Weeks 6 and 7 – reflections

My blog writing has gone amiss thanks to a nasty infection, but as viewers of the weekly feedback videos will note I have emerged from my bed. In fact I am just on my way back from a meeting with the UK Department of Energy and Climate Change (DECC) which was very well aligned with our MOOC content this week. The meeting was all about what we would need to do to stay within 1.5C global warming – the target that is aspired to in the Paris agreement made late last year. The short answer is a lot, and quickly. To have a chance of limiting warming to 1.5C above preindustrial levels would require a heroic global effort starting now, maxing out all of our mitigation (greenhouse gas emissions reduction) options, and then doing a huge amount of deliberate carbon dioxide removal from the atmosphere. In other words it needs all the elements we have been discussing this week – including generating renewable energy and reducing energy demand in the built environment – plus a hefty dose of carbon dioxide removal ‘geoengineering’. (Either that or ‘sunlight reflection’ methods rear their head.) Interestingly DECC want to know how doing all this would impact different countries around the world, as well as what climate impacts it would save in those countries. I think those of you taking the MOOC from around the world can help answer that…

Professor Tim

Week 5 reflections

I’ve been really enjoying the interactions around this week’s course material, but I’m also aware that many of you taking our MOOC are beginning to get thoroughly depressed by what you are learning about climate change. I know the feeling, and I wanted to say that it isn’t hopeless – there are lots of things we can all do to make a difference – and we are stronger doing things together to start addressing such a seemingly huge problem. Hopefully the course is the start of that – a community of interested learners have come together from all over the world to wrestle with what could be the defining issue of our time. We also need are own local communities. For example, where I live and work in Exeter I enjoy being parts of arts project ‘Kaleider’ (kaleider.com) which is supporting all kinds of creative responses to climate change, like the project ‘Ancient Sunlight’ (http://kaleider.com/projects/ancient-sunlight/). As we progress into the last couple of weeks of the course we will focus more on empowering practical action. But I have to admit that if I designed the course again I would devote more time to the solutions side of climate change. We know so much about what can and needs to be done.

Professor Tim

Week 4 questions answered

This week we have been learning about climate models and their projections, and also considering proposals to geoengineer the climate. Here are answers to a few of the questions that have come up repeatedly:

Why do models diverge from natural factors after 1970, when emissions have been going on before that?

Prior to this the sulphate aerosol produced from burning ‘dirty’ coal and other fossil fuels acted to cool the climate by roughly the same amount as the CO2 emitted from burning the same fossil fuels. However, sulphur dioxide emissions and the resulting sulphate (after oxidation in the atmosphere) caused acid rain (as well as choking smogs) and in the 1970s it became clear that acid rain was seriously damaging forests (as well as buildings), especially in Europe. This led to stronger legislation to ‘clean up’ fossil fuel burning with e.g. sulphur dioxide scrubbers fitted to major power stations. This reduced the cooling effect but left the CO2 warming effect. Some fossil fuel burning e.g. ship fuel was exempt, and hence ships have continued to produce cooling ‘ship tracks’ of aerosols and clouds that counteract the CO2 they emit. This ‘ship track’ effect is very similar to some geoengineering schemes for deliberate sunlight reflection.

Many of your questions have focused on the other type of geoengineering – carbon dioxide removal, and in particular the chemistry of ‘artificial trees’ that absorb atmospheric carbon dioxide using sodium carbonate, e.g.: How does this work, chemically, and do we have enough sodium carbonate to compensate for the fact that it’ll saturate quickly to make this a viable option?

The key point here is that the ‘sorbent’ for CO2 – here sodium carbonate – is regenerated when the CO2 is desorbed from it (and then transferred to some form of storage). This step costs energy (as does pressurising and pumping into storage the resulting CO2) and that is key to setting the cost of this CDR option. There are other passive sorbents that different groups are working on – but still there must be some energy investment in concentrating CO2 from its very dilute form in the atmosphere to concentrated form – because this amounts to working against the second law of thermodynamics. In one scheme that energy comes from the latent heat of water but that in turn makes the CO2 capture step very demanding of water.

