2015 Week 2 – questions answered

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

2015 Week 1 questions answered

Week 1 of the course has already thrown up a variety of questions, so here goes with some answers…

1. What is the process of ice sheet sublimation? We’re all familiar with evaporation, where a liquid turns into a gas – e.g. liquid water turning into water vapour (a gas). Sublimation is when the molecules in a solid (in this case, ice) go straight into the gas phase (water vapour), without turning into liquid first.

2. Does increased snow/ice melt due to climate change and a warmer world lead to more precipitation as snow? Not really. More snow/ice melt produces more liquid water, much of which ends up in the ocean, from where it can be evaporated and then return to the land as precipitation. Overall we observe more evaporation and precipitation in a warming world (the hydrolgogical cycle ‘spins’ faster). But because it is getting warmer more of that precipitaiton tends to be rain rather than snow.

3. How did the process of advancing/retreating ice start? Sea ice (for example) tends to expand in winter during the polar night (darkness). In the spring and into summer as the days get longer, this tends to melt more ice. That in turn triggers the ‘ice-albedo positive feedback’, absorbing more sunlight and accelerating the ice melt. On a much longer timescale we can think about similar processes governing the creation and melt of giant ice sheets on land. More on all this next week.

4. How is atmospheric circulation driven by release of latent heat? Atmospheric circulation is driven, in general, by the different amounts of heating of different parts of the atmosphere. Most importantly, more sunlight is absorbed at the equator than the poles, and that drives an atmospheric circulation that tends to redistribute heat towards the poles. In addition, when water condenses in the atmosphere (such as when clouds form), this releases latent heat. That tends to cause the air to expand and rise upwards, starting the process of convection (a kind of atmospheric circulation). On small scales this is what creates the remarkable tower like structures of clouds. On larger scales the release of latent heat in monsoonal rains (e.g. in India) fuels an upward flow of the air over land, sucking in air from over the ocean, which returns at height in the atmosphere – this is the monsoon circulation.

5. Can we stabilise climate and emissions whilst population and consumption increases? Only if the consumption is no longer of fossil fuels, or meat eating, or any of the other processes that produce emissions of ‘greenhouse’ gases. More on this in the coming weeks.

6. Who is going to support the investments required globally to prevent the acceleration of climate change? This requires a change in our economic system so that there is a price on the pollutants that are accelerating climate change and a corresonding reward for technologies that can provide energy and other services without contributing to climate change. On a national scale, e.g. in Germany, there have been some success stories with governance arrangements that have incentivised the solar energy industry and are helping it gain market share over fossil fuel burning. Globally there are some signs that solar energy is taking off, but the overall picture is not good. More on this in the coming weeks.

Professor Tim

Further reading

For those of you wanting to keep learning about climate change and related topics, we have compiled a list of further reading – just hit the ‘Further reading’ link to the right and you’ll get a pdf with active weblinks in it. This includes texts recommended by the presenters of each section, and the pick of some the best articles that have been tweeted or shared on the Facebook page during the course. Enjoy!

Tom Powell, Steve Beckett, Chris Boulton and Professor Tim

2014 Week 1 – reflections on ozone

Practice what you preach, as they say. So I have decided to do some reflective learning and blog about my experience with our Climate Change MOOC. It’s been great this week, seeing so much engagement with the course material on the discussion threads. But it left me looking for one place where I could respond to some of the issues that were catching fire, without having to repeat myself. Hopefully this will be it. So here goes with this week’s favourite topic – ozone…

Yes, we threw a curveball in the very first question on the course. A lot of people were surprised to hear that ozone (chemical formula O3) is a greenhouse gas, especially knowing that we have been trying to protect the ozone layer for the last twenty years. So, let’s tackle that one head on.

The first point to grasp is that ozone is present in two layers of our atmosphere – the well-mixed bottom layer that we breathe, known as the ‘troposphere’ – and the next layer up, known as the ‘stratosphere’ (because it is vertically stratified, i.e. layered).

It is in the stratosphere that the ozone layer forms, and it is the absorption of high energy (UV) sunlight by the ozone layer that heats up the stratosphere and gives it its stratification (with temperature increasing as a function of height).

Down in the troposphere ozone is a short-lived gas, concentrated near the surface, and produced as a by-product of chemical reactions acting on a range of mostly human pollutant gases, including oxides of nitrogen, carbon monoxide, methane, and other ‘volatile organic carbon’ species.

In both atmospheric layers, ozone functions as a ‘blanket gas’ absorbing heat radiation coming off the Earth and thus helping warm the surface. However, the warming associated with the stratospheric ozone layer is natural, and the ozone layer is doing a wonderful service shielding us from ultraviolet radiation, which we couldn’t live without. The ozone in the troposphere on the other hand has been increased in concentration by human activities, thus contributing to climate change – and it has some other nasty effects, like inhibiting plant productivity.

The depletion of the stratospheric ozone layer that was caused by human-produced chlorofluorocarbons (CFCs) did, as would be expected, tend to cool the planet, but only by a small amount when globally averaged. That cooling was more than outweighed by warming due to the CFCs themselves, which are potent ‘blanket gases’. And both effects are small compared to the contribution of carbon dioxide (CO2) to recent warming.

Interestingly, the creation of the ozone hole, as well as letting more UV radiation down to the Earth’s surface, has affected the climate regionally in Antarctica and the Southern Ocean, tending to keep things cool there, and leading to a strengthening of the winds encircling the planet above the Southern Ocean. Those strengthening winds have in turn tended to blow more sea-ice away from the areas where it is made around Antarctica, causing the surprising increase in area of Antarctic sea-ice that is so beloved of climate sceptics.

Hopefully that gives some glimpse of the beautiful, interconnected complexity of the climate system. Happily the stratospheric ozone layer is on the mend, but unfortunately the compounds we replaced CFCs with (the HCFCs) are still potent ‘blanket gases’. One day we’ll learn…

Professor Tim