Climate change reversal seems more plausible than I thought.

One of the more frustrating aspects of climate change, and in particular of climate change coverage, is its inexorable progression towards doom. We are inundated with stories of natural disasters and biodiversity loss. What’s more, we are frequently reminded that we’ve passed tipping point after tipping point, and of how little we can do to save ourselves. UN climate conferences centre on how many degrees warmer the planet will get by when. Universities offer degrees in climate adaptation rather than climate correction. I frequently hear that “even if all emissions stopped tomorrow, the planet would continue warming”. 

Urgency and alarmism are warranted - climate change is a big deal. But the lack of optimism in these narratives means that most people I meet buy into this idea that we are cruising towards environmental dystopia, that at best we may only avoid speeding into hell. 

In a modern world where we try to optimise so much, I am struck by how little credence is given to the idea of a total climate solution. In our own lives, we do not just pursue the least bad outcomes. People want to lose weight, not just stop gaining it. They want advanced technology, not technology that simply works. The pursuit of improvement drive us. Accepting suboptimal outcomes breeds apathy. 

Yet climate change, ostensibly our biggest problem, seems to be treated differently. We talk a lot about ways to reduce emissions and to live with climate change, which are of course important. But most discussion of reversing the damage done by greenhouse gas emissions is dismissed as unrealistic at best, or nefariously distracting at worst. 

I think we need a light at the end of the tunnel so I thought I’d explore the ways humanity could actually “solve” climate change.

First, what is the scope of the problem? Human activities have resulted in the cumulative emission of around 1.8 trillion tonnes of CO2 into the atmosphere since roughly 1850, resulting in a total warming of 1 to 1.5 degrees C. Humans are currently emitting about 40 billion tonnes of CO2 per year, meaning that if we carry on at this rate, another 3.2 trillion tonnes will be emitted by 2100. I expect that number will decline as decarbonisation progresses. So, for the purposes of this thought experiment, I’ll assume that about another 2 trillion tonnes will be emitted by 2100, meaning that 4 trillion tonnes is the total amount of carbon we will need to remove from the atmosphere in order to go back to “square one”. 

How could we do this? 

Plant A LOT of trees

This is the easy answer. We all know that plants photosynthesise carbon out of the atmosphere into their biomass, and that planting trees is a relatively cheap and easy way to capture carbon. But of course, we humans continue to cut trees down to use land for our own purposes. We’re so good at this that since 1900, humans have deforested roughly 25% of the earth’s forested area and over 50% of its natural grassland area. 

Let’s assume though that we go all in on reforestation to remove emitted CO2. How much could we remove? Forests’ carbon storage capacity varies depending on their age, health, and dominant tree species, but a standard estimate is that a hectare of new forest stores roughly 50 tonnes of carbon over 100 years. Using this figure, we would need roughly 60 billion hectares of long-term reforestation to cancel out those 3 trillion tonnes of CO2 emissions. 

This is, unfortunately, impossible. The land surface area of the entire planet is only around 13 billion hectares. Roughly 5 billion hectares are in agricultural use so even if we assume that we might double agricultural efficiency to free up land for reforestation, we could still reforest only about 2.5 billion hectares, an area equivalent to the USA and India combined. This would remove about 125 billion tonnes of carbon from the atmosphere, meaning we would still be 3.9 trillion tonnes short. 

There are lots of other good reasons to try to reforest more of the planet - species conservation, air quality improvement, regional cooling, erosion mitigation, and simply because forests are beautiful - but it seems that reforestation alone will not be enough to undo climate change. 

Image: Matthias Neill

Suck carbon out of the atmosphere

Direct air capture (DAC) is an emerging technology whereby machines use chemical processes to separate the carbon from carbon dioxide and transform it into solid or liquid forms, allowing it to be stored underground or in the ocean. DAC facilities typically employ large air intake systems that look a bit like air conditioners, which then pass air either through reactive filters or solutions with which the carbon in CO2 bonds.

Image: Climeworks

DAC technology is still in its early days. There are currently just 27 DAC facilities in operation and only 3 of these are capturing more than 1,000 tonnes of CO2 per year. Climeworks, a Swiss company, currently operates the largest, a 4,000 tonne-per-year facility in Iceland and it was their work that initially got me interested in DAC last year. Despite early progress in getting the technology off the ground, and increasing innovation in the chemical methods of carbon separation, it’s fair to say that DAC has no meaningful impact against emissions yet. Existing facilities remove only 10,000 tonnes of carbon per year - just 0.025% of total annual emissions. 

This is likely to change. 130 large-scale DAC facilities are currently at various stages of planning and most should come online by around 2030. In total, these could remove around 65 million tonnes of C02 per year, though the IEA estimates of their potential are more conservative. Over 100 years, these facilities could remove around 6.5 billion tonnes of CO2. That’s nothing compared to the 4 trillion tonnes I outlined earlier but if other green technologies are any guide, DAC technology will likely improve such that more efficient DAC facilities will continue to come online over the 21st Century. 

What’s more, some quick maths suggest that we only need about 400 times more DAC capacity than the 130 large-scale facilities already in development. That is really not that much, compared to everything else humans have built. I estimate that we need around 50 to 60 thousand large-scale DAC facilities to remove those 4 trillion tonnes of CO2 and that conservatively, the average large-scale DAC facility covers an area of roughly 5 square KM. Based on these figures, it would only take 250 to 300 square kilometres of land to accommodate all those required DAC facilities, an area slightly larger than Brooklyn, New York. Even if I’m wildly off in my estimations, it seems quite plausible that we can remove all emitted CO2 with roughly a city’s worth of built-up land, which gives me great hope that this approach could work. 

