 If you work on photon-related research, you know about Eli's name. When I was still a graduate student, that was many years ago. And he was already so well-known. He's the father of Photonic Bangal concept. If you look back at history, the Photonic crystal, this research field, is super exciting. So that was the time when I was a student. And then when I joined Stanford faculty 18 years ago, I started to work on solar cells. I discovered, look at that. There's a limit named after him called Jabra Novich Limit. His name is everywhere. You cannot really escape from Eli's footprint if you work on something related to photon. And well, he also introduced the idea of string semiconductor lasers. He was directing this NSF Center for Energy Efficient Electronic Science. He funded companies to commercialize high-efficiency solar cell technologies. He's the member of NAS, National Academy of Science, National Academy of Engineering, National Academy of Inventors. Well, he has a lot of wisdom. Today's topic is very different. I look at his recent work. He moved into thinking about CO2 capture using biomass. I was a little bit surprised. I said, if there's no photons, Eli can also do it. There's a little bit of photon right there because photosynthesis using photons. Well, Eli just told me he said, well, 40 years ago he actually looked at his problem before. Without further ado, and Eli? Okay, I'm glad to see so many people are interested in this subject. So I should first mention my co-author, Harry Deckman. This is somebody I used to work with 40 years ago when I was working on solar cells. And so it might come as a surprise if you work on solar cells. It does have some implications toward photosynthesis and biomass. So the first thing to notice is that whereas we used to talk just about carbon neutrality, it recently is five years ago. Since that time, people have come to realize it's not enough to be carbon neutral. You actually have to pull carbon dioxide out of the air. And there are a number of reasons for it. First of all, for aviation, the amount of hours you can stay in the air depends upon the energy to mass ratio of the fuel. It turns out jet fuel is just for the highest practical fuel, with the highest energy to mass ratio. And that enables us to have very long intercontinental flights. Sort of a nice calculation if you're a very ambitious physics student is given the energy to mass ratio, which first you have to figure out how much gallon of gasoline weighs and you have to figure out how much energy it has and so forth. Given that energy to mass ratio, why is it that the longest flight that you can take is 19 hours? And that's enough that there are scheduled flights from Dallas to Singapore. So that's made possible by the energy density of hydrocarbon fuel. So it's going to be difficult to get rid of hydrocarbons in that way. Then there's another reason why we can't get rid of hydrocarbons. And that's for summer, winter energy storage. And that's true even here in Northern California. It does get cold. We do heat our houses. And one of the things you notice, I used to be at Bell Labs. I would land at Newark Airport and I would look down and all I would see was a sea of oil storage tanks. And what they're doing is they're storing the oil they make in the summertime, the heating oil. They're storing it for the winter time. And that form of energy storage is so inexpensive that it adds about a penny a gallon to the cost of the fuel. So it's not enough to have batteries. It's not enough to have seasonal storage. And it's a very difficult economic problem because you put all this money into it and then you get to use it only once a year. So that's a problem. And there's another problem. And of course that's sort of a reason why we might be stuck with hydrocarbons. Another reason is that there will always be some countries that do not cooperate. And so if they don't cooperate, what are you going to do? You have to pull their carbon dioxide out of the air. There are other ways of dealing. Maybe you send them a bill. But it has to be done. So those are three reasons. And a more speculative reason, there are some climate models that say that positive feedback of CO2 and methane from permafrost can lead to a thermal runaway. And so in that case we have to go into the past and pull out the old carbon dioxide too. And so it says carbon neutral is not good enough. I think people agree with that now. And carbon dioxide removal is needed. So as a result, a number of reports have come out about five years ago from the different national academies. This is the American one. This is the European one. This is a Physics Today article. And this is something that actually our government has just devoted vast amounts of money, well $1.2 billion to pull carbon dioxide out of the air and pump it underground. The cost is at least $600 a metric ton. I'll explain about the costs later on. But what the government is paying, they're paying $1,000 a ton of CO2. So that's actually a very high cost. So they're directly removing the CO2 is probably not the most efficient way. So if you look at the European report, they recognize this as a problem. So they suggest six ways of doing it. And I'll just mention briefly manipulating the forests, land management to convince the farmers to do the farming differently, burn the biomass and capture the carbon dioxide. Then there are rocks, which absorb carbon dioxide. And I just heard about that from my host, Yitre. And so there's some hope there. And direct air capture and storage, that's the one I just showed