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Ice ages

An ice age is a long interval of time (millions to tens of millions of years) when global temperatures are relatively cold and large areas of the Earth are covered by continental ice sheets and alpine glaciers.

Within an ice age are multiple shorter-term periods of warmer temperatures when glaciers retreat (called interglacials or interglacial cycles) and colder temperatures when glaciers advance (called glacials or glacial cycles).

At least five major ice ages have occurred throughout Earth’s history: the earliest was over 2 billion years ago, and the most recent one began approximately 3 million years ago and continues today (yes, we live in an ice age!)

Currently, we are in a warm interglacial that began about 11,000 years ago.

The last period of glaciation, which is often informally called the “Ice Age,” peaked about 20,000 years ago.

At that time, the world was on average probably about 10°F (5°C) colder than today, and locally as much as 40°F (22°C) colder.

-from the Utah Geological Survey


When were they?

There have been several major ice ages during the past 2.4 billion years

We are currently in the middle of the latest ice age: This is why both poles are covered in ice, and why most of Canada, northern Europe and northern Russia, are covered in ice.  Over the last 450,000 years we have cycled in and out of several glacial periods.

* Glacial period – large glaciers descend south

* Inter-glacial period – large glaciers retreat noth

Climate swings in the past 12,000 years

Causes of ice ages

* changes in ocean and atmosphere circulation patterns

* varying concentrations of atmospheric CO2

* volcanic eruptions.

* Milankovitch cycles

* plate tectonics creates cycles of ocean basin growth and destruction, aka Wilson cycles.

[Wilson cycles] affect ocean and atmospheric circulation patterns. When plate-tectonic movement causes continents to be arranged such that warm water flow from the equator to the poles is blocked or reduced, ice sheets may arise and set another ice age in motion.

Today’s ice age most likely began when the land bridge between North and South America (Isthmus of Panama) formed and ended the exchange of tropical water between the Atlantic and Pacific Oceans, significantly altering ocean currents.
– Utah Geological Survey

Milankovitch cycles

Milutin Milankovitch studied patterns in:
(a) shape of earth’s orbit (eccentricity) – 100,000 year cycle
(b) tilt of earth’s axis (obliquity) – 41,000 year cycle
(c) direction of earth’s axis (precession) – 23,000 year cycle

He discovered that there was a correlation between these timescales and ice ages; more specifically, he was able to make mathematical predictions that could be tested by climate scientists: These cycles change the amount of sunlight absorbed by the Earth, thus driving changes in climate large enough, he proposed, to cause ice ages.

These cycles have been modeled by physicists to a high degree of accuracy; they find that the changes by these cycles alone are insufficient to explain the full range of ice-ages, but they do appear to be a meaningful contribution, perhaps a major trigger, that combines with other effects.

Orbital permutations

Another contribution

Milankovitch cycle app


What Causes Milankovitch Cycles?

– from skepticalscience.com

The changes in eccentricity of Earth’s orbit are due to alterations in the gravitational tugs induced by other planets. Jupiter has a very moderate eccentricity, but if it were larger, it would drive larger changes in Earth’s eccentricity.

It therefore seems likely that exotic cases of highly eccentric orbits may be prominent in other solar systems, where various gaseous planets are known to exhibit large orbital fluctuations.

Obliquity and precession variations arise due to the torque exerted by gravity (i.e., a force that acts perpendicular to he spin axis of the top) which ultimately comes from the pull of the Sun and Moon on Earth’s equatorial bulge. Precession also varies due to the tilting of the Earth’s orbital plane, as shown above.

The periodicity of Milankovitch cycles is therefore subject to change over geologic time, as the length of day of Earth changes, and the moon becomes further separated from Earth.

A shorter Earth-Moon distance would cause the precessional movement to have been larger and the precession and obliquity cycles would have been shorter, as would have occurred in geologically distant paleoclimates. For example, in the Upper Carboniferous (~300 million years ago), the ~41,000 yr obliquity cycle would have taken about 33,000 years (see e.g., here)



Wilson cycles

Through the course of a Wilson cycle continents collide and split apart, mountains are uplifted and eroded, and ocean basins open and close.

The re-distribution and changing size and elevation of continental land masses may have caused climate change on long time scales.

Computer climate models have shown that the climate is very sensitive to changing geography.

It is unlikely, however, that these large variations in the Earth’s geography were the primary cause of the latest long-term cooling trend as they fail to decrease temperatures on a global scale.

westerly winds could be diverted by plateau uplift. WGBH Educational Foundation

westerly winds could be diverted by plateau uplift.
WGBH Educational Foundation

Likewise, changing topography cannot, by itself, explain this cooling trend. Computer model experiments performed to test the climate’s sensitivity to mountains and high plateaus show that plateau uplift in Tibet and western North America has a small effect on global temperature but cannot explain the magnitude of the cooling trend.

Plateau uplift does, however, have a significant impact on climate, including the diversion of North Hemisphere westerly winds and intensification of monsoonal circulation.

Another theory explaining these changes in climate involves the opening and closing of gateways for the flow of ocean currents. This theory suggests that the redistribution of heat on the planet by changing ocean circulation can isolate polar regions, cause the growth of ice sheets and sea ice, and increase temperature differences between the equator and the poles.

NOVA PBS: What causes ice ages?

Tectonic Collisions

In Discover magazine Nathaniel Scharping  writes (3/14/19)

At geological time scales, what really controls the climate isn’t the atmosphere, it’s the ground. Most of Earth’s carbon dioxide is held underground, in reservoirs of natural gas and oil, but also in the rocks themselves. As the planet’s tectonic plates slide and churn against one another, they bury carbon deep beneath the surface while exposing fresh rock that will soak up more carbon over time.

