Science allows us to make educated guesses regarding the temperature of the earth before humans invented measuring devices like the thermometer.
To see far into the past scientist have different techniques to estimate past temperatures, for example by examining the properties of atoms in the layers of ices sheets at the poles and of those of sediments that deposit in layers as the world moves through time.
This allows us to estimate the variation of temperature, from thousands to millions and billions of years ago.
This graph plots the findings of past temperature variation on Earth during the last 541 Million years before present. The graph is in a logarithmic scale which allows us to see detail in the present and at the same time see far into the past.
The line 0*C on the graph shows us the average temperature of present.
During this period (The Phanerozoic Eon) life has been plentiful with a wide variety of Fauna and Flora.
Extinction of the dinosaurs happened around 65 Million years ago after having lived for about 200 Million years in a climate averaging 3 degrees Celsius higher than today. The temperature peaked at about 50 Million years ago soon after at a maximum during the Eocene Epoch (green graph) 6*C higher than today's average.
Since then the temperature has been coming down. And at about 20 Million years ago, the two poles of the Earth have experienced an uninterrupted glaciation till present.
Now if we zoom in to closer to present,
During the last million years, the earth has been experiencing a recurring cycle in which glacier and inter-glacier periods succeed each other. About 12 thousand years ago the earth came out of its last glacier period which itself lasted about 90 thousand years. This cycle of about 100 thousand years has been repeating for over a million years (it is due to the variation of position of the earth as it rotates around the sun).
Humans (Homo Sapien) have since there apparition on earth about 200 thousand years ago have therefore already gone through two times 80 thousand years of glacier period. During which the temperature was 2*C lower then today's average. Much of Europe and north America was covered in ice, and water withdrew from the oceans leading to a much lower sea level and to more land surfacing around the globe.
With out exact possibility to predict when, we know that the Earth will experience this shift once again, and so humans will have to adapt as they are already used too.
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