you, $1,000 a ton. And then fertilizing the ocean. So some of these are viable, but there's one very important thing that they're missing. And it's very surprising, the safe burial of the agricultural biomass. They're missing that, which is rather surprising because it was almost 40 years ago, Freeman Dyson, who is one of the most famous physicists of the 20th century and very imaginative person. I'll be sinking out of the box. And he wrote a paper already, I guess he wrote in 1976, and suggesting that we should grow trees and bury the logs. And it turns out, I think that's the first paper I saw with that suggestion. It was kind of early. And what's happening today, there are at least seven companies who are doing exactly that. These are just the ones I know about. I'm sure there are more. They're bearing wood and getting rid of the carbon that way. So indeed, the idea of carbon removal is now an X prize. So they're in the midst of identifying the best way of doing this. And I have here just a little, half a slide, a little calculation of the entropy associated with going from very dilute CO2 in the atmosphere to getting very dense CO2 in wood. And it represents a considerable amount of free energy. This is a little bit of an underestimate because this is just concentrating the atmosphere. They have to get it into the wood. Where do you get this free energy from plants? Because they use sunlight and they do photosynthesis and they create energy and they also capture the carbon. So this can be done. I think it's close to a volt per CO2 molecule, electron volt per CO2 molecule that you have to provide. So much of what I'm going to be describing to you is built on the very pioneering work in biomass energy. Bioenergy and we're very fortunate and a lot of that work came from professor Steve Chou here in the front row. And they solved many of the problems associated with this. Among them is to identify which is the best crop in each climate. So for example, Miscanthus is a very good crop for the Midwest. But you have Florida, of course you'd have a different climate, different crop, and Mexico and so on. And in England, I was ready to give up on England because it's so far north and it doesn't get that much sunlight, they have something for England as well. And the biofuel people figured out which crop, the productivity of the crop, what it costs and so forth. So these are some of the crops they identified and some of the things like the more in the tropical regions, alfengrafts, a lot of emphasis on algae, a little bit misplaced. But what is the main point is that if you grow a non-food crop, there are great advantages, they are more productive for the farmers. So you get more tons of biomass per acre. And then the insects, it's not food for the insects either. So the farmer doesn't have to use any insecticide, maybe only a little bit, maybe none at all. And your productivity is much greater. So for example, these biofuel crops, they're three times more productive than maize. So what is maize? So I gave this talk in Europe and if you tell them wheat, they get corn. They get very confused by corn. They think corn is wheat and so you have to use a different word in maize. So there's just something to learn about the difference in the English language. So you could extract a lot of carbon from an acre, a lot of acres. So it would become a very large agricultural enterprise. So I was motivated to this. I started this 40 years ago. So I had a mentor 40 years ago. I started working on solar cells and a mentor. And this guy was Al Rose and he was a big guy from the RCA labs. Well, probably none of you even know what RCA is anymore. He was, in its time, it created radio, then it created television, then it created color television. Al Rose, shortly after he graduated, he was recruited by Vladimir Zorkin, like the original people back in the 1930s. And he invents the Viticon camera so he becomes a very big guy in television in very early days. And one day he comes to me. He's my mentor. I passed some gas stations on the way to work and the price of gas is high. Why don't you look up the price of energy from corn on the Chicago Board of Trade? So now that's a little bit difficult. So you have to figure out, okay, you go to Wall Street Journal, you get the price of corn, then you have to figure out what bushels are, then how many bushels are in a ton, then you switch to metric tons, and so on and so forth. And then you also figure out the free energy, the Gibbs free energy in the corn when you burn it. And compare it to the price of gasoline, so then you have to know how many megajoules there are in a gallon of gasoline. But you do all this stuff. So I did all this stuff. I didn't realize that he had already published this and I don't know if he was teasing me, but he already worked it all out. And here's the answer. The price, the energy price of corn per megajoule, at least at that time, probably still pretty close today, was roughly the same as the price of gasoline per megajoule. And this sort of stunned me that agriculture could possibly be so efficient that it was competitive in terms of the energy industry. So ever since then, I have had a great respect for agriculture. In fact, occasionally one gets invited to the University of Illinois, and I remember checking in at a motel there and turning on the TV, and what you had was the crop reports. And I was fascinated. I was just glued to the television. They were talking about the crop reports and the weather and things that farmers are concerned about. So what do we do with