What is an Ice Age? And how many Ice Ages have there been on Earth? I came across these questions, while writing a blog post about Sundaland. During the Ice Ages the sea level was much (~ 120 meter) lower than at present and the islands of the Malay archipelago were connected to Thailand, Cambodia and Vietnam. This landmass was called Sundaland. In my blog Sundaland (still under construction) I write in more detail about it.

If what follows is too detailed for you, just jump to the summary

During the 4.5 billion years that Earth existed, Its climate has fluctuated between Greenhouse and Icehouse. During a Greenhouse there were no glaciers, no icecaps (South Pole), no Ice sheets (Greenland) no permanent sea ice (North Pole). Earth was mostly a greenhouse , about 85 % of the time. Temperature was (a lot) higher than at present. Sea levels higher, sometimes 300-400 meter. Lots of carbon dioxide in the atmosphere. Tropical rain forests on the South Pole 😉

But there also have been Icehouses, often called Ice Ages, where glaciers, icecaps and ice sheets were permanently present. Scientists have identified 5 of them. Here they are marked on the 4.5 billion year timescale that Earth existed.

The Hadean, Archean and Proterozoic are the first three “aeons” of Earth’s geological history. We are now living in the last aeon , the Phanerozoic, which started ~ 542 million years ago, the name is too long to be named in the image. Before I “zoom in” on this last aeon, a few comments, related to blogs I wrote in the past.

• Only a few million years after the formation of Earth, in the Hadean, our Moon was “born” as a result of a collision of Earth with another planet. Where does the Moon come from? .
• The oldest fossils date back to the Archaean, 3.4 billion years ago, when Earth was still young. Therefore many think that life comes easily and must be ubiquitous in the universe. I am sceptical, see my blog The Drake Equation.
• It took a long time before those simple cells evolved and developed a nucleus that contained the DNA, about 2,2 billion year ago in the Proterozoic. And it took even longer for multicellular organisms to develop, about 600 million year ago, at the end of the Proterozoic. See my blog The Tree of Life.

The Proterozoic had two Ice Ages. The Huronian actually consists of several separate Ice Ages and lasted about 300 million years The Cryogenian also has two separate Ice Ages, together lasting about 85 million years. The Cryogenian was severe, there may have been periods that Earth was completely covered with ice, a so-called Snowball Earth.

.Keep in mind that Earth looked very different in those days because of plate tectonics and continental drift. See my blog The Paleomap Project. Here is one of Scotese’s maps: Earth during the Cryogenian Ice Age.

Now let’s zoom in on the Phanerozoic, from 542 million year ago until present. The ‘official’ start of this aeon is 538.8 million year ago. This beginning was chosen because around that time a sudden , explosive diversication of life forms started, the Cambrian explosion. Multicellular organisms evolved into a multitude of species. Here is an artist impression.

The Phanerozoic had three major Ice Ages, the Andean-Saharan, 440 million years ago, lasting 40 million years, the Karoo , 300 million years ago, lasting 100 million years and the current one, the Late Cenozoic Ice Age , which started 34 million years ago and is still ongoing.

Here is a (complicated) graph of the global temperature during the Phanerozoic. Complicated because the timeline has been split in five parts, zooming in.

The first part (in red) covers from 542 Million years (Ma) until 66 Ma. The temperature data are less reliable, but you can see the Andean-Saharan (440), the Karoo (300) and an unnamed one (~180). Next comes the green part, timeline enlarged about 10 times, from 66 Ma until ~ 5 Ma. Around 66 million years ago a huge asteroid collided with Earth, causing the extinction of the dinosaurs and giving mammals the opportunity to develop. Earth was a hothouse then with a maximum temperature around 55 million years ago (PETM). After that maximum, global temperature started to decrease. Around 34 million years ago, Antarctica got an icecap and Earth became officially an Icehouse, the Antarctic Glaciation . The next zoom in (again x10, in black) shows how the cooling of Earth continued. Around circa 2.58 million years ago the Pleistocene started, ice “everywhere”, also in the Arctic region.

The next part, in blue, shows in more detail the last one million years of Earth. It’s clearly an Icehouse but there are periods which are colder (glacials) alternating with warmer ones (interglacials). If you would be able to watch Earth during this one million years, you would see the icecaps and ice sheets advancing during glacials and retreating during interglacials. The last of these “warmer” interglacials was the Eemian (130-115 thousand year ago. Followed by the last glacial (26-20 thousand years ago). It is this last glacial that is often, called the Ice Age.

At present Earth is in an interglacial, as can be seen in the last part of the graph (also in blue). This interglacial started around 12 thousand years ago and is predicted to continue for many thousands of years. These predictions are based on the theory of Milankovitch cycles, a bit to complicated to explain here. It may last 50 thousand years, or even longer because of human interference (climate change!). After the interglacial, a new glacial will start, because Earth is still an Ice House.

Summary:

• Earth is an Ice House already for 34 million years. An Ice House (also known as Ice Age) consists of many glacial (colder) and interglacial (warmer) periods, each lasting thousands of years.
• The last glacial period occurred 26-20 thousand years ago and is often called the ICE AGE.
• At the moment Earth is in an interglacial and that will last for many thousands of years, possibly even longer because of human interference.

Of course there is a lot of information available these days about future climate developments. Not always reliable !

Here is a very dishonest one, A New ICE AGE Is Coming: Prepare To Freeze By 2050! A lot of factually correct information, leading to a ridiculous ending. Click on the link to watch the video, AYOR!. His advice: Pump more greenhouse gases into the atmosphere to avoid the impending next Ice Age (~ 2050!).

Here is a much more interesting one, New Evidence We Are Entering An Ice Age Termination Event Glacials have been ended numerous times by termination events. We are in an interglacial now, but it looks different this time. Could this be the start of a Hot House. We just don’t know.