How Earth got its ice caps and helped life to prosper

A new 3D computer model of the planet through time has helped answer a contentious question: How did the Earth’s ice caps form?

“The study looked at the Earth’s long-term stable climate,” explains the University of Adelaide’s Andrew Merdith, lead author on the new study in Science Advances.

“Over tens to hundreds of millions of years, Earth’s climate swaps between greenhouse climates and icehouse climates. The difference is that in an icehouse climate, you have a permanent ice cap over one of the poles.”

Icehouse periods are further divided into much smaller interglacial and glacial periods, which we commonly think of as ice ages.

But for the majority of the planet’s 4.6-billion-year history, its climate has been dominated by greenhouse periods. There are only five, significantly shorter icehouse periods known, including one that started 34 million years ago and continues today.

Now, this new study has found that these cool periods are rare events because they are the result of multiple complex global processes working together. In other words: it’s a lucky coincidence.

Cold quest

The main factor driving changes to the Earth’s climate is the concentration of greenhouse gases in the atmosphere, particularly the balance of carbon dioxide (CO₂). When the CO₂ concentration drops low enough, this triggers an icehouse period.

But it has long been unclear what causes these drops. Researchers have previously suggested many ideas, from lack of volcanic activity (which spews CO₂ into the atmosphere) to increased storage of CO₂ in rocks via silicate weathering, to the growth of forests sucking up CO₂.

But which one is the primary driver of icehouse climates?

To answer this, Merdith led a team from the University of Adelaide and the University of Leeds to test the combination of all these climate drivers in a new, long-term 3D model of the planet.

Most previous models attempted to answer this question in one of two ways. Firstly, from a palaeoclimate point of view, where researchers ran numerical simulations of the climate – though these only cover a few thousand years.

Secondly, by running carbon cycle models, which attempt to balance carbon sources and sinks, over longer time periods.

“What our model did, which was the new part, is that it tried to couple the two together,” Merdith says.

In addition, the other models don’t have spatial input, which means they can’t capture potential driving factors such as shifting geography. But the Earth’s continents move, slowly but significantly changing the distributions of rocks that contribute to the silicate weathering cycle and thus to the sequestration of CO₂.

Merdith and team captured the movement of continents over millions of years in their model by incorporating global paleoclimate data.

“We were able to test lots of these different hypotheses that had all been looked at independently, and we were able to put them all into the one model and then do a series of sensitivity analyses – like turning one mechanism off in the model at a time – to see under what circumstances we could produce an icehouse,” he explains.

The results? None of the climate drivers could have triggered an icehouse period on their own.

Instead, the cool conditions require a combination of multiple global processes occurring at once, explaining why they are so rare in comparison to greenhouse periods.

“We now know that the reason we live on an Earth with ice caps, rather than an ice-free planet, is due to a coincidental combination of very low rates of global volcanism, and highly dispersed continents with big mountains,” says Merdith.

This combination created conditions for a lot of global rainfall. This increased the rate of the global weathering cycle, in which carbon dioxide is removed from the atmosphere and becomes bound up in rocks.

“The important implication here is that the Earth’s natural climate regulation mechanism appears to favour a warm and high-CO₂ world with no ice caps, not the partially glaciated and low-CO₂ world we have today,” Merdith explains.

“We think this general tendency towards a warm climate has helped prevent devastating ‘snowball Earth’ global glaciations, which have only occurred very rarely and have therefore helped life to continue to prosper.”

Ancient history, current implications

Benjamin Mills, co-author on the paper from the University of Leeds, says their results provide important context for the climate crisis we are currently facing.

“Earth’s current ice-covered state is not typical for the planet’s history, but our current global society relies on it,” he warns.

“We should do everything we can to preserve it, and we should be careful with assumptions that cold climates will return if we drive excessive warming before stopping emissions.

“Over its long history, the Earth likes it hot, but our human society does not.”

Model matters

According to Merdith, confirming their findings will require looking into the tangible geological record and comparing evidence of glacial periods written in the rocks.

“Our model makes a prediction on when the cold times will be and where the cold places should be,” he explains. “And then we can compare the model output to what we see in the geological record to see if it’s done a good enough job or not.”

There are also several limitations to overcome in the next steps of research.

The main limitation is the coarse resolution of the spatial information. The model itself runs over 540 million years, but there is only ‘new’ information about the location of the continents every 30 or 40 million years.

Improving the resolution will require more computing power and more time to run the model to produce shorter gaps between information, for example with a frequency of 20 or 10 million years.

This model also didn’t pay enough attention to plants, Merditch says.

“Mostly what we tested was the inorganic carbon cycle, which is carbon and rocks interacting with one another. But there is another carbon sink, and that’s how plants and the terrestrial biota sink carbon.”

Adding a more complex and realistic model of these processes will help make their results more accurate.

Other ice caps