the biomass? So I say we put it in a dry environmental chamber. In fact, I can give you the punchline of the talk, is that if you keep biomass dry, then it will last for millennia and I'll show you the evidence for that. And so this suggests, then, a way to safely pull the carbon dioxide out of the air. And part of keeping it dry is you have four millimeters of polyethylene. This is a very tough plastic, much thicker than you're used to, and very strong. And it's similar to the way landfills, but the most modern landfills are sealed this way with very thick polyethylene. And the goal there is to keep the garbage from contaminating the groundwater, and we have the exact opposite goal. We don't want the groundwater to making our biomass wet, because once it's wet, it decomposes and gives you back your carbon dioxide and methane, which is really bad. So we published this in this article, so you can look it up. If you want to look it up, just look up my name and my co-author has a very easy name, Deckman. If you can spell my name correctly, you'll find this article. Now, we build very much on the knowledge that was gained from biofuel research. But there's an issue in biofuels is that you're starting with cellulose, so you're growing cellulose with the approximate chemical formula like this, and the biomass starts out half oxidized. So in order to turn this into fuel, you actually need to grow two acres of biomass to get one acre of fuel, because you've already half oxidized the carbon, as opposed to just putting the biomass into the ground where you get full credit for the acre right away. So the biofuel requires twice as much farming, but nonetheless we learn a tremendous amount from the biofuel. First thing to notice, why doesn't it work if we just throw the logs underground is that there are microbes and different things, fungi and so on, and they will basically anaerobically digest it. Of course, if it's aerobic, it'll decay into carbon dioxide, but if it's anaerobic, it still decays, because you have plenty of these microbes, insects and fungi that can eat it and turn it back into CO2, which wouldn't work out. And this is roughly how it goes. You start, let's say, with a chemical formula for cellulose, and unfortunately the microbes get to it, it turns into carbon dioxide and methane, so you don't want that. So what about wood? So something interesting about wood is that it is 40% cellulose, and cellulose is, that's what this is, and chemists can work with it, but it's also 30% lignin, which is a very tough, durable compound, which is similar to the grain of wood. I'm just looking at the podium here, and the wood has a kind of a grain, and that comes from lignin. And what happens is the microbes, they tend to decompose the cellulose right away, the lignin stays behind, and so when you get older, you're all going to have houses, and you might dig up in your backyard maybe doing some gardening, and you'll find a board that the contractors buried because they were too lazy to get rid of it, so they just left it underground, and you pick up the board, and some of the older people here will have had the experience, you pick up the board, and it looks like a board, but kind of where's the weight? It's very, very lightweight, but it has the shape of a board, and the cellulose is gone, and what you've picked up is mostly lignin. So it's not like totally pointless, you get 30% sequestration from the lignin, but it's 30%, so that presents an economic disadvantage. So there's one more thing scientific to understand here is a concept called water activity. So we all know what relative humidity is, and luckily we're in a relatively dry part of the country, but sometimes it's really humid, and then we say, well, how do we describe relative humidity if it's not the atmosphere? What if it's a piece of food, like a steak? I have here a picture of a steak, and you could ask, well, if water activity is like relative humidity, if it's a steak, when it's dry, we say it has a low water activity, and indeed when you dry food like this, it tends to be preserved. So you could apply this to any biomass, and when it's very, very dry, it tends to suppress all biological metabolic activity. In fact, this has been researched, and I'll mention that at about 60% water activity, metabolism comes to a stop. I already mentioned who has researched this. So this has been researched by the Food and Drug Administration. If you want to put food on the supermarket shelf, and sort of the inspector looks at it, and he says, well, what's the water activity? He says, oh, it's below 60%. He says, okay, he wraps himself in around it and put it on the shelf. So the principle of drying food is, of course, well known. Another part of the government, NASA, has studied this in great detail. And why? Because they're looking for life on other planets. What does life require? Well, it requires a little bit of moisture. So as you go below 95% many bacteria, poop out, yeasts go away around 85%, and various other yeasts, and mildew, and finally at 60% water activity. Metabolism comes to a stop, and the natives in the Pacific Northwest knew this. This is like just a picture of salmon drying, and they would, of course, easy to catch salmon, and they would dry it, and it was preserved. So that's basically the punchline of the talk. If you dry it below 60% water activity, whenever I say water activity, just think relatively immediately, below 60% water activity, you can preserve carbon or cellulose biomass, you can preserve it. And so now this is a little bit confusing, because