In a comment published on 27 November in the prestigious science journal Nature, Timothy Lenton and others write that
Atmospheric CO2 is already at levels last seen around four million years ago, in the Pliocene epoch.
This was before the current ice age, known as the Quaternary Glaciation, which began 2.6 million years ago. Note that we are currently living in an interglacial phase of this ice age.
Note also that, if the industrial revolution had not happened, it is almost certain that global temperatures would not be rising but falling, as they have been doing for about the last 5 thousand years. The earth was probably heading for another glacial period. Indeed, a “little ice age” had already started, with several particularly cold intervals. For example, one began in 1770 and caused crop failures in Europe that contributed to the French Revolution.
A graph showing this temperature decline published on the US government Climate.gov website, adapted from Figure 1(b) in Marcott et al. looks like this:
Lenton et al go on:
[Atmospheric CO2] is rapidly heading towards levels last seen some 50 million years ago — in the Eocene — when temperatures were up to 14 °C higher than they were in pre-industrial times.
This would cause a total transformation of the whole earth. There would be no ice anywhere on the planet. Large parts of the land would become desert.
If this seems an unlikely scenario, think again. It turns out that temperatures up to 14 °C hotter than today have occurred throughout the history of the earth.
Glaciations, such as the one we are living in, are relatively rare. For more details see the Wikipedia article Geologic temperature record.
The authors of the commentary conclude:
In our view, the consideration of tipping points helps to define that we are in a climate emergency and strengthens this year’s chorus of calls for urgent climate action — from schoolchildren to scientists, cities and countries.