How Long Has Ice Been Around

New Scientist Default Image — Don't forget your woolly mittens

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Primitive humans, clad in animal skins, trekking across vast expanses of ice in a desperate search to find food. That'southward the image that comes to mind when nigh of us call up near an water ice age.

But in fact there have been many ice ages, well-nigh of them long earlier humans made their first appearance. And the familiar picture of an water ice age is of a insufficiently mild ane: others were so severe that the entire Earth froze over, for tens or fifty-fifty hundreds of millions of years.

In fact, the planet seems to have three main settings: "greenhouse", when tropical temperatures extend to the poles and in that location are no ice sheets at all; "icehouse", when there is some permanent ice, although its extent varies greatly; and "snowball", in which the planet's unabridged surface is frozen over.

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Why the ice periodically advances – and why it retreats once more – is a mystery that glaciologists accept merely just started to unravel. Hither's our recap of all the back and forth they're trying to explain.

Snowball Globe

two.4 to 2.ane billion years ago

The Huronian glaciation is the oldest ice historic period we know about. The Earth was just over two billion years old, and home only to unicellular life-forms.

The early on stages of the Huronian, from two.iv to 2.iii billion years agone, seem to have been particularly astringent, with the entire planet frozen over in the first "snowball Globe". This may take been triggered past a 250-one thousand thousand-yr lull in volcanic activity, which would take meant less carbon dioxide beingness pumped into the temper, and a reduced greenhouse effect.

Deep freeze

850 to 630 million years ago

During the 200 1000000 years of the Cryogenian flow, the Earth was plunged into some of the deepest cold it has ever experienced – and the emergence of circuitous life may have caused it.

Ane theory is that the glaciation was triggered by the development of large cells, and possibly likewise multicellular organisms, that sank to the seabed after dying. This would have sucked CO₂ out of the temper, weakening the greenhouse effect and thus lowering global temperatures.

There seem to take been two singled-out Cryogenian ice ages: the so-chosen Sturtian glaciation betwixt 750 and 700 million years ago, followed past the Varanger (or Marinoan) glaciation, 660 to 635 1000000 years ago. There's some evidence that World became a snowball at times during the large freezes, simply researchers are still trying to work out exactly what happened.

Mass extinction

460 to 430 million years ago

Straddling the late Ordovician period and the early Silurian period, the Andean-Saharan water ice age was marked by a mass extinction, the second most severe in Earth'south history.

The die-off was surpassed only by the gargantuan Permian extinction 250 million years ago. Simply as the ecosystem recovered after the freeze, information technology expanded, with land plants becoming common over the class of the Silurian period. And those plants may have caused the side by side smashing water ice age.

Plants invade the land

360 to 260 million years agone

Similar the Cryogenian glaciation, the Karoo ice age featured ii peaks in water ice cover that may well have been singled-out ice ages. They took place in the Mississipian catamenia, 359 to 318 million years agone, and again in the Pennsylvanian 318 to 299 million years ago.

These ice ages may have been the effect of the expansion of land plants that followed the Cryogenian. As plants spread over the planet, they absorbed CO₂ from the atmosphere and released oxygen (PDF). Equally a result CO_two levels fell and the greenhouse effect weakened, triggering an ice age.

In that location is some evidence that the water ice came and went in regular cycles, driven by changes in Earth's orbit. If true, this would hateful that the Karoo ice age operated in much the aforementioned fashion as the electric current one.

Antarctica freezes over

14 million years ago

Antarctica wasn't always a frozen wasteland. It wasn't until around 34 1000000 years ago that the first small glaciers formed on the tops of Antarctica's mountains. And information technology was 20 1000000 years later, when world-wide temperatures dropped by 8 °C, that the glaciers' ice froze onto the rock, and the southern water ice sheet was born.

This temperature drop was triggered by the rise of the Himalayas. As they grew higher they were exposed to increased weathering, which sucked CO₂ out of the atmosphere and reduced the greenhouse effect.