you have, on the one hand, you have on the horizontal axis water activity, so here's 60%, and you're safe below 60%, but then there's weight percent, which is different. So at 60% water activity, it means the weight percent of water is maybe in the 10% to 14% range. And so this is kind of interesting, because when corn, when farmers in the Midwest, when they grow corn, they can sell it on the Chicago Board of Trade, but then there's a requirement, the Chicago Board of Trades will take your corn, but it has to have less than 14% water. And so then you can ship it around, and it will last. So that's something to know about. So what about then putting the biomass underground in an environmental chamber? It doesn't have to be very big, because you can build it up maybe to 30 meters tall and covered over. The land area is one-tenthousandth of the land area needed for the agriculture, so that's good. And it's stable, and I'll show you the proof of that. So one of the reasons it's stable, and the reason why this is done, is water does not easily get through polyethylene. And the amount of water it gets through is equivalent in a year, let's say it was fully wet on one side and on the inside, it's equivalent to 1.7 microns of liquid water every year. And when you have this much dry biomass, it can easily take that up and still stay below 60%. It lasts for thousands of years, below 60% water activity. So here I think I can just go to the bottom line. It's a big problem, and it's commonly accepted that if we can deal with 20 billion metric tons of carbon dioxide every year, we can do this. So fortunately, I go back to the biofuel research, and they have already searched and done all the research on the crops, the growth, the productivity and so forth, so here are the written reports. Where are you going to get the land? And so here's another report from the Department of Energy. And I have this way of sort of describing the amount of land that you would need. So the horizontal axis is representing the whole land area of the earth. And then some of it is useless. The poles are covered in ice. There are parts that are very barren and so on and so forth. But a good part of it is agriculture, forestry, shrubs, and a little tiny segment for freshwater lakes and cities and so on. Now how much is used for growing our food? So we tend to grow our food in row crops. So this is the amount. It turns out 11% of the earth's surface is row crops. So if you're bored driving through Iowa, nonetheless it's 11% of the earth's surface is row crops like that. A lot of it is for pastures, for our farm animals and so on and so forth. And this blue rectangle is what you would need to capture 20 gigatons of carbon dioxide. So it would be a giant agricultural enterprise, but something that is within, it's available, and you can actually make the rectangle, the blue rectangle, bigger and do a lot more than just capturing one year's carbon dioxide. So what is this about agriculture? Well, we're pretty good at it. We've been doing it for 10,000 years. And so we need to know the cost. The biofuel research has given us the cost, but there's controversy about costs. And so I like to do a sanity check. As a professor, sometimes my students tell me things and I don't know if it's right. So I do a sanity check. So I look up the price of corn on the Chicago Board of Trade, back to that again, and I can figure out what the farmers are making per acre, how much money they make per acre, and how much of what they produce and what it's going for and so on and so forth. So for approximately $500 an acre, the farmers break even, well, do better than break even, that includes the profit and also includes the rental of the land. So $500 an acre, that's what it costs. So from that, you can figure out what is it going to cost for the CO2. And so it works out to be, and this was already done by the biofuel people, so about $30 a ton just for the agricultural part. So it's $30 a ton. And then for the sequestration, the environmental chamber, the trucking to the environmental chamber, all of that stuff, and the cost of the fertilizer and the CO2 cost of the fertilizer and the N2O problem, et cetera. So that's another $30. So it ends up that for different crops, it's roughly, it's not that different. For different crops, it's about $30 plus $30, so $60 a ton of CO2. And now I convert that, so this is like the advanced placement test for the high school students, convert $60 a ton to dollars per gallon of gasoline. That's a simple units conversion, and it comes out to be $0.53 a gallon of gas. So I would say that's a tolerable cost, but it would be a big commitment, but not a crazy commitment, because if we want to deal with 20 gigatons, and the world GNP is $100 trillion, but 20 gigatons at $60 a ton, that comes out to be $1.2 trillion. That's only 1.2% of the world GNP. And the GNP of the world, the productivity, because we get better at doing things, productivity jumps about 2% a year. So the 1.2% cost doesn't seem so onerous. So we set the paper off, and the referee said that, well, it's an idea, but you need some proof from maybe a natural experiment, an experiment in nature that if you keep the biomass dry, it'll last a very long time. And so he said, go back and add that to the paper. So we actually knew about it, but then we added it, and this is a very fascinating story. So this is a very famous tourist attraction in Israel, and it is a Mesa. As you can see, it's a Mesa. The Dead Sea has shrunk a little bit because they're using more of the water for agriculture, but