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Researchers estimate ancient temperatures using data from climate proxy records, i.e., indirect methods to measure temperature through natural archives, such as coral skeletons, tree rings, glacial ice cores and so on.
Finding out what the temperature is is ridiculously simple these days. However, ascertaining temperature was more challenging two centuries ago. The modern instrumental temperature record could only tell us about the planet’s temperature trends over the last ~180 years.
In that case, how do scientists and researchers discuss the climatic conditions thousands or even millions of years ago? How can they tell what the temperatures of Earth were in the ancient past?
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Global temperature in present times is estimated by utilizing specialized thermometers installed on ships, buoys, and several weather stations that are operational worldwide.
However, determining the temperature of previous centuries is more complex than it seems due to our limited resources to ascertain past weather conditions conclusively with any degree of certainty.
Climate Proxies
In order to deduce the ancient temperatures of this planet, scientists rely on a number of indirect methods and techniques called climate proxies. (Source)
These are preserved physical characteristics of the ancient past that help scientists estimate the corresponding weather conditions of that particular era. Since reliable records of the Earth’s historical temperatures only began to appear in the 1860s, climate proxies are the only way researchers can estimate our planet’s weather conditions before that era.
Some common examples of climate proxies are rocks, ice cores, tree rings, fossils, lake and sea sediments etc. They act as “natural climatic archives” as they contain imprints of the ambient temperature conditions on them.
Ice Sheets
The ice sheets in the polar regions, including Greenland, Antarctica, and North America, can provide us with valuable information about historical temperatures. Each year, snowfall forms a new layer of ice on top of the previous one. These layers are preserved and visible, allowing us to study them.
As snow layers formed at different temperatures have unique chemical properties, we can gather insights into the temperature conditions of the time period in question by analyzing these layers.
Tree Rings
The rings visible in a horizontal cross-section cut through the trunk of a tree are commonly called tree rings. It’s interesting to note that tree rings can be wider or narrower depending on the existing climatic conditions when the tree was growing.
Therefore, fossils of trees can help scientists estimate the trends of changing weather conditions.
Pollen Grains
The best thing about the pollen produced by plants is that it can help identify the parent plant species and is highly resistant to decomposition.
Since pollen production largely depends on the existing weather conditions, their abundance or absolute absence in certain geographical regions can help us establish how warm/cold those areas were in the past.
Fossil Leaves
The carbon dioxide content of the ancient atmosphere can be determined by studying fossil leaves’ isotope composition and stomata (tiny pores found in leaves and stems that assist in gas exchange).
Lake And Ocean Sediments
Deep sediments that are found at the bottom of water bodies, such as lakes and oceans, are a great source of knowledge regarding ancient temperatures. The most important of those sediments are the layers formed by the shells of small, surface-living animals that are deposited over millions of years.
Scientists examine the oxygen isotopes present in these sediments, which gives us some solid quantitative information about the weather conditions dating back to the age of the dinosaurs!
There are some other climate proxies through which scientists have been able to establish the existing weather conditions thousand of years in the past. Although none of these techniques provide absolute results, they definitely provide us with enough information to make educated guesses about the climate of the planet millions of years ago.
