MadSci Network: Earth Sciences

Re: How has surface temp and atmosphere varied since Earth was formed to now?

Date: Sun Nov 8 21:59:06 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Earth Sciences
ID: 906486418.Es

What you are asking is a very long and complicated story. And when 
scientists are unravelling a story like this about the distant past, there 
are some things and times that they know a lot more about than others. 
There are many clever ways of collecting evidence, but the sort of evidence 
we can get can only tell us a few little bits when we go to the very 
distant past.

The earth is, according to the best evidence we can collect, about 4.5 
billion years old. As far as the composition of the atmosphere is 
concerned, there are three main periods, with small variations within each 

In the very early times, the atmosphere was made of mainly hydrogen and 
helium, with a little bit of methane. The atmospheres of the giant outer 
planets, Jupiter, Saturn, Uranus, and Neptune are still very much like 
this. But the Earth was a smaller and warmer planet, and its gravity was 
not strong enough to hold on to these very light gases for very long -- 
millions of years maybe, but not billions. The hydrogen/helium atmosphere 
was lost very quickly on the overall time scale of things.

So the atmosphere gradually thinned and changed its composition. Hydrogen 
and helium were lost, and were replaced by gases that came out of 
volcanoes, and from the weathering of rocks. The main gases in the 
atmosphere were nitrogen, carbon dioxide (although that was probably mostly 
dissolved in the oceans), and a little bit of argon. The atmospheres of 
Venus and Mars are still pretty much like this today. 

But there were two things that were a little bit different and quite 
remarkable about the earth. The first is that the temperature was just 
right for liquid water to remain about on the Earth's surface, and for 
oceans to form. Venus was too hot, and Mars was too cold. The second was 
that life got started, probably in some of the shallow water of the oceans 
near the edge of the land. Some of the early life forms learnt a remarkable 
trick. Water contains hydrogen and oxygen atoms; carbon dioxide contains 
carbon and oxygen atoms. By rearranging the chemical bonds between these 
atoms, organic substances like sugar and starch, which also contain carbon, 
hydrogen, and oxygen atoms, could be made. It required energy from the 
sunlight to make this happen, and when it was done, there were quite a lot 
of oxygen atoms left over, which joined up in pairs to make oxygen gas. The 
first living creatures to do this were a sort of seaweed slime, known as 
blue-green algae; later the whole great variety of green plants developed 
on the basis of the same sort of chemistry. The sugar and starch were 
useful both as growth material for the living creatures, and as a source of 
energy that they could use.

As green plants became more numerous, and particularly when they began to 
colonize the land surfaces, the composition of the atmosphere changed 
again, because all of the carbon dioxide was being used up, and oxygen was 
being added as a waste product. And of course other creatures multiplied 
which could use the sugars and starches in green plants for food, and 
oxygen from the air to react with it to produce their energy. So for about 
the last 2 billion years the atmosphere has contained a fairly large amount 
of oxygen, along with nitrogen and a little argon, and only a tiny bit of 
carbon dioxide.

I do not have a really good reference I can give you. The one I mainly use 
"Chemistry of the Natural Atmosphere" by Peter Warneck, Academic Press 
1988, chapter 12, is very technical, and I think you would find it too hard 
to read. "The Ages of Gaia" by James Lovelock, 1986, is a bit less 
technical, but still quite heavy going.

Now, to get on to the temperature of the Earth: because there is plenty of 
evidence that liquid water has always been present in large quantities on 
the earth's surface (water worn pebbles are quite different to pebbles 
produced by glaciers, wind erosion, or other weathering processes, for 
example), we know that the average surface temperature on the earth has 
never been more than about 30 deg C nor less than about 5 deg C. 

I will start out with the most recent tiny fraction of the Earth's history.
For the last 300 000 years or so, we have a very detailed record from an 
analysis of ice cores drilled in Antarctica and Greenland. In these two 
places, fresh snow accumulates on top of old ice, and gradually compresses 
it down into an icesheet that is now several kilometres thick. It forms a 
frozen record of the recent past -- the shallower ice refers to more recent 
times, the deeper ice to older times. For the top bit, you can apparently 
count ice layers like tree rings, because of changes in ice structure due 
to slight surface melting in the direct sunshine of summer months; deeper 
down this structure disappears, and you have to rely on other clues to get 
accurate dates. Air composition shows up in trapped air bubbles in the ice; 
temperature in the relative amounts of two different isotopes of oxygen in 
the ice; dust content of the ice tells how strong the prevailing winds 
were, and marks major volcanic eruptions; and trapped pollen grains tell 
what sort of plants were growing in countries not too far from the ice -- 
obviously there would be almost none of them in ice ages, and a lot more in 
warmer times.

This record shows us three 'ice ages', each of which lasted for tens of 
thousands of years, and three warmer and briefer 'interglacials' where the 
climate was much warmer for a relatively shorter time. We are at present in 
the third of those interglacials.

When we go earlier than this ice record, we have to rely on the evidence of 
fossils to make deductions about the climate. One good example is that 
where I am living in Victoria Australia, we have fossils of dinosaurs from 
about 70 million years ago. At that time this part of the land was very 
close to the South Pole. But the dinosaurs look as though they were adapted 
to living in forests. So it seems fair to suppose that at that time the 
world was a fair bit warmer than it is today. The South polar icecap either 
did not exist, or was much smaller.

It is only from those sort of deductions that we know much about the 
temperatures in those earlier times. The other thing we do know is that the 
average surface temperature, now about 15 deg C, never got to be above 
about 30 deg C, because then there would have been extra carbon dioxide and 
water vapour in the atmosphere which would have led to runaway warming to 
even hotter temperatures.

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