An ice age is when one or both poles are permanently frozen. Like right now.
For most of the earth’s life, poles were not frozen. It was always summer
- hot, humid and good for life to flourish. Called greenhouse phases, the average
temperature of the earth was 30C (currently 15C). Carbondioxide (CO2) was above 300 ppm (could
reach 800 ppm or more). There was a greenhouse phase from 600 to 360 million
years ago (Mya) i.e. lasting 240 million years and from 260 to 40 Mya (220
million years).
The Ice age previous to ours, called the Karoo Ice age, lasted from 360
to 260 Mya. Rodinia supercontinent split apart around 700 Mya. The pieces came
together around 335 Mya to form another supercontinent called Pangea. Since the
Gondwana part of Pangea was in the South
Pole region around 360 Mya, it got cold and the Karoo Ice age resulted. But
only the poles got cold, like present day earth and unlike snowball earth of 2400-2100
Mya (Huronian) and 700-600 Mya (Cryogenian) when glaciation was extensive. During
Karoo ice age there were carboniferous rain forests in the tropics which ultimately
converted all carbondioxide to oxygen. CO2 fell from 800 ppm to below 300 ppm
and oxygen levels went up causing more severe glaciation of poles around 300
Mya which ended the carboniferous era.
Evolution of termites ended the Karoo ice age, around 260 Mya. Termites
digest lignin of even buried wood and their gut has methanogenic archaebacteria.
Conversion of CO2 into wood and then coal occurred in swamp lands which by 260
Mya got submerged undersea, stopping the removal of carbon. Methane from
termites accumulated. Forests died. CO2 and methane levels rose causing the
global warming which ended the Karoo ice age. Later, 250 Mya there was a mass extinction
of insects (Permian Triassic Extinction). We don’t know whether a meteor or
volcano or climate change caused that.
It was hot and humid from 260 Mya. No ice caps. Cold blooded dinosaurs
evolved and could roam the hot earth with ease. They got wiped out by a meteor
falling into the Yucatan region around 65 Mya. It caused an impact winter
because of dust in the atmosphere. All photosynthesis stopped. Winter lasted
only a few thousand years but at the end of it, only creatures smaller than 25
Kilos survived. All else perished. We say the Era changed from Mesozoic to
Cenozoic but the impact winter effects vanished fast and earth was hot again.
Things changed a bit when Pangea, which had split about 150 Mya, moved our continents to locations similar to present day (See figure – you can see India move up).
The fourth and last Ice Age of Pleistocene started 2.5 Mya when the
isthmus of Panama developed and stopped mixing of water between Pacific and Atantic.
The north pole also froze. Wooly mammoths, wooly rhinos, sabre toothed cats etc
evolved, just like the ice age movies.
As long as the Isthmus of Panama remains, earth will remain in an ice
age.
Like the Karoo Ice age, Pleistocene was not a snowball earth. Africa,
India, South America and South East Asia had climates pretty similar to what we
have today, just a bit more arid with less rainfall. Homo erectus evolved 1.5
Mya in Africa and spread throughout the world, a scavenging nomad.
In the last 2.5 million years of the Pleistocene, there were interglacial
periods when ice cover retreated. It happened every 100,000 years or so,
brought about by solar maxima (Milankovitch cycles). Earth has a tilted axis
which undergoes precession. Just like a spinning top, the axis moves in a
circle every 26000 years. So every 13000 years earth faces more towards the sun
and gets hotter. After another 13000 years it gets gets colder when axis tilts
in reverse. Since orbit around sun is also eccentric, every 100,000 years or so
the orbit and precession combine to make it hot enough for glaciers to melt a
lot. Sometimes, sun spot activity also combines to accentuate or minimise these
effects. These warm interglacial periods usually last for around 15000 years.
That’s where we are right now – an Interglacial period called Holocene.
Holocene started about 11600 years ago (or about 9000 BC) when we were at
solar maximum with respect to axial tilt. It melted the glaciers and ended the
ice age. Melting of float ice in sea doesn’t raise sea level but ice melt from European,
Siberian and Canadian land mass does. Sea level rose by 60 meters or so (about
the height of Qutab minar at 73m). Britain became an island because of channel ice
melt. Sri Lanka became an island because sea levels rose to create Palk straits.
Most of the giant mammals became extinct. Humans found conditions good for
agriculture along the rivers and we became civilized, only ever knowning the
interglacial period of Holocene in history.
The previous interglacial period is called the Eemian. It started 130,000
years ago and ended 115,000 years ago. It was hotter than the Holocene by about
1-2 degrees. CO2 was about 280 ppm, same as pre-industrial Holocene. It is
during the Eemian that Homo sapiens developed.
Its interesting to note that during the Eemian, hippos roamed the Thames
in London. Seas were 6-9 meters higher than today, including Baltic sea, making
Norway and Sweden an island.
The Eemian ended when the earth’s
axis tilted away, as expected, having lasted 15000 years. It iced over for
100,000 years after that. When we talk of “The Ice Age”, we normally mean the
last 100,000 years. Global temperatures were around 10C and CO2 levels were
around 200 ppm. Most human migrations from Africa occurred in this period as
they went where prey were easier to catch, including the ice age mammals of
Europe, Asia and the Americas. A few also came to India.
Many people don’t realise
that we are now close to the solar minimum due to axial tilt. We have enjoyed
11600 years of warm interglacial period. In a few thousand years, the earth is
due for another ice age. Our only long term hope is to prevent it using climate
control technology.
One might think that global
warming is good, as it prevents another ice age. But things aren’t so simple. As
long as we stay where we are now, its “probably” fine.
But if earth warms too
much, it causes a catastrophy. Global warming increases precipitation of rain
and snow. Within a few hundred years, snow accumulates in the poles and an ice age
develops. However, thats far away.
In the short run, within our
lifetimes, a hot earth makes everything very unpleasant indeed. Cyclones,
floods, blizzards, we get them all. The last few years have already indicated
these dangers. We are a few crop failures away from a global famine. Any
drastic global warming results in an out of control climate disaster.
We want things to stay
stable until we do enough research and learn to control the climate properly,
which might take a 100 years. Until then, lets keep our methane and CO2
emissions curtailed.
Just like every previous
climate change, the issue is evolve or perish. The evolution demanded of us is
to become better planet scientists able to control the factors which dictate
our climate. The Pleistocene Ice Age we live in has given us just enough time to
be able to do it – provided we don’t mess it up with backward Anti-Science
mentalities.
Ref:
1. Pedersen RA, Langen PL,
Vinther BM. The last interglacial climate: comparing direct and indirect
impacts of insolation changes. Clim Dyn 2017;48:3391-3407
2. Salonen JS, Helmens KF,
Brendryen J et al. Abrupt high-lattitude climate events and decoupled seasonal
trends during the Eemian. Nature Communications 2018;9:2851
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