Temperature changes show up in differing proportions of isotopes of oxygen trapped in the frozen layers. Pollens tell something of plant growth on adjacent lands. Rock dust tells of dry times on northern continents. Volcanic ash points to eruptive activity. Even differences in electrical conductivity within the ice provide hints.
More salt blown up from the sea increases the conductivity of ice laid down in that layer.
All of these clues tell about atmospheric circulation, and trapped bubbles reveal something of atmospheric chemistry. The GRIP team concentrated on a period between ice ages. This interglacial time, known as the Eemian, took place between 115,000 and 135,000 years ago, and its record lies between 2780 and 2870 meters deep in the ice. It's been a time of reasonably stable climate. Most scientists assumed not only that our stable climate was the natural order of things until the ice returned again, but that other interglacials were also times of climatic stability.