Varin's Blog

How do we remove excess carbon from the atmosphere? This question is at the heart of the climate change crisis we face. Currently at COP26 in Glasgow, world leaders are debating and discussing many different aspects of climate change, like lowering GHG emissions from factories, landfills, cars, planes, our homes and offices, reducing our meat consumption, going to plant based foods, from dealing with cows belching methane to removing plastic from the oceans, from lowering our oil addiction to producing better energy, and storing it and many others. All of this is great, and if we can mobilize action on these fronts, super. But, if we want to make substantial progress, I think the main question to address is: how do we remove excess carbon from our atmosphere? In this series of blogs I want to address this question, from my point of view as a 14 year old.

There is a lot of talk, and also some action, around using trees to remove carbon. Last year, in my final year of middle school, I did some experiments to understand the size and scale of this, inspired by Fermi Estimates (https://en.wikipedia.org/wiki/Fermi_problem). Here is what I found:

According to NASA (Source), in 2014 we released ~35 billion metric tons (or roughly 38.5 Billion US tons, or Gigatons, which is what I will use here) of Carbon Dioxide from fossil fuels into the atmosphere. In 2019, before Covid, this number went up to about 43 billion tons (Source). According to experts (Source) there is a total of almost a 100x as much, or ~3,500 Gigatons of CO2 in the atmosphere. Of this ~1,100 gigatons of CO2 have been added due to human activity since 1880. Note that in a CO2 molecule, about 27% is carbon, so the 3,500 gigatons of CO2 in the atmosphere, means ~950 gigatons of carbon, of which ~300 Gigatons has been added since 1880. And every year, this nunber goes up by another ~35-40 Gigatons or roughly ~10 Gigatons of Carbon! There are about 3 trillion trees in the world (Source: gotreequotes.com), and because of deforestation in the Amazon rainforests, Indonesia, and many other parts of the world (Source: WWF), this number is decreasing (for example, 90% of forests in Tanzania and Kenya, and two-thirds of the forests in Borneo, have already been lost). There are many initiatives to plant more trees around the world (1t.org and onetrilliontrees.org). So in order to understand the scale of planting trees to combat climate change, we need to know: How far can trees go in reducing the excess carbon in our atmosphere? Is this feasible? And what would it take?

One question people often ask is, since the trees are on the ground, and the CO2 is in the atmosphere, miles above the trees, does it even make sense to use trees for carbon extraction? Since the air constantly keeps moving, do the air molecules come down and make it to trees? To answer this question, I did an experiment to see how air molecules travel within a closed room. It turns out air molecules are constantly moving around, at 300-400 meters per second (Source: Department of Education, Australia), or 675 to 900 mph! I tested this by checking how long it takes perfume to travel from one corner of a room to another. I tried this by having my helper release perfume at the top of one corner of a 25 foot by 15 foot room. I stood on the other end of the room (about 232 inches away) and could correctly recognize the smell of the perfume 70 seconds later. So the air moved at ~3.3 inches per second. In a different room, it traveled 170 inches in 52 seconds, again almost the same 3.3 inches per second. While this seems slow (and keep in mind that this is without the influence of wind, jet streams, etc.), most atmospheric CO2 resides in the lower stratosphere, which is ~9.3 miles above sea level. This means that it would take ~2 days for a molecule of CO2 to travel from the lower stratosphere to the leaves of a tree. This is assuming that there is no movement in the air that ever influences its movement to the surface, so 2 days is pretty good by that standard.

Next, to understand how much carbon a tree can absorb, I measured the increase in the carbon content in two trees in my backyard over ~12 weeks. These are a type of cherry tree called drooping/weeping cherry. Their average current weight is ~25 lbs. I measured the weight by the thickness of the trunk (about 1.5 inches) and their height (about 8 feet), and then adding a small percentage of weight from the leaves, and using the average density of a cherry tree (750 kg / m^3) to calculate the weight. 25lbs seems about right for my trees. As the trees get older, a lot more carbon will come into the branches and of course the trunk will keep getting bigger. I took three measurements, every 4 weeks, and the volume of the trees grew by about 10% over the 12 weeks. My estimate is these trees added about 5 lbs of weight, of which about 2.5lbs was the weight of carbon, because a tree is about 50% carbon (Source). So over 12 weeks, they absorbed roughly 2.5lbs of carbon from the air, and over a year they will absorb roughly 10 lbs of carbon from the air.

