Today, it is possible to measure temperature in any part of the world using precise thermometers that are often found on ships and meteorological stations, so it is relatively easy to calculate the global average, and then monitor whether that average is changing. However, as technological capabilities have changed over time, old preserved data differ in how much they can be used to draw conclusions about the climate in the past.
The turning point is usually taken to be 1880, so that today’s comparisons of the global average temperature are made exclusively taking into account the years that followed.
This moment was chosen for the beginning of modern measurement not because thermometers similar to today’s did not exist before, but because before this period, there is not enough data from many parts of the world, and the vast majority of stations were only in Europe and America.
Also, the reliability of the measured data before 1880 may be questionable, since the uniform standards for measuring temperature, which are adhered to today by all meteorological and climatological stations in the world, have not yet been clearly defined.
However, in order to properly research climate change, it is not enough just to know how temperatures have moved in the last 160 years, but we must go even further back in time and look at temperatures from hundreds of thousands or even millions of years ago.
As there is no time machine to go back in time, scientists have found other ways to “travel through time” and discover what kind of climate ruled our planet in the distant past.
What does ice say?
One way to learn about the climate in the past is to study ice cores.
When it snows, air-filled bubbles remain in it. In addition, in very cold areas, so much snow falls that new layers are constantly deposited over the years, centuries, and even millennia. But even in those thick ice sheets and glaciers, the bubbles still remain.
In order to find out what kind of climate prevailed in the past, the researchers managed to extract the so-called ice core by careful drilling, on which different layers can be noticed, each of which originates from the snow that attacked certain seasons. When they reach this nucleus, it is the bubbles that will provide the most information because air that can be chemically analyzed is trapped in them. In this way, the researchers find out how much carbon dioxide, nitrogen or oxygen could have been in the atmosphere in a certain year when the layer was formed.
Knowing the ways in which they circulate through the climate system, or their contribution to the greenhouse effect, various conclusions can be indirectly reported about the climate in the past, and even the temperature of the entire planet.
The older the layer of ice on the land mass, the further its analysis can go into the past. Thus, the ice core in Greenland can be used to monitor temperatures up to 250,000 years ago, and in Antarctica as much as 800,000 years. However, millions of years ago in this way it could not be returned.
Preserved in fossils
Plants and animals that inhabit the Earth today can be related to certain climatic conditions that suit them, or other parameters of the ecosystem in which they live. In a similar way, organisms that once lived can reveal something about the planet’s past. Therefore, one way to obtain temperature data from the past is to analyze fossils.
Fossils can also reveal some oddities to us. Thus, the remains of palm stumps were found in Alaska, and parts of alligators were fossilized in the Arctic, which indicates that tropical conditions once prevailed in these icy areas 52 million years ago.
However, with this method, only rough data can be obtained, whether it was hot or cold in a certain area. For more details it is necessary to use other sources. In addition, fossils can take us as far back as 542 million years, because older fossils of complex organisms have not been found. As each of the methods of studying the distant past of the climate has some limitations, the most successful are those researches that combine data obtained in different ways.
Difference in atomic mass
In order to go even further into the past and follow what happened to the temperature on our planet, we can also use isotopes of chemical elements that appear in nature.
Oxygen is most often used, ie. the ratio of the isotope oxygen 16 and oxygen 18 in the water in the ocean. When the climate on our planet is colder, the ocean is relatively “richer” in oxygen 18, while in periods of warm climate, this ratio changes and the amount of isotope 16 in the ocean increases, so it becomes relatively “poorer” in isotope 18.
The way to reconstruct the relationship between these two isotopes in the oceans in the past, and thus to estimate what the temperature was, is to examine it.