Ice core researchers look for the presence of calcium dust in the ice when they reconstruct ancient climates. Calcium comes from limestone which is deposited in the ocean in large quantities. During ice ages, sea level decreases as water is trapped in ice sheets around the globe. This exposes the continental shelves rich in calcium and prone to erosion. The dust is blown into the atmosphere and eventually falls to earth with rain or snow, preserving it as a record within the ice.
Some gases accentuate the warming or cooling of the atmosphere.
Those that warm the atmosphere are called greenhouse gases. These include water vapor, carbon dioxide, and methane. Gasses that are present in the atmosphere fall to earth with snow and fill the air spaces between the snowflakes on the ground. As the snow accumulates over time and turns to ice, the gas is trapped in small bubbles. These bubbles can be sampled using sophisticated equipment to tell us which gases were present in the atmosphere when that ice was formed and therefore what the climate was like.