One of the many issues with verifying the validity of climate change is the question of how we can actually measure it in the first place. The measurement must be of two factors: temperature and time. If the planet is in fact warming then scientists must verify that by seeing if the temperature is increasing over time.
One of the best ways of doing this does not come from any sophisticated instrument but from the natural world itself. Ice sheets hold the key to this issue. The glacier ice sheets of Antarctica and Greenland are organic archives of the climates of the distant past. They even predate the history of the written word. By studying an ice sheet a scientist is able to determine the climate of the planet hundreds of thousands of years ago in the past.
This is because the ice sheets contain very small air bubbles. Within these lie the information needed to determine what the atmosphere was once composed of. It means scientists can collate this with with how the temperature has risen over time. So if the atmosphere was once very hot and also full of carbon dioxide we can determine that this gas is a contributor to global warming.
This is significant for people in the current era as the amount of carbon in the atmosphere is constantly increasing. As it does so too does the temperature of planet Earth. This makes archiving the climates of the past important in order to better understand our own.
The dating of each ice sheet is incredibly complex and relies on a number of different methods all used in conjunction with one another. There are scientific formulas that have been developed to calculate the age of each ice piece. Most of this useful work is carried out by the Norwegian Polar Institute.