Week 2 questions answered

Thanks to everyone who watched and enjoyed our weekly feedback video. Here I’m going to attempt to answer some additional questions we didn’t get to there…

How does CO2 get taken up into the Earth’s crust?

The phrases `carbonic acid dissolves silicate rocks´ and `CO2 goes into sedimentary rock´ appear in the course. To say a bit more about this, it is a two step process. Firstly carbon dioxide (CO2) reacts with rainwater to form a weak carbonic acid solution, which in turn dissolves silicate rocks, liberating Ca and Mg ions into solution together with bicarbonate ions. Then once the Ca, Mg and bicarbonate ions have been washed to the ocean they can be combined to form calcium or magnesium carbonates – by organisms making their carbonate shells or coral reefs. Some of this carbonate gets buried and ultimately forms new sedimentary rocks in the Earth’s crust. So we have gone from a silicate rock that contained no carbon to a carbonate rock that does contain carbon and that carbon has been taken out of the atmosphere/ocean system. The overall equation is, e.g.: CaSiO3 + CO2 => CaCO3 + SiO2

What triggers natural planetary-wide shifts?

A variety of things have triggered e.g. mass extinctions in the past. Sometimes it involves an asteroid impact from outer space. Sometimes it involves periods of prolonged intense volcanic activity – but that probably has to interact with natural ‘feedback processes’ in the Earth system to become truly catastrophic. For example at the ‘great dying’ – the end Permian extinction 251 million years ago – a ‘mantle plume’ of magma pushed up under present day Siberia and caused massive lava flows several kilometres thick covering a huge area. The magma appears to have come up through sedimentary rocks that were rich in organic material and this got cooked, adding a mixture of gases to the atmosphere including CO2 and methane, warming the climate and acidifying the ocean. Still that doesn’t explain why the oceans were also catastrophically de-oxygenated – something which usually requires an input of nutrients. Scientists are also still puzzling over what could explain evidence that the ozone layer was depleted at the time.

If we rewind further to snowball Earth around 720 million years ago one theory has it that another large outpouring of lava (basalt) which weathers very quickly could have consumed so much CO2 on the timescale of millions of years that it tipped the Earth into a snowball. This would involve the silicate weathering mechanism described above, and I mention this longest-term effect of volcanic eruptions in the feedback video. If the idea is right it highlights that the ‘trigger’ of catastrophic climate change in this case was pulled rather slowly – it takes hundreds of thousands of years for a ‘large igneous province’ (basalt outpouring) to be created, and then it takes a few million years to shift the balance of the carbon cycle to lower CO2. However, once the ‘ice-albedo’ feedback goes into runaway it only takes a year or less to cover the ocean in sea-ice.

How do we know about Milankovitch Cycles and where are we on the current cycles?

Changes in the Earth’s orbit and the tilt of its axis are periodic and are governed by ‘classical mechanics’ – basically the gravitational interactions between the different planets in the solar system (and the Sun). Although it is not possible to analytically solve the ‘many body problem’ of multiple gravitational bodies interacting, it is possible to make very accurate numerical predictions of the effects of different planets on one another’s orbits. Hence we can predict and hindcast changes in the Earth’s orbit for many millions of years in either direction. What we learn is that the Earth is currently in an unusually circular (less elliptical) orbit and therefore the precession of the equinoxes around the elliptical orbit has less effect on the climate than it usually does. For this reason the present inter-glacial period is predicted to be an unusually long one (even in the absence of human activities).

I hope that gives some feel for the awesome complexity of the Earth system and its interactions with its planetary neighbours!

Professor Tim


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