The northern hemisphere remained relatively ice-complimentary for longer, with Greenland and the Arctic becoming heavily glaciated simply around 3.2 million years ago.

Latest accelerate of the water ice

2.58 1000000 years agone

The Quaternary glaciation started just a few million years ago – and is withal going on. So its history is relatively contempo, in geological terms, and can be studied in far more particular than the others'. It's evident that the ice sheets take gone through multiple stages of growth and retreat over the form of the Fourth.

During "glacial" stages, the temperature was low and water ice extended far away from the poles. During "interglacials", the temperature was somewhat warmer and the water ice retreated. Brief, inconclusive periods of advancing ice – typically lasting less than 10,000 years – are chosen "stadials"; conversely, periods when the water ice retreated, but just briefly, are called "interstadials".

The main trigger for the Quaternary glaciation was the standing autumn in the level of CO_ii in the atmosphere due to the weathering of the Himalayas. Nonetheless, the timing of the glacials and interglacials was driven past periodic changes in Globe's orbit that change the amount of sunshine reaching diverse parts of the planet. The outcome of these small orbital changes was amplified by positive feedbacks, such every bit changes in greenhouse gas levels.

During the first two-thirds of the 4th, the ice advanced and retreated roughly every 41,000 years – the same tempo every bit the changes in the tilt of Earth'southward centrality. Well-nigh a million years ago, the water ice switched to a 100,000-year wheel for reasons that were until recently a mystery. Now more than detailed information about the timing of the ice's movements may take helped glaciologists detect an answer.

To make matters more complicated still, the water ice didn't advance and retreat simultaneously all effectually the globe. Often it would brainstorm advancing on one continent, with the others only being covered thousands of years later on, and then linger on a few continents several millennia after it had disappeared from the others.

So in that location were really many overlapping glaciations within the Quaternary, each separately named: the Bavelian and Cromerian complexes of glacials and interglacials; the Elsterian glacial; the Holsteinian interglacial and the Saalian glaciation, among others.

Betwixt 130,000 and 114,000 years agone, the water ice retreated during the Eemian interglacial – and then avant-garde once more to create the glacial that most people know every bit "the ice age".

Our water ice age

110,000 to 12,000 years ago

The absurd temperatures of the Quaternary may take allowed our brains to become much larger than those of our of hominid ancestors. While that'south still open to argue, it's plausible that the most contempo glacial period left its marking on our species.

Neanderthals, with whom we shared the planet until simply before the last glacial maximum, 20,000 years ago, may have struggled to survive every bit the rising and falling ice ate away at their habitat – although many other explanations for their extinction take been suggested. What is across dubiousness is that Human sapiens survived and turned to farming soon later the ice retreated, setting the stage for the rising of modern civilisation.

As the glacial period drew to a shut and temperatures began to ascension, there were two final cold snaps. First, the chilly "Older Dryas" of 14,700 to 13,400 years ago transformed most of Europe from woods to tundra, like modern-day Siberia. After a brief respite, the Younger Dryas, between 12,800 to 11,500 years ago, froze Europe solid within a affair of months – probably as a result of meltwater from retreating glaciers shutting down the Atlantic Ocean'south "conveyor-belt" electric current, although a cometary impact has also been blamed.

Twelve grand years ago, the nifty ice sheets retreated at the outset of the latest interglacial – the Flandrian – allowing humans to return to northern latitudes. This catamenia has been relatively warm, and the climate relatively stable, although it has been slightly colder than the terminal interglacial, the Eemian, and ocean levels are currently at least 3 metres lower – differences that are beingness closely scrutinised by researchers great to empathize how our climate will develop.

Just this respite from the water ice is likely to prove brusque-lived, at least in geological terms. Human effects on the climate nonetheless, the cycle will go on to plough, the hothouse menstruation will some day come to an end – and the ice sheets will descend again.