There are many ways to create a paleotemperature record and form a chart of temperature and atmosphere concentrations dating back thousands of years. One of the most common ways, and most commonly known, is ice core stratigraphy, or the use of ice core data to determine temperature and atmosphere conditions.
So how does this work? When ice forms, it traps bubbles of gas inside, and as this ice accumulates on top of more ice it compresses, retaining this trapped gas. Therefore, scientists can go to Greenland or Antarctica and drill a core of ice dating back hundreds of thousands of years, formed from compressed snow and ice. Using very careful methods, they can extract the gas from the core and analyze it for concentrations of carbon dioxide, methane, and other greenhouse gases.
Then scientists have to “date” the ice cores, or determine when the gases they are analyzing came from, to accurately get an understanding of how the atmospheric conditions have changed over time. This is why it is known as ice core stratigraphy; they can use the strata, or layers, in the ice as an indicator of time. There are layers of annual freezing and melting that are identifiable in the ice core; by looking at these layers we can get an estimate of when the gases were trapped in the ice.
However, this is not always accurate; it’s hard to count year-by-year and sometimes the individual layers are not visible enough to create an accurate estimate. So, scientists look at impurities in the ice. Scientists can cut out a portion of the ice, melt it down, and then analyze the water for impurities. These impurities vary annually. For example, in the spring in Greenland storms bring in large amounts of dust, which can be identified by high calcium concentration and insoluble dust (the dust that does not dissolve in water) can be seen by shining light at the water and looking at how the light bounces off the insoluble dust particles. Other elements and compounds, such as sodium, ammonium, and nitrate, also vary by season. Looking at the relative concentrations of these impurities gives scientists a more accurate estimate of when the gases were trapped in the ice.
So now, we have atmospheric conditions and when this atmosphere existed. Now, to create a chart of how these atmospheric conditions relate to temperature, we need a paleotemperature record, or an idea of how temperatures fluctuated over time. To do this, scientists look at isotopic concentrations. We have discussed the concept of an isotope in previous posts; isotopes of elements differ only in number of neutrons (which leads to a difference in mass). Isotopes of elements exist in different amounts. The most common isotope of oxygen is oxygen-16, which means an atom of this oxygen isotope weighs 16 atomic mass units. There does exist an isotope of oxygen known as oxygen-18. Water molecules can have either oxygen-16 or oxygen-18, and those molecules with oxygen-18 are heavier than those with oxygen-16. Thus, when those molecules evaporate, they are the first to rain back down, because they are heavier.
So, oxygen-18 and oxygen-16-carrying water molecules evaporate (mostly from the equator because that is where the waters are warmest) and as they move away from the equator in clouds, they cool. As they cool, they begin to precipitate out some of that water from the clouds, and the first types of water to precipitate out, as we discussed above, would be the water molecules containing oxygen-18. So, we would see the highest concentration of oxygen-18 near the equator, and as those clouds move farther to the poles, they are much more concentrated in oxygen-16-carrying water molecules. Thus, all the ice cores that we are digging up in Greenland and Antarctica are very heavily concentrated in oxygen-16.
However, this cycle does change with temperature. When the temperature is much warmer, we should see more oxygen-18 making it to the poles because the cooling phase, when most of they oxygen-18 precipitates out, happens later and later in the cycle because the atmosphere is so warm. Thus, scientists can use the isotope concentrations at the poles to create a temperature record.
Some scientists can also use fossils of marine creatures from a long time ago to date these temperatures. At the equator, when it is warmer, we will see a lower concentration of oxygen-18 because it is carried all the way to the poles (or close to the poles). Marine creatures can take oxygen from the water and incorporate that into their skeletons. Therefore, when they are fossilized, those isotopes are trapped in their skeletons. Scientists can then extract those isotopes and use radiocarbon dating (see the previous post) to date when that temperature was.
This is just one of the ways we can get an understanding of how the atmosphere and temperature changes over thousands of years.
Svensson, et al, “A 60,000 year Greenland stratigraphic ice core chronology,” http://www.clim-past.net/4/47/2008/cp-4-47-2008.pdf