Nesting material from desert rodents can be used to estimate moisture and temperature conditions. Sediment deposits in lakes and oceans are used extensively to determine a wide variety of climate parameters on times scales that extend back millions of years. Glaciers and ice sheets are especially valuable because they record a great variety of climate records. The snow contains not only water, but also a mixture of chemicals that were collected by the snow as it fell through the atmosphere. Wind-blown dust and atmospheric chemicals also fall to the surface of the snow when there is no precipitation.
Atmospheric gases fill spaces between the snow crystals.
As the surface snow is buried, it is compressed and forms ice. The chemicals and dust that the snow collected when it was falling and exposed on the surface are preserved in the ice. The atmospheric gases that filled the space between snow crystals are trapped in bubbles. At favorable locations, recognizable annual layers of ice containing water, chemicals, dust, and gases lay on top of each other like a stack of annual weather summaries. By drilling into the ice sheet and recovering an ice core, samples of the water, chemicals, dust, and gases preserved in the ice sheet can be collected and analyzed.