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That rapid change in the conversation will soon lead to questions about geoengineering, says the assistant professor at Cornell University in New York state. A specialist in stratospheric aerosols, Visioni brings together engineering and atmospheric chemistry expertise to answer some of those questions.
We spoke to him about the challenges of stratospheric aerosol injection (SAI), the significant role engineers have to play, and his predictions for the future.
Stratospheric aerosol injection (SAI) is the technique that gets the most attention. What are the opportunities and challenges involved?
There's always a big initial challenge when thinking about the actual technical aspects of a deployment of any geoengineering technique, because there’s a delicate in-between investigating something from a theoretical point of view and the actual details of a deployment, so a lot of people feel quite uncomfortable thinking about it. But nonetheless, it comes to a point where, for any of these techniques, one needs to start asking questions about how they would actually work.
One main issue is we currently do not have planes that can carry the load that would be necessary all the way to the stratosphere. The main thing with SAI is that you need to be above the tropopause, you need to be in the stratosphere, which is where the aerosols have the longest lifetime.
However, the problem is you need to get it to at least 20km at low and mid latitudes, and that's currently not feasible with any planes that we know of. The main reason why that is the case is because simply there's never been any need to bring such huge loads all the way to the stratosphere.
There is an interesting discussion to be had about ‘Well, stratospheric aerosol injection is something you would have to do globally.’ However, you can think about starting at higher latitudes, so closer to the Arctic, where the tropopause is lower, and therefore, one can start thinking about ‘What would it take to repurpose current planes, or do we have the technical capacities to start an SAI deployment closer to the Arctic?’
At that point, if you start thinking about the technicalities of it, you start thinking ‘Let's say that Country X has space at 60º North to start building an airbase, to start a deployment. How would you bring the sulphate there? How many planes would we need? How would you monitor meteorological conditions close to their location? How do the droplets change day-by-day? What does it mean to have a ceiling for a specific plane when you're talking about normal aviation, versus something like this?’ There are a lot of interesting technical issues there.
What kind of planes would be required, and how much would they carry?
The idea would be something like a [Boeing] 777 in general. Let's run some numbers – normally, every year, human activities throw into the air 100 million tonnes of sulphate, from many, many different kinds of activities, especially those that have to do with fuels. These sulphates produce aerosols, but these aerosols are very close to the surface so they immediately get rained out.
The idea would be that with less than 1% of that, so let's say a million tonnes brought all the way to the stratosphere. Their lifetime would be much longer, in the order of a year or so. Releasing at those altitudes would result in an actual perceivable cooling.
Now a million tonnes is a big number – but in isolation, it’s a small number compared to all of human activities. But the important thing is that if you can imagine a plane like a 777 capable of carrying a load of around 100 tonnes, it ends up being 10,000 flights that would be necessary. So if you think that there are, let's say, 300 available days in the year, that ends up to around 30 to 40 flights per day that would be necessary – if you can imagine a plane that has that kind of load.
What about the technical challenges of deploying the particles?
The idea behind SAI comes from the observation that volcanic eruptions release SO2 in the form of a gas in the stratosphere. If you were to release SO2, that would have some very positive aspects. First of all you can compress the gas a lot. And second of all, once the gas decompresses, once you opened it out, it will diffuse. So you wouldn't need to worry about loitering. It's going to take a few days before it oxidises, so the processes that lead to the formation of these aerosols would take some more time.
The question then is ‘How safe is it to have very high highly compressed gas? How does it change when you consider temperature and pressure conditions in the stratosphere? Would it make sense to maybe have elemental sulphur and burn it to produce SO2 on the plane, what would you need?’
If you were releasing the aerosols as aerosols themselves, you need to think about spreading them, because otherwise they would just clump together. Then you want them to be the right size, you don't want them to be too big. At that point, the technological challenges become higher, because you need a nozzle capable of spraying aerosols of that exact size. How do you do that, how would that work in the stratosphere, and so on and so forth.
There are lots of engineering challenges involved – will engineers need to discuss these topics more widely in the years to come?
Absolutely. We’re in this kind of weird space where everybody wants to say, for obvious reasons, ‘We're not actually claiming we should do this. We're only investigating these things at a high level to figure out whether it would be feasible.’ So there's a challenge around talking too much about that – when we talk about things in such an idealised way, it's also hard to argue for more involvement from engineers.
But this is clearly changing. When I talk to people, a lot of them recognise that climate change risks are a huge problem that is not something we're going to easily manage, especially in the next 50 years. A complete reduction of emissions is, long term, the only way we can reduce those risks for good. Perhaps carbon dioxide removal will eventually also help. But for now, as emissions are still large, and carbon dioxide removal looks hard to scale, how do we manage those risks? In this sense, I think that we need to really look into things like geoengineering, which might help bridge this gap.
If there's one thing that engineers have taught me, it's not just about developing the technical capacity. If you are thinking about something bigger, you also need to think about what observational systems do you need, how would your plans change depending on what you learn? The mission-driven research of engineers is much more suited for that than climate scientists.
How do you feel about the future, and whether geoengineering will be required?
So the thing that I feel, like many others really, is that there's an incredible change in how people are perceiving climate change. In a few years, it's moved from something that will happen to somebody else in the future, to something that is happening right now. I'm thinking of heatwaves, flooding, extreme weather events. There's a lot of that.
The other thing that I see is the huge increase in interest, not just from scientists, but from lay people and from governments – even though they might not be ready to admit it – about the topic of geoengineering. So while right now it is true that nobody is demanding to do geoengineering, I think there's a high chance that this could change in the next five years. It could be a country, it could be a group of people, but I find it unlikely to believe that there won't be such a call in the next five to 10 years.
That doesn't mean it’s going to happen. There's a lot of things that hypothetically and theoretically we might think are good, but then for a lot of of political and ethical reasons don’t happen. One can think of GMOs, lab-grown meat. There's tons of reasons why something might make sense in theory but doesn't happen in practice.
On the other hand, we know that fundamentally it would work, and the technical challenges are complicated, but not unsolvable. It's hard, but not as hard as building a nuclear power plant. And so there's this feeling that well, if governments don't get their act together and don't actually make some meaningful decisions, somebody might just go out and do it.
And so eventually, in fact in five years, I feel like we will see actual real demand to deploy, or to start thinking about deployment, and that's where my research sits – once we’re there, people are going to start asking questions. What kind of answers can we provide to them right now?
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