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First land plants plunged Earth into ice age

Never underestimate moss. When the simple plants first arrived on land, almost half a billion years ago, they triggered both an ice age and a mass extinction of ocean life.

The first land plants appeared around 470 million years ago, during the Ordovician period, when life was diversifying rapidly. They were non-vascular plants, like mosses and liverworts, that didn't have deep roots.

About 35 million years later, ice sheets briefly covered much of the planet and a mass extinction ensuedMovie Camera. Carbon dioxide levels probably fell sharply just before the ice arrived – but nobody knew why.

Tim Lenton of the University of Exeter, UK, and colleagues think the mosses and liverworts are to blame.

Moss versus rock

It's not the first time that plants have been fingered as a cause of glaciation. Researchers already suspect that the rise of vascular plants in the Devonian period, some 100 million years later, triggered another ice age. The plants' roots extracted nutrients from bedrock, leaving behind vast quantities of chemically altered rock that could react with CO2 and so suck it out of the atmosphere.

Non-vascular plants like mosses don't have deep roots, so it was thought that they didn't behave in the same way. Lenton suspected they might have played a role nevertheless. To find out, he set up an experiment to see what damage a common moss (Physcomitrella patens) could inflict on granite. After 130 days, rocks with moss living on them had weathered significantly more than bare ones – and about as much as they would have if vascular plants were living on them. "The secret seems to be that the moss secrete a wide range of organic acids that can dissolve rock," Lenton says.

When Lenton added this effect of non-vascular plants to a climate model of the Ordovician, the CO2 dropped from about 22 times modern levels to just eight times modern levels. That was enough to trigger an ice age in the model of Ordovician Earth.

In his experiments, the non-vascular plants also released lots of phosphorus from rocks. Much of this would have wound up in the ocean, where we know it can trigger vast algal blooms. As other bugs feasted on the algae, they would have used up the oxygen in the water – suffocating oxygen-breathing animals and accounting for the mass extinction of marine life known to have occurred at the end of the Ordovician.

Although the first land plants were responsible for these mass deaths in their ocean-dwelling neighboursMovie Camera, Lenton says they themselves probably came out of the Ordovician ice age largely unscathed. That's because the ice was concentrated around the South Pole, while the plants lived in the tropics.

Life may also have caused an even harsher cold snap much earlier in Earth's history. The first complex animals appeared some time around 800 million years ago, and may have sucked so much CO2 from the atmosphere that the entire planet froze over in a "snowball Earth".

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1390

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Not So Powerful Co2

Thu Feb 02 02:19:05 GMT 2012 by Ian W

So eight times the current level of CO2 is not powerful enough to unleash runaway global warming - indeed it cannot even prevent an ice age. Whereas nowadays an eighth of that level is cause for exceptional concern?

This is not a logical argument - even though it is perfectly possible to write software to support that illogicality.

This comment breached our terms of use and has been removed.

Not So Powerful Co2

Thu Feb 02 19:12:50 GMT 2012 by TwoZeroOZ

Good question, unfortunately, your own use of logic is as bad as you're claiming others of.

The simple logic of "The past was likely very different from today, therefore you can't make present-day assumptions and apply them to the Ordovician period".

As it turns out, this logic actually holds true in reality: During the Ordovician period, co2 was indeed much higher, yet somehow resulted in an Ice Age - how can this be? Well, as most of us know, the energy from the sun was significantly less that many millions of years ago as compared to today. At the levels of sun output during the Ordovician age, glaciation would have occurred at anything less than a whopping 3000 ppm atmospheric co2.

Not So Powerful Co2

Sat Feb 04 19:21:57 GMT 2012 by David Ellard

I'm afraid the effect is too small. It's thought that the early sun was 30% fainter than now but that's at the beginning of Earth's existence. If the effect is linear with time (I haven't been able to check this) then the sun would have been only about 3-4% fainter than now in the Ordovician age.

Applying the Stefan-Boltzmann formula would give an absolute temperature 3.5/4 = 0.9% = about 2.5C cooler than now.

This is rather less than the temperature difference we are talking about in the case of the ice age(s) under discussion.

So the original point still stands.

Not So Powerful Co2

Fri Feb 03 13:58:02 GMT 2012 by MarkmBha

Well said!

I agree fully with your statement.

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Moss age <i>(Image: Philip Silverman/Rex Features)</i>

Moss age (Image: Philip Silverman/Rex Features)

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