What is the potential that stored carbon, from carbon capture schemes, could release? What impact would this have on the climate?

If liquid CO2 is stored underground at appropriate pressure and temperature then it can be very stable there. If carbon is stored as charcoal in soils (‘biochar’) then this can break down slowly. We certainly wouldn’t want to put CO2 somewhere that it could escape easily or abruptly – not only would this have the potential to asphyxiate animals locally (as happens in natural CO2 seeps) – but it would obviously contribute to climate change.

Professor Tim

Week 3 questions answered

I am writing my blog this week from the American Association for the Advancement of Science meeting in Washington DC, where I’ve just been talking about the implications of climate tipping points for policy and societies. It was a packed session on tipping points in ‘social-ecological systems’ and there were many audience questions about climate change. So, what better time to answer some of your burning questions that we didn’t get to in this week’s feedback video…

Is it misleading to start recent temperature change graphs from 1850 as this is when the Little Ice Age ended? Surely we’d expect a temperature rise then?

I don’t think it’s misleading. The observational temperature record starts when there is sufficient coverage of thermometer measurements to reasonably accurately reconstruct northern hemispheric temperature. It happens that this was roughly half way through the nineteenth century, around the end of the Little Ice Age. But this is a coincidence not a deliberate effort to show a temperature rise. It’s also important to note that the Little ‘Ice Age’ was a regional phenomenon – there was a marked cooling in Western Europe and the North Atlantic region – but other parts of the planet didn’t cool. The warming observed now is global and most of it has occurred since about 1950 – what goes on between 1850 and 1950 does not make a big difference to the figures.

Why are temperature anomalies recorded relative to the 1961-1990 average?

It was not until this 30 year interval that we could make a really precise estimate of global temperature – thanks to an expansion of surface measuring stations. The temperature in earlier intervals is less precisely known because there were fewer weather stations.

If we have more CO2 in the atmosphere, doesn’t this mean more photosynthesis (as CO2 is a limiting factor) and a negative feedback begin?

Yes it does, and this response – called the ‘CO2 fertilisation effect’ – is a key reason why there is a land carbon sink – i.e. a net land uptake of CO2 of around 2.5 billion tonnes of carbon per year. This is indeed a negative feedback that is slowing the rate of rise of CO2 concentration in the atmosphere, and the corresponding global warming.

Is there much hope after the COP21 event that countries such as the US and China will begin reducing their emissions?

In their ‘INDCs’ (intended nationally determined contributions) China do not commit to reducing their emissions for some time – instead they commit to lowering the amount of carbon they emit per unit of GDP (but growth of GDP is expected to outweigh this for some time). The US have voluntarily commit to reducing their greenhouse gas emissions by 26-28% from 2005 levels by 2025. US emissions have already been declining slightly, but the decline will have to accelerate considerably to meet this target. The best hope globally for me is if solar energy technologies and other sustainable energy technology continue to drop in price. Then economics will compel a switch away from fossil fuel burning. But there is so much relatively cheap coal left in the ground that what nations really need to wrestle with is an agreement to leave most of it underground (or we need a global price on carbon high enough that it compels us to capture and store carbon dioxide from all power stations where fossil fuels are burned).

Professor Tim

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


Week 1 questions answered

Thanks to everyone for your comments and responses to the week 1 course content. We hope you like the feedback video. Here are some answers to questions that have come up and we didn’t address there…

What is the role of cloud cover – does it provide a cooling effect?

Yes, especially low cloud cover. Clouds reflect sunlight back to space, preventing it reaching the ground where it would have a chance to be absorbed and to heat the Earth. Clouds make the biggest contribution to determining the Earth’s albedo (reflectivity) as seen from space, because at any one time they typically cover about half of the Earth’s surface and they are very white (reflective).

