I used to think that our current climate was relatively normal and that there was an occasional ice age every few million years to mix things up. It was somewhat of a surprise to me to find that according to some definitions, we are living in an ice age! Not the coldest part, nor the coldest ice age, but while we have glaciers and significant ice at the poles, that’s what it is. There have been some very long periods when Earth has had no ice to speak of, even at the poles.
Earth is about 4.5 billion years old. It doesn’t seem to have settled yet to a 'normal' temperature. This graph is an estimate of temperature changes, relative to today, over just the last 65 million years. Temperatures have averaged as much as 12 degrees hotter, and as low as 8 degrees colder than today.
Source: http://commons.wikimedia.org/wiki/File:65_Myr_Climate_Change.png
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 4 degrees centigrade lower, and as much as 8 degrees lower than today.
Most references to an 'ice age' refer to a colder period between the ‘warm’ inter-glacial periods, but I guess 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 has never covered any more than Northern America and Northern Europe/Asia with ice sheets more than we have today.
Lets take a closer look at that period:
Source: http://commons.wikimedia.org/wiki/File:Five_Myr_Climate_Change.png
How do they know the temperatures from way back then?
Different methods, are used but a key one is -
Different methods, are used but a key one is -
Sediment layers at the bottom of the ocean contain varying quantities
of different shellfish remains dependent on the temperature of the ocean
at the time the sediment was laid down.
of different shellfish remains dependent on the temperature of the ocean
at the time the sediment was laid down.
Note the temperature range compared to todays average in the left edge of this graph.
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 has lasted about 10,000 years (that’s just the tip of the last peak in the graph above - see more detail in the graph below). 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.
We humans started to get more organised at the start of this inter-glacial, about 10,000 years ago. Many became less nomadic and started farming. You could say it was the dawn of the earliest civilisations. There’s an interesting timeline in this linked article comparing human history with temperature variations.
www.vox.com/2016/9/12/12891814/climate-change-xkcd-graphic
Judging by past patterns our current, comfortable, inter-glacial climate would be near its end! That would mean temperatures dropping, yet the news tells us the temperatures are rising! 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 ‘civilisation’ has happened during a brief inter-glacial period, in the middle of an ice age, and has taken only a short moment in Earths history. Our 'normal' climate is an island in a sea of rises and falls.
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