the Dead Sea used to come right up to the base of this Mesa. This Mesa was very easy to defend, and it's famous as the last holdout of the Jewish zealots against the Roman legions. But there was something else that was here, and they had a king, a king herit. And like most kings, he was afraid of being overthrown, so he built himself a castle up here, and actually this is also part of it, and obviously a very easy to defend place. And actually sometimes it's called a palace, but it looks to me like more like a defensive fortification. So he builds it up there, and he never needs it. No one tries to overthrow him, but it's a very remote location because first of all, it's very dry, it's very hot, hard to get to, and then you have to climb these vertical cliffs. So it was ignored. It was neglected for 2,000 years. And then an archaeologist in the mid-1960s, he says, I'm going to excavate King Herod's castle. So this is what King Herod was supposed to look like. This is the archaeologist Egal Yedin, and he goes and he excavates it, and he publishes a lot of archaeology papers, but along them he finds these seeds. And these are seeds for a common tree in that region, which is the date palm tree, and you can see they're sort of big like pecans. And then he eventually passes away, and if somebody is holding all these archaeological treasures, and this woman, who is a medical doctor in Jerusalem, she hears about this, and she says, she sweet talks to people who are in charge of this, and they give her some of these seeds, and she sends the seeds out to be carbon dated, and they come back, yep, they're 2,000 years old. And then she gives the seeds to the horticulturalist, and says, can you plant these? And she germinates the seeds. And so let me go to the next page, which is figure five of our paper, which is an 18-year-old date palm tree germinated in the year 2005 from seeds that were 2,000 years old. And what's the special feature? It was in an extremely dry climate, okay? The top of that mesa is at sea level, the bottom of the mesa is at the deep below sea level, the Dead Sea. So this was too good to miss out, so we included it in the paper, and the referee accepted it. And so this tells you a little bit about, yeah, we have some proof, which is not always available in many of the options for dealing with the carbon dioxide problem. Okay, so there's something else to know about agriculture, and that is that it tends to get better, because we're human beings, we tend to put some effort into it, it changes, it gets better over time. So this is in agricultural productivity in the least developed countries, and they traced it from 1960 to 2010, and it roughly doubled. And this is common. Many things that people do, there's a learning curve or an experience curve, and you get better at it. Now, some of it leads to Moore's Law, but this sort of says that maybe there's a Moore's Law for agriculture, because things get better by a factor of two over a 50-year period. And you know what got better, so we have better seeds, we have fertilizer, we have many other things. Now, so we went and looked up the productivity of land, and this is the oldest data we could find is from England. The productivity for wheat in England. Now, going back to the 1700s, and they didn't even use acres back in the 1700s, so we had a lot of unit conversions that we had to give. But of course, so we started the graph at 1800, where the data was pretty good, and in the early 1800s, we're all taught in high school that Reverend Malthus warned us that the industrial revolution was pointless because we're not going to have enough food, people are going to have too many children, and there's not going to be enough food. But it didn't work out that way, and this sort of tells the story, is that between 1800 and 1900, the productivity of land in England doubled. And then what happened in 1905, the Haber process to create artificial fertilizer, and then in the next 50 years, it doubles again. So from 1900 to 1950, it doubles again, and then the next 50 years, it doubles again. So confirming this Moore's Law. And the question is, always with Moore's Law, is this going to come to an end? And I would say no, there are going to be further improvements because we're going to be able to manipulate the genome of the agriculture, of the biomass, with CRISPR, we'll be able to manipulate it more easily, and we still have a lot of headroom. Now, why is there headroom? So a question that is worth answering is what is the efficiency of, let's say, growing corn? And they've studied this, it's differential efficiency, just a little bit of extra biomass for a little extra sunlight, and it comes out to be about 2% efficient. So solar panels are roughly 29% efficient, and so I would say that's another three factors or two. So I'm very optimistic about agriculture improving, but all the estimates I've given you are from the existing type of agriculture, nothing about the future. I just wanted to share it with you because it's so interesting. Here's another interesting fact. 1.5 trillion acres of land have fallen out of production because of the improvements in agriculture, because of the Moore's Law for Agriculture, and so we can use 68% less land to produce the same amount of food. So there's something to think about. So how are we going to solve the climate crisis? So it needs to be scalable. Agriculture is one of mankind's largest industries, so it's scalable. The cost, I've told you the cost, would be 1.2% of the world's GNP spread over the next century. Stability. So this is