![Oxygen isotope measurements in foraminifera from deep-sea cores.](http://static.sciencelearn.org.nz/images/images/000/001/806/full/Oxygen-isotope-cycles20160826-15594-133sjie.jpg?1674166904)
Oxygen isotope measurements in foraminifera from deep-sea cores. Oxygen isotope measurements, combined from many cores, show cycles of warming and cooling. The diagram only shows the last million years, but cycles going back 5 million years have been measured.