How Stable is the Climate?

Tom Hunt, CCL’s co-coordinator in the Illawarra, says he used to think that our current climate was relatively normal and  was ‘somewhat surprised’ to find that we are living in an “ice age”! In this post he shares what he’s found out about ice ages and our place in the current one.


Define Ice Age

While our present climate is neither the coldest part nor the coldest ice age, while we have glaciers and significant ice at the poles, an “ice age” is by definition what it is. By contrast, there have been some very long periods when Earth has had no ice to speak of at the poles. So, while Earth is about 4.5 billion years old, it doesn’t seem to have settled permanently – yet – to a “normal” or stable temperature.

For example, the graph below is an estimate of Earth’s surface temperature, relative to today, over the last 65 million years (Note this is still only 1/70th of Earth’s history).

Source: ​

[This graph is a compilation of estimates measured in different periods from different research and approaches. For instance, the Equivalent Vostock <delta> T (degrees C) scale on the right shows the temperature of that part of the ocean where the Vostock core was drilled through history compared to today’s temperature there, based on the Benthic Carbonate measurement. (There is supportive evidence that this relationship of the Temperature to Benthic Carbonate concentration is valid). We can see that temperatures and variability have changed significantly.]

Humans “arrived” on the planet about 2.5 million years ago around the beginning of the current “ice age” (based on the above definition). During most of this period the average global temperature has more commonly been around 4 degrees centigrade lower, and as much as 8 degrees lower than today.

While most references to an “ice age” refer to a colder period between the warmer inter-glacial periods, I suggest it’s fair to consider all of the period where there’s been continuous ice at the poles an ice age. Polar ice has been on the planet longer than humans, yet in all that time it appears to me that it has never covered any more than Northern America and Northern Europe/Russia with ice sheets.

[One might well ask – “how do they know the temperatures from way back then?” Different methods are used but a key one involves looking at sediment layers at the bottom of the ocean, which contain varying quantities of different shellfish remains depending on the temperature of the ocean at the time the sediment was laid down. The sediments often form annual layers and can be carbon dated.]


Roughly every 40,000 to 100,000 years through this ice age there have been short “inter-glacial” periods where much of the ice melts as the temperature rises several degrees before it drops back to a long chilly period again.

I say “short” inter-glacial. That’s relative of course. The current inter-glacial period has lasted about 10,000 years (but that’s just the tip of the last peak in the graph above). This period has been unusually stable with only small rises and falls (in the global average) of less than a degree. Inter-glacial periods generally don’t last much longer than this.

Human civilization as we know it only started around 10,000 years ago, at the beginning of this stable interglacial period. There’s an interesting timeline in this linked article comparing  human history with temperature variations.

Judging by past patterns our current, comfortable, inter-glacial climate would be near its end! That would mean temperatures dropping slowly, yet the news tells us the temperatures are rising, and not so slowly! History tells us that temperatures can change, and can change a lot, but is it possible we humans have had an effect?


Summing up

The whole of human “civilization” has happened during a brief warm period in the middle of an ice age, and has taken only a short moment in Earth’s history. What appears to be our “normal” climate is a small island in a sea of change.

And the real sea also rises and falls – a lot – with the changing climate, as we shall see in the next blog post in this series. 

You may question something I have said here. Some things appear to provide contradictory evidence but having checked out all the arguments I’ve heard about I am still confident in what is presented here. But I’m open to new ideas, and am always willing to do more research or answer your concerns. Please let me know at , Tom Hunt. 

1 Comment

  1. Nigel Howard 4 weeks ago

    Nice work Tom. I thought I understood this reasonably well, but you’ve added a few aspects that I hadn’t thought of – thanks. Regardless, since man has evolved 20,000-2Myears ago, the climate has never been warmer than now and over the last 100years the correlation with man’s emissions and developing prosperity (=consumption) is utterly compelling of our influence, so this gives no room for complacency that our influence is trivial. The rate of change also appears unprecedented.

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