At the same time clouds have a warming ‘blanket effect’ because they trap heat radiation coming up from the Earth and send some of it back down again. This should be familiar if you think about the difference in temperature between a clear night (cold) and a cloudy night (warmer).

So, which effect wins out? Well, it depends on the altitude of the clouds. Low level clouds have a net cooling effect on the Earth, whereas high-altitude clouds have a net warming effect. Globally, the cooling effect wins.

Interestingly the reflectivity of clouds depends on whether the cloud water is arranged in lots of small droplets (highly reflective) or relatively few large droplets (less reflective). Amazingly this property of clouds is partly controlled by biology – tiny algae in the ocean make a gas called dimethyl sulphide which oxidises in the atmosphere to form tiny particles on which cloud droplets form.

Can you give an example of where we can see the Planck feedback in action?

We can see it in action all the time. If I take a hot coal out of the fire it cools down because it radiates away heat – until it reaches the temperature of its surroundings. As day turns to night the ground is no longer heated by the Sun and it starts to cool down by radiating away heat. If I put some food in the fridge (a cold place) it cools down by radiating away heat. Equally if I bring a cold object out of the fridge it starts to warm up because it is giving off less heat than it is gaining from its surroundings.

At the surface of the Earth another really important feedback is the water vapour feedback we met in the video – the loss of ‘latent’ heat caused by evaporating water. There is also the loss of ‘sensible’ heat – that is where heat is transferred to the atmosphere (assuming it is cooler than the surface).

What role is man playing in these naturally balanced feedback cycles?

Human activities are both affecting natural feedback cycles in the climate system and creating some new effects. For example, some of our atmospheric pollution is cooling things down and acting to partially counteract the blanket effect of increasing ‘greehouse’ gases. In particular, burning coal and other fossil fuels produces sulphur dioxide, a gas that oxidises to sulphate which forms tiny particles called ‘aerosol’ in the atmosphere. This aerosol scatters sunlight acting to cool the Earth, and it also forms the nuclei on which cloud droplets can condense – and those cloud droplets also overall act to cool the Earth.

Other human activities are adding to the overall warming. For example, by deforesting the tropics humans are tending to dry out the land surface and reduce cloud cover, and this makes the deforested land heat up faster.

Hopefully this gives just some flavour of the wonderful complexity of the climate system.

Professor Tim


Welcome to all the new learners joining our MOOC on Climate Change: Challenges and Solutions. This is the place to come for my weekly efforts to answer your questions – especially the ones we don’t get to in the weekly feedback videos. If you scroll down you will see my weekly blogs from the last two years, when we ran the MOOC and the content was very similar – so there could be some helpful material down there…

Professor Tim

2015 Week 5 questions answered

Week 5 has thrown up a bumper crop of questions so here goes with some answers:

1) If the Himalayan glaciers melt, what effect will that have on SE Asian rivers and reservoirs? Are there any mitigation measures?

As the glaciers are melting it will increase river flow, especially during the summer, once they have melted it will decrease river flow, and that will be particularly obvious in the dry (no monsoon) season, affecting e.g. irrigated agriculture. Aside from mitigating overall climate change, another option is to reduce soot emissions (mostly from biomass burning in wood stoves in India) as soot deposition on the glaciers is darkening them and contributing to the melt.

2) What caused the above normal melting of Greenland in 2012?

There was an unusual ‘blocking’ high pressure weather system that stayed in position on Greenland, with blue skies allowing temperature to build up and melt right across the ice sheet surface – check out the NSIDC report.

3) Will marine organisms be able to adapt in time given the rapid acidification of the oceans?

It probably depends on the organism – microbes can adapt fast, but e.g. long-lived animals with shells adapt much slower. If corals are suddenly placed in a high-CO2 experimental environment they can really struggle, but given decades to adapt they may cope better – we’ll have to wait and see to find out.

4) What are the effects of acidification on marine plants and fish?