According to usda.gov, a mature tree absorbs 48 pounds of carbon dioxide per year. This equates to ~13 pounds of carbon per year. I can see how in 10 years my trees will be absorbing that much carbon too. That carbon extraction rate means that in order to absorb the extra 10 billion tons of carbon that we emit into the atmosphere annually, we need to plant around 1.6 trillion trees! This excludes the remaining 300+ gigatons of excess carbon in the atmosphere that we’ve been emitting into the atmosphere since the 1800s, and to actually get rid of this, we would have to plant several times more. There is a great initiative underway called 1 trillion trees that aims to plant 1 trillion trees by 2030 (Source: onetrilliontrees.org). Assuming all deforestation stopped happening right now, urban sprawl and land use for agriculture stopped increasing, and that the amount of carbon emitted per year stopped increasing, this would require a minimum of 15 million square kilometers, and 10 years from the time of planting for the trees to start absorbing 48 pounds of carbon dioxide per year, which, again, translates to ~13 pounds of carbon per year. Once they are all at that maturity, they will be able to take out a (relatively) lackluster ~6.5 gigatons of Carbon out of the air every year. So, while these initiatives to plant this many trees are really needed, they are not enough, especially when you consider that as our population and our energy usage, grows, the amount of carbon we emit into the atmosphere each year will increase and the impact that these 1 trillion trees have will become less significant.

Anyway, planting even 1 trillion trees (let alone 1.6) is really, really hard. First, it would take years for those trees to mature, and those trees wouldn’t start absorbing carbon at the previously mentioned rate of 13 pounds of carbon per year until the 2030s (assuming we instantly planted those 1.6 trillion mature trees today). And, it will be hard to find enough space where these trees can be planted. So I looked up some data to understand the feasibility of planting a trillion trees.

According to ourworldindata.org (Source), ~29% of the Earth’s surface (or ~149 million square kilometers) consists of land, and the rest is all ocean. Approximately 71% of this land is habitable (as in, trees can be planted there). The rest is either made up of glaciers or barren land (deserts, salt flats, etc). Of the habitable land, ~1% is used for urban and built-up areas, another ~1% consists of freshwater (rivers, lakes), ~11% consists of shrubland, ~37% consists of forests, and a gargantuan ~50% of all habitable land goes to agriculture. This is roughly 51 million square kilometers of land used for agriculture! Of that, ~77% (that is ~40 million square kilometers) is used for livestock (raising cows and other animals for food), while the remaining ~23% is used for crops. Despite this, meat and dairy products only provide ~18% of the global calorie supply, and ~37% of the global protein supply. Everything else comes from plants.

A general guideline for planting medium-sized trees is to plant them 25-40 feet apart (Source: Arbor Day Foundation). Even if we were to plant our 1.6 trillion trees just 7 feet apart, each mature tree would need 150 square feet of space. So, to plant 1.6 trillion trees, we would need ~22 million square kilometeres of space, a space about as large as the entire north America, including the US, Canada, Mexico and more. Slowing urban sprawl doesn't help us get more land to plant trees, and replacing shrubland with forest is pointless since shrubs also extract carbon from the atmosphere. This means that the only two sources of additional land for forests are 51 million square kilometers of agriculture and 28 million square kilometers of habitable barren land. Transforming barren land is rather simple in principle, because usually simply planting trees in barren areas will make the areas not barren (provided they get enough care, fertilizers, water, etc). If we could lower our meat consumption through lifestyle changes and also by adopting new types of food, we can also remove the immense amount of land used for livestock production and replace it with forested land.

How would we plant this many trees? If every human on the planet took it upon themselves to plant trees, a trillion trees would mean each one of us needs to plant ~140 trees. 1.6 trillion trees means each of us needs to plant ~210 trees. If we did a tree per month, this will take more than 17 years. Perhaps in more developed countries we can do more, and we definitely should. Clearly this will be a mammoth task that will have to be institutionalized somehow, and (multiple) organizations will need to be incentivized to plant and maintain trees. Every country should sign on to a certain number of trees, and they should all agree on incentives for people to do so, on providing seeds, best practices, etc. In India, the place where my parents grew up, the “Green Revolution” was achieved basically within 20 years. Do we have the collective will to get something similar done to save our planet?

While I think that planting trees is a good idea, and beyond the climate benefits, trees improve the aesthetics and biodiversity around us, I think that when it comes to the central question we are contemplating here, trees alone will not get us there. We need new, technological solutions to extract carbon from the air (for example, DAC facilities) that are more efficient, cheaper and faster than planting trees. I look forward to sharing more thoughts on that in my next blog.

So the conclusion here is: Yes, we need to plant more trees, a lot of trees. We need roughly 2 trillion more trees to make a real dent, so let’s start now. And beyond planting trees, we need to do more, a lot more. We have one Earth, and one fleeting chance to rescue it. Very soon it may be too late. So let’s do it, now. Leaders of the world, this 14 year old joins millions of others to request you to please come up with concrete goals and timelines, and execute. Talk alone gets nothing done, the action that comes from it does. And there is already too much hot air around us.