The glacial-interglacial cycles
are best captured by this record of the Vostok CO2 concentration through time,
and the global ice volume, and Antarctic temperature, also from that same ice core.
It's got this intensely close correlation between the CO2 concentration
and the Antarctic temperatures.
The way that we think this all works is that the orbit of the Earth around the
Sun affects the intensity of sunlight, as it hits
the Northern Hemisphere of the Earth, in the summertime.
The summertime is important because that's when you
decide whether to melt last winter's snow or whether to keep
it around and continue to accumulate it to make an ice sheet.
And the Northern Hemisphere is important because
that's where the land is, where we can put these ice sheets.
During the glacial time there was
a huge ice sheet in North America called the Laurentide ice sheet,
and in Europe, there was the Fennoscandian ice sheet.
There aren't optional ice sheets
in the Southern hemisphere in the same way.
It's the Northern hemisphere
that's kind of the sweet spot for global climate.
The changes in the ice sheet somehow provoke a
positive feedback in the carbon cycle that
amplifies the climate change and globalizes it.
And it's not very well understood how exactly that happens.
One false conclusion that is often drawn from this record comes from a time
right in here, at the end of ice age, called the deglaciation,
a time when it was just starting to transition
from a full glacial maximum to an interglacial climate.
And if you look in detail at this time the temperature from
Antarctica starts to rise first, before the C02 starts to go up.
It's hard to get this information,
because the temperature comes from
the oxygen 18 which is in the H2O which is in the ice,
and the CO2 is from little bubbles which
don't necessarily have the same age.
In fact its pretty clear that, so you you put snow on
top of snow, until the snow is compressed enough to seal off
and make bubbles, the gases can still exchange with the atmosphere.
There's a difference between the Ice Age and the
Gas Age which make this a little difficult.
But as best we put together, it looks as though the
temperature started warming a little before the CO2 started to rise.
That leads people to conclude erroneously
that since temperature is apparently driving CO2 in this part
here that means that CO2 somehow couldn't drive temperature.
Whereas, you now know that a feedback is simply a loop of cause and effect.
There's no reason why cause and effect has to be a one-way street.
And it took much longer than 800 years for the full CO2 and
temperature responses to develop.
The fact that the temperature in
Antarcita might have provoked the CO2 to start rising
does not mean that the temperature rose all the way
without any help from the rising CO2.
A feedback loop allows these things to go in that direction.
The temperature changes during the ice age cycles
are about five to six degrees Centigrade,
which is a remarkably small temperature change
when you consider that the whole world looked different.
Sea level was 120 meters lower.
You could have walked from England to France or Russia to Alaska.
and that was only with a five or six degree C temperature change.
For a comparison, the temperature change predicted for the year 2100
under business as usual, is about something like that.
That same order of magnitude.
The temperature changes of the last 1000
years are comparable to what we've
seen so far and that's really small potatoes
compared to what is forcast for the future.
About half
of the warming on the deglaciation or, in other words, half the reason
why it was colder during the ice age, is due to the lower CO2.
The other half is due to the shininess,
the albedo, of those ice sheets.
They can put together an analysis
of the glacial climate and try to derive
value for the climate sensitivity. Delta T(2x),
how much you expect the earth to change
its temperature, in equilibrium from doubling the CO2 concentration.
And the values they come up with for the climate sensitivity are
completely consistent with those from diagnosing the instrumental record period.
And they're also what the climate
models are tending to produce. If CO2 were not a greenhouse
gas, we wouldn't be able to explain even why the earth isn't in a deep freeze.
And then if CO2 didn't have about the impact on climate that we think it does,
we wouldn't be able to explain all these past temperature changes.
The fact that we
can only explain the instrumental temperature record with
climate models using the greenhouse effect and the
rising CO2 concentrations is the smoking
gun for climate change, but we know the
gun works because, if things didn't work the way we
think they do, we wouldn't be able to
explain anything about the climates of the past either.