of human-caused global warming in the 19th century, the surface layer of Earth's oceans had undergone 1,800 years of a steady cooling trend, according to a new study. During the latter half of this cooling period, the trend was most likely driven by large and frequent volcanic eruptions.An international team of researchers reported these findings in the August 17, 2015 issue of the journal Nature Geoscience. The study also indicates that the coolest temperatures occurred during the Little Ice Age–a period that spanned the 16th through 18th centuries and was known for cooler average temperatures over land.