You do know that ice builds up in layers like rings in a tree. When you find certain Isotopes in ice cores and tree ring samples you can deduce what happened. Not being 100% but close. Science is some amazing stuff >>t was first discovered by an analysis of tree rings, of all things. Scientists found that the level of carbon-14, an isotope of carbon, was much higher in rings from that year than usual. Some years later, looking at air samples from ice cores, scientists saw that there were elevated levels of beryllium-10 and chlorine-36 as well. >>The common factor in all these elements is that they are created when extremely high-energy subatomic particles hit Earth's air and ground. They slam into the nuclei of atoms and change them, creating these isotopes. The only way to get particles at energies like this is from space, where powerful magnetic fields in exploding stars, for example, can accelerate the particles to such high speeds. We call these isotopes cosmogenic, made from space.