something that is often overlooked, is that you can sequester, but is it going to stay there? And we have proof that it stays there, provided you provide dryness. So dryness is the big thing. You also need predictability, and society will not accept a method that, well, you have to come back 100 years later to find out if it really worked or there are no bad side effects, etc. So that type of predictability is very essential, and any approach to solve this problem, you have to already know in advance. You can't wait for the experiment to be done. And then rapid implementation. So agro sequestration, it can start with the next growing season, so it can start in April next year. So first some apologies. I'm a high-tech guy, but I'm offering a low-tech solution. But a problem of this nature, we cannot take a chance. We have to be very certain how things will turn out hundreds of years from now. And I've given you an example that the agro sequestration is a proven method for at least 2,000 years. Now, as far as what's going on in the real world, it's kind of shocking that 40% of US corn, by the way, US is the largest corn producer in the world, about 40% of it is used for ethanol, and ethanol is a really dumb idea, and it's far from carbon neutral even. And so it gives you an idea that that would be like 40% of Iowa is available just from that. But it doesn't mean you would want to use Iowa, but obviously we... Well, the government has different parts of the government already contracting for biofuel use, miscanthus. I've been told there are thousands of acres in Iowa that are already being used to grow miscanthus, and this would be... How are you going to solve the political problems? This is politically acceptable to farmers because it's another cash crop, and so that part is going. So my conclusion is that agro sequestration at this point in time appears to be the lowest cost scalable technology that fulfills all those requirements. There might be others, but they would have to be cheaper than the agro sequestration. And in an emergency, it could even remove historical carbon dioxide. We have to be aware of complacency, the moral hazard that we can depend upon this. We have to continue research in new forms of energy. Conservation needs to continue. Alternative energy sources need to be... continue to be developed. Decarbonization will also be needed and other forms of sequestration. So we definitely need an all-of-the-above approach. We should not be tempted to say this solves our problem. So now one of my experiences is that if you really want to get something done, you have to start a company. So we have started a company, and it's called Agro Capture Corp. And we are recruiting at all levels. So I see a lot of students here. Maybe if you're close to graduation. We're recruiting all the way from a member of technical staff all the way to CEO. So please send CVs to myself and I hope to hear from some of you. So my concluding slide is that we're living with previously uncharted carbon dioxide concentration in the atmosphere. Carbon neutral is not enough. And we have to actually be carbon negative. That used to be controversial, but I think it's more or less accepted now. So we need carbon dioxide removal. And let me ask for your questions and to thank you so much for listening. Thank you very much. You're like, what was a fantastic job of talking about some very technical things in plain and simple language, at least from my point of view. So I thought it was terrific. It also is a great, a good new old idea if I catch and capture it or go for it, which does make it tried and true. Easy to understand. Sorry, sorry. We now have about 10 minutes of questions in the room. We usually give preference to student questions first. Any student questions from our audience here? Hi, thanks a lot for the talk. It was very interesting. I was doing research in carbon dioxide removal for the past two years and I came across one of the solutions that might be possible as well. And with regard to the stability, I wanted to ask if you looked at the stability of the polyethylene that is about to be buried as well, because I know there is some biodegradation of polyethylene as well and I could imagine that there is some societal concern about microplastics polluting soils and so on. So I was just asking myself if you considered these topics as well. That's a really good question. And we think of that very thick, tough polyethylene that it's immune to many of these problems. But you're making a good point. We need to look at that more carefully and the one thing that has been looked at is the diffusivity of water through the polyethylene. That came out to be a very low number which I indicated. I once visited Dow Chemical because they were making roof shingles that had solar panels on them and one of the scientists there pulled me over and said, you know, it's not the solar panel. They were the high technologies. The high technology is in the plastic that stays up on the roof for 50 years with the sun beating down on it and does not degrade. So the hope is that usually that's it. It's photo degradation. But by being underground, maybe that won't be a problem. But there might be some other issue. So it's a very good point. Thank you as well for your talk. Thank you. As an undergraduate student who doesn't know much about long-term effects of this nature, how long does your company plan to continue sequestering carbon underground for how many decades or centuries before? So I should say the company is just a start-up. So we haven't sequestered