We are a long way off from meaningful scale-up of DAC technology, but the same was true of solar power in the 1980s. Today, solar panels are abundant, cheap, and financially competitive with other forms of electricity generation. Policy making in developed countries is catching up to the potential of DAC. America’s Inflation Reduction Act increases tax credits for carbon removed via DAC, Canada is drafting legislation to subsidise DAC facilities via tax credits, the UK has set aside a dedicated pool of about USD 25 billion for DAC projects. The EU and Japan are also flirting with supportive policies. With continued innovation, investment, and government support, I am optimistic that DAC technology can follow solar energy's growth trajectory over the next 40 years. 

Expose a lot of rocks to a lot of air

One of the coolest things in nature is the process of terrestrial weathering. When rocks such as granite or basalt, which contain silicate minerals like calcium, magnesium, and silicone, are exposed to air and water, they slowly release these minerals. Silicate minerals then react with atmospheric carbon dioxide to form new carbonate minerals such as calcite or silica. Over geologic time scales, these carbonate minerals formed by weathering are washed into the oceans where they pile up on the sea floor to eventually form sandstone. Because carbonate minerals are also chemically basic, they also present the added benefit of counteracting ocean acidification.

Terrestrial weathering is a near perfect carbon sequestration method. The process requires no energy input and any carbon captured is stored out of the atmosphere for millenia. Fortunately for us, the rocks involved are abundant. Unfortunately for us, terrestrial weathering occurs far too slowly in nature, removing only about 1.1 billion tonnes of carbon from the atmosphere per year. 

In order to intensify this process for our own benefit, we need only employ a basic trick from school chemistry that speeds up any chemical reaction: increase the surface area of the reactive elements. In nature, it takes eons for mountains, cliffs, and and landmasses to be weathered away and exposed to air. But we can speed this up by grinding rock, thereby increasing the surface area exposed to the air.

When I first learned about the potential of terrestrial weathering, I envisioned a Russia-sized area covered in boulders. In actuality, this would look more like coating huge areas of the earth in rock dust. This may sound impractical, and messy, but luckily for us silicate mineral dusts actually have a practical application as fertiliser, and have been used as such since the dawn of agriculture. Basalt, in particular, seems to be best mineral from a carbon sequestration and agricultural utility perspective and is also abundant on every continent. 

Image: Institute for Sustinable Energy and Environment

The efficacy of weathering is complex and depends on the minerals involved, local soil chemistry, temperature, and organic interaction with the chemical process. This means that it’s hard to say  how much carbon we can remove with weathering. Estimates vary widely, from 700 million tonnes per year to 95 billion tonnes per year, depending on the minerals involved. The IPCC seems to estimate that weathering can remove 2 to 4 billion tonnes per year, if we apply basalt rock powder to about 50% of the farmland of the USA, China, Brasil, and India. No small feat, but not impossible either, given that we already apply water, fertiliser, chemicals, and seed to far larger areas. If we extrapolate this out over the course of the next century, we are looking at between 200 and 400 billion tonnes of sequestered carbon using just 30% of the earth’s agricultural land. I am, of course, glossing over the huge effort it would take to mine, grind, and distribute all of this rock dust but because we already do this sort of thing at scale, I’m less concerned with the feasibility. We can mine a lot of minerals if we need to. 

My takeaway on terrestrial weathering is that we are incredibly lucky that the mineral composition of our planet, and the natural chemical reactions occurring on it, work in our favour this way. If all we have to do is learn more about how to effectively grind up the right rocks in the right places, I am confident that humanity can make this work as the bedrock (pun intended) of carbon sequestration efforts. 

In summary…

I haven’t explored the potential of other more exotic forms of carbon removal, like fertilising the oceans to encourage massive algae blooms, or widespread biomass removal and storage, because I don’t see these happening any time soon. From what I’ve been able to discern, the three methods of carbon sequestration outlined above are our best bets to undo climate change. 

At the outset, I mentioned 4 trillion tonnes of carbon as our removal goal. Summarising the potential of forests, DAC, and weathering, I’m left with the following best guesses: 

• If we really focus on reforestation, we might manage to remove somewhere between 10 and 100 billion tonnes of carbon over 100 years.

• If we apply enhanced weathering to all of the earth’s agricultural land, we could remove between 600 billion to 1.2 trillion tonnes of carbon over 100 years. 

• DAC technology isn’t yet on track to play a meaningful role, but if scaled up substantially, it seems that it could remove trillions of tonnes of carbon over the next 100 years over a relatively small land footprint. 

Mathematically, it looks like we could remove 1 trillion tonnes of carbon without much innovation or adjustment to our lifestyles, and probably more if DAC innovation goes well. I’m excited by this. As a species, we’re pretty bad at giving things up but pretty good at embracing new technologies that make our lives better. As the urgency of climate change ramps up I expect to see an increased focus on the nature-based and engineered carbon removal techniques I’ve discussed here. This will be an effort extending beyond the lifetime of anyone reading this in 2024, but it will be the most magnificent effort upon which our species has ever embarked. 

And we might achieve it all with trees, big fans, and ground up rocks.

Notes

• I have not factored in the carbon sequestration of existing forests, mineral weathering, and ocean algae. These systems surely improve our odds when combined with the potential methods of carbon sequestration outlined above.

• None of this is an argument against decarbonisation. If anything, the challenge of removing emissions drives home the need to reduce them as fast as possible.

• I am not a climate scientist and this was largely a thought expirement. My estimates may be way off.

• Much of what I've written about here is drawn from the works of Hannah Ritchie, James Jerden, and the IEA, and I wish to credit them for the research and influence on me.