An ice age is any time period when glaciers exist upon the earth, even if the icesheets are few and far between.
Ice ages fluctuate in intensity. Periods when ice-packs spread far across continents or the sea are called glaciations. Intervals when the glaciers retreat to the poles and to mountaintops, with a relative warming of the climate, are called interglacials.
We are currently in an interglacial in the middle of the Quaternary Ice Age, which began 1,700,000 years ago. The Quaternary ice age glaciations have expanded to extreme reaches four times so far in its duration. The most recent glaciation ended around 12,000 years ago.
Interglacials like the one we are presently basking in -- the Holocene Interglacial -- commonly run 10,000 to 20,000 years. The Holocene interglacial could end within a century... or last several more millennia.
The Earth has experienced several ice ages during its 4.57 billion year existence. Lumped all together, the ice ages total about 12% of that time frame.
For 88% of Earth's history, the globe has been devoid of glaciers. Any snow that fell in winter would melt in the summer.
During the majority of Earth's existence, the average global temperature has been significantly higher than today, helping to prevent the growth of glaciers. For example, during the Mesozoic era, 220,000,000 to 70,000,000 years ago, the average global temperature was 6-10°C higher than present.
Though the age of our current ice age seems lengthy at 1,700,000 years, the glacier-free era that came before the Quaternary lasted nearly 200 times as long: a quarter of a billion years.
We have to look back 260 million years to find the preceding glacial period, called the Karoo Ice Age. It took place during the Carboniferous Period: 350,000,000 to 260,000,000 years ago.
the Carboniferous Period
Due to plate tectonics, all the major landmasses on earth had collided to form one supercontinent called Pangaea. Pangaea's southern edge infringed upon the south pole, which helped trigger the Karoo Ice Age. (Some factors in the catalyst of an ice age: land mass covering the pole; high latitude ocean currents blocked by continent; ...)
The Karoo Ice Age locked up so much water in its glaciers that sea levels dropped much further than any caused by the Quaternary Ice Age. Great expanses of former continental shelf became exposed. Ferns, horsetails, and primitive trees colonized these tracts of lowland throughout the tropics.
Heart of the matter:
As a plant grows, it absorbs carbon dioxide from the atmosphere and emits oxygen while using the carbon to build its living framework.
When a plant decays after death, it releases its store of carbon dioxide back into the atmosphere.
As the Karoo Ice Age cycled back and forth between glaciations and interglacials, the sea level rose and fell, alternately drowning and revealing the lowland marsh and forest lands. The thick bark of primitive trees resisted decay in the acidic waters. Then erosion silted layer after layer of sediment over the plant matter.
Huge tracts of non-degraded, carbon-containing vegetation were buried under thick caps of limestone and sandstone -- thereby robbing the atmosphere of most of its CO2 assets. (According to Wikipedia, depleting to just a third of former levels!) Oxygen built up in the atmosphere. Whereas today's air contains 21% oxygen, the atmosphere of the Carboniferous period rose to 35%.
With the rich oxygen mix and accompanying higher atmospheric pressure, insects and amphibians (and other creatures that absorb oxygen through the skin) began to grow to incredible size. One variety of griffinfly had a wingspan of more than two feet!
Fast Forward Again
The first dinosaurs appeared around 230,000,000 years ago, also benefitting from the high oxygen/low carbon dioxide atmosphere. By then, Pangaea had crept away from the south pole, and Earth's climate turned hot and dry with no glaciation anywhere. All the glaciers had melted completely away. So passed the Triassic, the Jurassic, and the Cretaceous Periods of the tropical Mesozoic Era, and on into the age of mammals.
Since carbon dioxide was at a minimum, it played no part in the global warming of the Mesozoic Era. If the greenhouse effect had any input into the steambath conditions of the time, water vapor is the only gas that could possibly have been the culprit.
The rock strata of Earth's surface do not form an airtight seal. Over the hundreds of millions of years since the end of the Karoo Ice Age, carbon dioxide has been seeping to the surface, slowly returning to its rightful place in the atmosphere. This adjustment in atmospheric gases probably was a prime factor in the downsizing of insects and amphibians in later ages, and hastened the end of the giant reptiles.
The burning of fossil fuel, too, releases long-pent-up carbon dioxide back -- to where? -- to its birthplace, to its source, to its origin: the atmosphere. There is nothing out of order, nothing inherently wrong with returning what was lost, with restoring a previous state to our ocean of air.
In fact, Earth's colonies of vegetation right now may very well be taking deep breaths of delight at the rich, life-giving replenishment of carbon dioxide.
Now, pray, do not leap about
in indignation and consternation.
THE ABOVE CONCLUSION IS NO EXCUSE
TO WANTONLY EXPLOIT EARTH'S RESOURCES!
upon clean energy sources
will provide mankind –
and myriad other inhabitants of planet Earth –
on the local, livable level.