Again it depends on the organism. Marine ‘plants’ means cyanobacteria and algae including macro-algae (seaweeds, kelp…). Some algae make their shells out of calcium carbonate, so they are potentially in trouble. Many others have different types of shell (or none at all), so they may get a relative advantage. For fish, the effects of elevated CO2 could include making it slightly harder for them to precipitate carbonates, which is something they do in their guts to regulate their salt balance.

5) Are there any ‘geoengineering’ methods that could be used to reduce the acidity of the oceans?

Yes, potentially – it has been suggested that lime or calcium carbonate could be deliberately added to the ocean (in powdered form) in order to reduce surface water acidification. The problem is you have to calculate how much CO2 is emitted in mining the lime/carbonates, crushing it, and transporting it out into the ocean on boats. L. D. Danny Harvey has done some calculations on this which show that overall it could still be overall ‘carbon negative’ as well as tackling ocean acidification directly.

6) In the past, acidity has supposedly reached higher levels. What were these levels, when did it occur and what impact did it have on marine organisms?

Care is required in thinking about this. The ocean is extremely well buffered, so the only way to lower pH dramatically is to add CO2 very rapidly. There are a few candidate events in Earth history where a natural release of CO2 has acidified the ocean – for example when a magma intrusion torched a massive volume of fossil fuels – but these events are few and far between. A well characterised one is 55 million years ago at the Paleocene-Eocene Thermal Maximum. It caused an extinction event, but not nearly as severe as some earlier extinction events where ocean de-oxygenation appears to have been a more potent killer.

7) Will scientists be able to maintain ‘seed bank’ equivalents of threatened marine organisms?

This is a good idea. We already do this to some extent in aquaria around the world – but generally for the ‘charismatic mega fauna’ (i.e. animals).

8) Would increasing acidity lead to greater dissolving of carbonate rocks? Thus reducing the acidification effect?

Yes, well done to whoever asked this question – dissolving carbonate rocks adds alkalinity to the ocean (2 moles of alkalinity for each mole of carbon added). This is exactly what happened at the aforementioned Paleocene-Eocene Thermal Maximum, and it is what is starting to happen under human-induced ocean acidification. The carbonate ‘rocks’ include ocean sediments themselves as well as harder rocks exposed on land – the chemistry is the same – its just the pH is already much lower in rainwater.

Hope this helps!

Professor Tim

2015 Week 4 – your questions answered

Here are some answers to some of the most popular questions in week 4:

1) What is required in Paris this year to improve the effectiveness of mitigation  measures?

There needs to a (long-overdue) global deal to reduce CO2 emissions. The countries that have historically emitted more should commit to reduce their emissions more, but no nation can be exempt from some action.

2) Clarification of the ‘global warming hiatus’ from 2000?

There has been a slow-down in the rate of warming suggesting that the ocean has been taking up heat faster than previously. Most of the excess energy coming into the planet goes into the ocean anyway, and there is natural variability in its heat storage. The extent of the hiatus varies somewhat between temperature reconstructions, because of a gap of weather stations e.g. in the Arctic which has been warming very fast. The slow-down may now be ending with 2014 being the warmest year on record in several reconstructions.

3) Would money be better spent focussing on developing renewable energy sources?

Money is being spent on encouraging the uptake of renewable energy technologies – for example, in the UK we have the feed-in tariff, which rewards people for installing solar photovoltaic cells on their houses. The signs are encouraging that global renewable energy capacity is starting to take off. I am looking forward to the ‘solar revolution’ – recent projections suggest that solar power (complemented by other renewable sources) will replace fossil fuels as the main global source of electricity during this century. With the right policy regime it will happen sooner rather than later.

4) Does solar radiation management interfere with the Earth’s magnetic field?

No, the Earth’s magnetic field is created by the flow of liquid iron in the Earth’s inner core. That’s one place that even the most ambitious would-be geoengineers can’t reach.

5) Why has the incorporation of Geo-engineering within IPCC scenarios not been more widely publicised?