That carbon can be liberated in large volcanic events, causing mass extinctions. But the process can also work the other way, where rocks pull carbon from the sky. A new study from MIT researchers claims that Earth’s last three major ice ages were caused by collisions of tectonic plates bringing fresh, carbon-hungry rock to the surface. Over millions of years, these rocks sucked up enough carbon dioxide from the atmosphere to cause temperatures to plummet and send glaciers marching outward from the poles.

The process is simple. Much of the rock in Earth’s mantle is composed largely of silicate, and when exposed to the air, it will naturally react with carbon dioxide, forming new minerals that sequester carbon as a solid. This process is much more likely to occur in the tropics where temperatures are higher and frequent rain will wash soils away to expose bare rock.

At certain points in Earth’s history, oceanic tectonic plates in the tropics have collided with continental plates, sliding over the top of them and exposing hundreds of thousands of square miles of fresh rock to the air. These pile-ups, called arc-continent collisions, create a generous supply of fresh rock. Weathering processes begin as they come into contact with air and over the course of a few million years, carbon is gradually drained from the atmosphere.

For their most recent study, published Thursday in Science, the MIT researchers first traced the origins of the Himalayas. Though the forbidding mountain range is far north of the equator today, it actually formed 80 million years ago in more southerly latitudes as the result of a collision between the supercontinent Gondwana and Eurasia.

The birth of the Himalayas turned out to have preceded a global ice age by a few million years — a short period of time in geological terms.

See Arc-continent collisions in the tropics set Earth’s climate state, Francis A. Macdonald et al., Science 12 Apr 2019, Vol. 364, Issue 6436, pp. 181-184,  DOI: 10.1126/science.aav5300

How do we know about past ice ages?

Scientists have reconstructed past ice ages by piecing together information derived from studying ice cores, deep sea sediments, fossils, and landforms.

Ice and sediment cores reveal an impressive detailed history of global climate. Cores are collected by driving long hollow tubes as much as 2 miles deep into glacial ice or ocean floor sediments. I

ce cores provide annual and even seasonal climate records for up to hundreds of thousands of years, complementing the millions of years of climate records in ocean sediment cores.

Within just the past couple of decades, ice cores recovered from Earth’s two existing ice sheets, Greenland and Antarctica, have revealed the most detailed climate records yet.


Last Glacial Period – 110,000 to 12,000 years ago – The most recent glacial period, within the current ice age.

26,000 – 20,000 years ago: Last Glacial Maximum (LGM) – the last period when ice sheets were at their greatest extension.

25,000-13,000 years ago: late glacial maximum – Beginning of the modern warm period, the Northern Hemisphere warmed substantially. The deglaciation following the LGM.

1400-1850 CE Little Ice Age (LIA) – A period of cooling in the northern hemisphere, but not a true ice age. Several causes have been proposed: cyclical lows in solar radiation, heightened volcanic activity, changes in the ocean circulation, an inherent variability in global climate, or decreases in the human population.


The Causes of the end of the last Ice Age


The following text is adapted, except where otherwise noted, from New Scientist magazine, 05 November 2012, Anil Ananthaswamy.

The last great ice age began around 120 000 years ago. One massive ice sheet, more than 3 kilometres thick in places, grew in fits and starts until it covered almost all of Canada and stretched down as far as Manhattan. Another spread across most of Siberia, northern Europe and Britain, stopping just short of what is now London. Elsewhere many smaller ice sheets and glaciers grew, vast areas turned into tundra and deserts expanded as the planet became drier.

With so much ice on land, sea level was 120 metres lower than it is today. Britain and Ireland were part of mainland Europe. Florida was twice the size it is now, with Tampa stranded far from the coast. Australia, Tasmania and New Guinea were all part of a single land mass called Sahul. The planet was barely recognisable.

Then, 20 000 years ago, a great thaw began. Over the following 10 000 years, the average global temperature rose by 3.5° C and most of the ice melted. Rising seas swallowed up low-lying areas such as the English Channel and North Sea, forcing our ancestors to abandon many settlements. So what drove this dramatic transformation of the planet?


Thickness of the ice sheets

Thickness of the ice sheets

Thickness of ice sheets Ice Age

From XKCD June 14, 2013

Were we heading into a new ice age?

Are we – or were we, before man-made changes occurred – heading into a new Ice Age?

“According to ice cores from Antarctica, the past 400,000 years have been dominated by glacials, also known as ice ages, that last about 100,000. These glacials have been punctuated by interglacials, short warm periods which typically last 11,500 years.

Figure 1 below shows how temperatures in Antarctica changed over this period. Because our current interglacial (the Holocene) has already lasted approximately 12,000 years, it has led some to claim that a new ice age is imminent. Is this a valid claim?”

* we’ve repeatedly moved in an out of inter-glacials.

* we’d eventually enter the next minor ice-age on a geologically short time scale.

* We may be statistically overdue for the next ice age.

* Some scientists say that climate data showed Earth beginning to enter this phase in the late medieval period (the little ice age)

* However, humans recently have introduced massive amounts of greenhouses gases into the atmosphere, creating an increase in the average temperature of the atmosphere and oceans.

* Thus: perhaps Earth could’ve soon entered a new ice-age, but now this will not be happening any time soon.  Rather, we’re likely entering a protracted period of global climate change, due to an average increase in worldwide temperature.


External links

Megafloods of the Ice Age

The Discovery of Global Warming February 2014: Past Climate Cycles: Ice Age Speculations

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