The rise in greenhouse gases in the atmosphere means far more energy is coming into Earth’s climate system than is going out, but half of that energy is missing and could eventually reappear as another sign of climate change, scientists said on Thursday.

In stable climate times, the amount of heat coming into Earth’s system is equal to the amount leaving it, but these are not stable times, said John Fasullo of the U.S. National Center for Atmospheric Research, a co-author of the report in the journal Science.

The gap between what’s entering the climate system and what’s leaving is about 37 times the heat energy produced by all human activities, from driving cars and running power plants to burning wood.

Half of that gap is unaccounted for, Fasullo and his co-author Kevin Trenberth reported. It hasn’t left the climate system but it hasn’t been detected with satellites, ocean sensors or other technology.