This refers to the widespread use of biomass energy with carbon capture and storage (‘BECCS’) in the IPCC’s scenario to stay within 2C of global warming (called ‘RCP2.6’). It deserves to be widely known about, because currently governments are doing very little to invest in research and development for carbon dioxide removal technologies, whilst at the same time they are nominally signed up to the ‘2C target’. This is simply inconsistent.

6) Could a geoengineering ’solution’ help one country and harm another? 

Yes. If there is unilateral deployment of sunlight reflection methods, for example the deliberate injection of aerosols into the stratosphere of one hemisphere and not the other this will have widespread repurcussions on other nations. See the discussion in this week’s feedback video for more on this.

More next week!

Professor Tim

2015 Week 3 – questions answered

Week 3 of our MOOC has thrown up lots of questions about observed changes in the climate and the carbon cycle. Here are some answers…

1) Could increased urbanization have an effect on global temperatures given the rate of increase and spread of this phenomenon?

It can have a very small effect – because the total urban area is a tiny fraction of the Earth’s surface. The more important issue is whether ‘urban heat island’ effects could bias the observational record of temperature change – by warming up weather stations that were once out in the countryside. The key point is this biasing effect was recognised by scientists and has been taken out of the compilations of the global temperature record.

2) Is thermal expansion of the oceans a long-term effect?

Yes, very long-term. The uptake of heat by the ocean will take thousands of years to complete, linked to the long overturning time of the deep ocean and the slow rate of diffusion of heat into the interior of the ocean. The thermal expansion is a product of that slow heating up. If it turns out that the Earth has a high ‘climate sensitivity’ this implies a very long timescale for heat uptake by the ocean to be complete (and ultimately a large temperature and sea level rise).

3) Can afforestation make a significant impact in reducing the impacts of climate change?

Afforestation has two key effects. It can store carbon in trees (and soils, depending on where the trees are planted), and it can alter the albedo (reflectivity) and other physical properties of the land surface. Taking up CO2 clearly reduces the impact of climate change, and I have estimated that globally afforestation may already be removing around 0.25 GtC/yr from the atmosphere. But this is small compared to 10 GtC/yr of emissions. And there is a problem – afforestation in the higher latitudes darkens the land surface (especially in winter, when the trees shade the snow) and this can cause them to warm the climate (more than they cool it by taking out CO2). In the future, the most optimistic scenario would be for afforestation to reverse the effects of historical deforestation, which has released about 150 GtC in total. But the regions where this was done would have to be chosen carefully to avoid the counterbalancing climate warming effects.

4) Will increasing Antarctic ice balance out Albedo lost from Arctic?

No. Although there has been some increase in Antarctic sea-ice linked to changing wind patterns in the Southern Ocean (in turn linked to the ozone hole and its interaction with the climate system), this is not set to continue in the longer-term. Meanwhile the Arctic sea-ice loss has accelerated and we are expecting largely ice-free summers in the Arctic ocean by roughly the 2030s.

5) What exactly is going on in the Antarctic? Some reports claim growth, others claim high rates of collapse + calving.

I am assuming this question is about the Antarctic ice sheet, not the sea-ice. Considering Antarctica as a whole, satellites that measure the effect of the ice sheet on the Earth’s gravity field find that it is shrinking overall. However, there are really two ice sheets – the East Antarctic ice sheet (larger) and the West Antarctic ice sheet (smaller) – which behave somewhat differently. Measurements of surface elevation (from e.g. high-precision laser altimeters) indicate that key parts of the West Antarctic ice sheet and parts of the edge of the East Antarctic ice sheet are losing altitude, i.e. losing ice into the ocean. This could already be irreversible for parts of the West Antarctic ice sheet which are grounded below sea level. However, the great bulk of the East Antarctic ice sheet is sat on a continent, and parts of its interior are gaining some mass due to extra snowfall. Still, we can be fairly sure from the gravity measurements that the overall balance is a loss of ice.

Keep up the good work!

Professor Tim