Ice cores are a solid record of the ancient environment. If you want to know how much carbon dioxide there was in the air 50,000 years ago, there is no need to speculate. You can find an ice core that old, find an air bubble in it, and measure its composition directly.
While there are other long-term records of the Earth's environment, such as a 114,000-year run of data from stalagmites in Chinese caves, ice cores run back further in time and provide more material to work with.
Ice cores lack some of the data found in the rock cores that geologists drill from the solid Earth, on land or in the ocean floor. These contain fossils that provide information about ancient life in a direct way. But ice cores are a uniquely sensitive indicator of the state of the Earth over the last several hundred thousand years.
Ice cores are drilled from areas that rarely or never see a thaw, to ensure that they offer as continuous a record as possible. They also have to come from areas where the ice stays still rather than flowing glacier-fashion.This means that favoured drilling sites are at the centre of Greenland and the Antarctic. Because the Antarctic gets less snow than Greenland, drilling into the ice there takes scientists back further in time. The oldest ice drilled there, in a 3km core, dates back 740,000 years, while a similar ice core from Greenland would be not much more than 100,000 years old at its base.
Because a 3000m ice core of the sort now being obtained is rare and precious, and weighs about 15 tonnes, it is important to treat it carefully. The US National Science Foundation has a massive cold store for the things, the National Ice Repository, in Denver, Colorado. Here their properties are measured from the outside and some luckless bits are melted, in the hunt for pollen, volcanic ash, meteorite dust or gas to analyse. Similar studies are carried out in Russia, the UK and elsewhere.
Measurements made of such ice cores reveal temperature indirectly, because as it gets warmer or colder, different types of oxygen atom vary in their abundance in the ice. This is called proxy temperature data. Other proxy data can be obtained about forest fires and volcanoes, by measuring dust in the ice, and about climate change via carbon dioxide in trapped air.
In recent years, techniques have been developed for finding out about ancient ice without destroying it. One is to measure its dielectric properties, how good (roughly speaking) it is at conducting electricity. Periods when there were a lot of active volcanoes can be spotted in such cores because volcanoes put sulphur in the atmosphere. It ends up as acid in the ice and increases its conductivity.
Although these ice cores are full of data about the remote past, they also tell us about our own behaviour. The zone dating back to the 1960s shows a severe peak in radioactivity because of US, Soviet and UK testing of nuclear weapons.