Hello MOOC-ers,

Here are some answers to some of the best questions you have been raising this week:

1) If the Greenland Ice Sheet collapsed, would the smaller pieces of ice breaking off from the main ice sheet not in fact increase albedo as there would be a greater surface area of sea ice? (and would this ice not persist for a significant period of time due to the high energy required to melt ice in water?)

The icebergs breaking off the Greenland ice sheet are a lot more reflective than the water they float in, but they don’t cover a huge area. In contrast, the sea-ice is much thinner than a typical iceberg, so the same volume of (sea) ice can cover a much larger area of dark ocean surface. Even the most catastrophic ice sheet collapse scenario would take centuries, so cannot add that much area of ice cover to the ocean, especially when compared to the massive areas of sea-ice that freeze and melt on a seasonal basis. Still, the sheer volume of frozen ice coming off Greenland could be important for climate in a different way: The icebergs coming off Greenland do melt, especially if they get carried southward on ocean currents, and this adds freshwater to the ocean. That in turn makes the surface waters of the North Atlantic less dense, making it harder for them to sink. That process of sinking drives what is called the ‘thermohaline’ or overturning circulation of the ocean – so Greenland melting could slow that down.

2) In one video you mention the Earth’s atmosphere filling with water vapour and essentially forming a ‘pressure cooker’ sort of situation. In relation this, is the atmosphere constrained in some way to fill a fixed volume (i.e. what prevents the atmosphere expanding outwards to reduce pressure of water vapour as the habitable zone moves further from the Sun)?

This is a great question. The atmosphere can indeed expand if it gets more mass due to the addition of water vapour. Most of the mass of the Earth’s atmosphere is near the surface, thanks to gravity holding it there – and the density of the air drops off with altitude in a characteristic way. When adding mass to the atmosphere most of it will be added near the surface, and the more massive and hotter atmosphere will have a thicker ‘troposphere’ (that is the bottom layer of the atmosphere). The crucial thing for habitability is whether water is lost, and the problem with creating a vapour-rich atmosphere is it tends to lose hydrogen atoms to space much more rapidly, thus drying the planet out.

3) Snowball Earth Hypothesis: Why is it not a particularly well-known theory and how widely is it accepted with many contrasting theories?

The snowball Earth hypothesis has only started to gain scientific acceptance in the last 10 years or so, and there are still many scientists who disagree with it. This is one big reason why it isn’t more widely known: scientists are still debating it. Most scientists agree with the evidence that there have been widespread glaciers near the equator in a couple of key intervals of Earth history. But they debate whether the ocean was completely frozen over with sea-ice. One alternative model is the ‘slushball Earth’ where there remains a band of open water around the equator. Some people like this model because it is easier to see how life survived in a slushball Earth. However, it is harder to explain some of the other evidence with this model, such as the extraordinarily long duration of glaciations (we now think 50 million years in one case!) and the appearance of massive ‘cap carbonate’ deposits afterwards (which in the ‘snowball’ model are deposited when all the CO2 that has built up in the atmosphere reacts with calcium and magnesium ions and gets deposited in the ocean).

4) What sort of time scale will it be before Mars becomes part of the habitable zone and will it be a slight possibility of colonisation on a new planet if technology allows?

Another great question. In fact some of the latest estimates from Jim Kasting’s research group put present-day Mars within the habitable zone. They may even put early Mars in the habitable zone. But Mars has another problem – it is tiny compared to the Earth, with correspondingly less gravity and a weak magnetic field that can’t protect it from the solar wind. This means Mars has lost almost all of its atmosphere and most of its water to space. In order for Mars to be habitable now (or in the past) it would need a thick atmosphere dominated by carbon dioxide – but it can’t keep hold of its atmosphere. The idea of making Mars habitable has been widely discussed in a field called ‘Terraforming’. James Lovelock and Michael Allaby wrote a book on ‘The Greening of Mars’ back in the 1970s, and Martyn Fogg has written extensively on it since. They discuss ideas like using a super-powerful greenhouse gas (e.g. CFCs) to warm things back up again. Apparently many people have signed up for one-way tickets to Mars on a prospective future mission. They are clearly more optimistic than I am about the prospects for making Mars habitable!

5) To what extent does the heat of the Earth’s core influence climate?

Remarkably little. When the Earth was very young and its surface was molten rock, then the climate was radically different – probably with a steam atmosphere. But since the surface has cooled down and solidified (in the first few tens of millions of years of the planet’s 4.5 billion year existence) remarkably little heat has been leaking out. Basically the flux of heat from the Earth’s interior to the surface is orders of magnitude smaller than the flux of heat from the Sun. The one time it could have been important is in helping stop the whole ocean from freezing in a snowball Earth. The key effects of the Earth’s internal heat are indirect ones. The inner core is liquid iron and its circulation is thought to create the magnetic field which protects us from the solar wind (etc). The mantle’s heat of course fuels volcanoes at the surface which can affect the climate (as we discuss in this week’s feedback video). The hot convection of the mantle is also linked to plate tectonics at the surface, which is essential for maintaining a habitable climate over geologic timescales – for example by recycling carbon deposited in ocean sediments back to the atmosphere.

Hope that gives some food for thought!

Professor Tim