How does the layers of ice show that?
Why do scientists study layers of ice in Antarctica to learn how Earth’s atmosphere has changed over time?
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The ice in Antarctica is made from snow, not liquid water. Ad it collects air is trapped and later compressed with the snow. THe air can be taken out of ice samples and analyzed. It is relatively easy to date the ice back for thousands of years. Under different conditions some of the proportions of the air entrapped, and the water as well, vary.
The variations in the trapped gases, and in some of the proportions of certain isotopes can be used to tell what the atmospheric conditions were like when the original snow fell.
Ratios of oxygen isotopes can be compared to “Standard Mean Ocean Water” to determine the temperature at which the ice formed. 18O/16O is higher when snow is formed in warm temperatures and lower in colder temperatures.
There may also be bubbles of gas trapped in the ice that con be measured for concentrations of different gases in the atmosphere.
Basically, the ice is like a sediment that formed in the air. Some of the original air is captured in the ice and can be analyzed. Also, the water can be analyzed and we can learn things about the climate and atmospheric chemistry from that. Even thickness variations in the layers tell a tale of the types of winters that formed them.
But what is really nice about ice, apart from all those other good and useful things, is that it makes layers that can be measured and counted, sort of like tree rings (but not exactly), and thus we can get an age of the samples relatively easily and reasonably precisely.
Of course, the ice core data only cover the very recent past of the earth, typically a period of several hundred thousand years, but the cores give a lot of nice detailed information for that short period.
as with above answers abouth gases, isotopes etc, but with the inclusion of minute particles such as volcanic dust etc which can help us understand y there may have been a fluctuation at a past time.