anything yet. But then it's really up to society and also up to the experts here at the School of Sustainability is how long does it need to be? But I think the estimates are pretty long. If you really want to say that you've solved the problem at least a thousand years and preferably longer. So I think that's... And the cause of that is that carbon dioxide stays in the atmosphere for such a long period of time. And so we really need to have good, stable, predictable long-term solutions. Hi. Hi. A really interesting presentation idea. Thank you so much for sharing. So I had a question about some of your thoughts about fertilizer inputs for some of these crops. You mentioned in your analysis that the fertilizer inputs for these sorts of crops would be less than for food crops. But I'm curious, how concerned are you about, say, the long-term sequestration about some of these nutrients underground? Fixed nitrogen is one thing. But what about minerals like phosphorus where the world supply is limited, say, and those things are considered less renewable than fixed nitrogen? So one of the things... If you look in the details of the paper and look at your information, we try to analyze everything. One of the concepts that we introduce is carbon efficiency. So if you sequester 100 tons of carbon dioxide, but you have farm equipment, you have tractors, you have fertilizer, and so forth. So the question is, what is the carbon efficiency of this process? So I think every scheme needs to do that. So we've taken into account these things. The fact that we have to provide some fertilizer and so forth. So it turns out our carbon efficiency is 95%. So if we sequester 100 tons and to grow and bury the 100 tons, it meant we needed farm equipment, we needed fertilizer, we needed all that stuff. So that's a very important factor to keep in mind. And I think one of the reasons it's so high is that if it's a non-food crop, you do get a lot more biomass. And some of the minerals that you're talking about, they can be replaced, but the question is the cost. So we've included the costs of replacing the minerals in the 95%. So we came to a very good conclusion and I was sort of satisfied with it, but my co-author said, no, no, it can be over 100%. How can you have over 100% carbon efficiency? He says, because we're putting roots into the ground and we're not taking into account the fact that we've sequestered some carbon into the roots. And I'm a little suspicious of that because we don't know how long that's going to last. So he was already ready to claim over 100% carbon efficiency, but I'm not going to go that far. Thank you very much. That was a fantastic presentation. Is there a value in burying these crops into the ground or is the value of the crops lost when buried? Well, there's value in the sense that the carbon sequestration can get carbon credits. The carbon credits cost a certain amount. So it's not like a pointless form of agriculture and the farmer gets paid per ton. But if you're asking is it valuable to humanity to have all this carbon or all this biofuel or feedstock to have it buried in the ground? I say it's valuable because you never know when the situation might change and we know where all the stuff is and at some point we might be so successful there might be an ice age and we want to try to reverse it. I'm just like being very futuristic. And so I feel comfortable knowing that it's there. More questions in the room? This will be the last question. A couple of comments actually to the question about the thing about Miss Kansas which Christian, they're perennials and after each season a lot of the minerals and stuff get drawn back to the roots because the plant is going to be there for 10 years. So a lot of the stuff it doesn't have to be the expensive stuff. Phosphorus is a problem but again another comment there is actually a carbon fixation in the roots it's at the level of a percent maybe a fraction that actually stays in the soil and so again these perennials actually do stick carbon back in the soil. Unlike the food crops which are depleting. So I should not have criticized my co-author. Maybe we can get above 95%. You should look at it because there is some fixation. Finally, when I was doing these calculations of biomass underground I found that Miss Kansas which is slightly fluffy you have to compress it to density of wood and that's another cost and if you talk about one of your slides you showed I think it's a couple of gigatons compared to the amount of land but that's one year but you want it year after year after year and so you really need high density and you're going to need deeper than 30 meters because if you want to store it for decades. We can do the fine tuning on the 30 meters offline the basically I'm in agreement with the things you said with regard to the cost of replenishing the soil we took those into account that's under control it's more the volume than I'm concerned about because in your graph you said here's the amount of row crop land this is one tenth row crop land for one year. Built into this that I haven't mentioned thanks to my co-author and so we have the drying function and the compressing function so your average density of garbage is about 0.4 and we need to compress to 1 so we need to really compress it a lot so those calculations were compressing the compression was in there and certain things cost associated with that we try to include all of that stuff but it's a very good point. Great, we're just about out of time so I'd like to thank Professor Yerovanovich for a big applause