Olivine is a green mineral that reacts with CO2 in the ocean to form a harmless silt. This reaction might be the key to slowing down climate change, or reversing it altogether.
A long cycle – as powerful as it is slow – is at work in the rocks, rainwater and air of Earth. Over eons, it moderates the planet’s temperature, makes molehills of mountains, and smooths out fluctuations in the climate.
Consider the lowly carbon dioxide molecule. When released into the atmosphere from fossil fuel combustion or a volcanic eruption, a molecule of CO₂ will only stay in the air for four to five years on average. By that point, it is more likely than not to have become part of a plant via photosynthesis, or have dissolved into rainwater and flushed into the ocean. When the tree it was incorporated into decomposes, or it roils up to the surface of the sea on the crest of a wave, it enters the atmosphere again, ad infinitum.
But there’s a catch to this process: weathering. Dissolved CO₂, in the form of carbonic acid (H₂CO₃), makes rainwater slightly acidic – even in preindustrial times, the pH of rainwater was well below six – and acids react with the rocks that comprise the Earth’s surface. Most of our planet’s crust (about 90 percent) is composed of silicates, minerals formed by bonding metal atoms to one of a number of combinations of silicon and oxygen.
When rainfall hits a silicate molecule, the metal reacts with the dissolved carbonic acid to form a carbonate mineral (containing the carbonate ion CO₃⁻² or the closely related bicarbonate, H₂CO₃), abandoning the silicate ion to an independent existence as silicon dioxide (SiO₂) – plain beach sand. Since both the carbonate and the sand usually end up flowing down into the ocean with the rainwater that birthed them, the reaction of dissolved CO₂ with minerals is one of the primary forces that files jagged Himalayas and Andes down into soft, rounded Appalachians and Ozarks.
As is often the case in chemistry, other factors come into play. More acidic rainwater and warmer temperatures cause the weathering reaction to proceed faster than it otherwise would. The result, over a time period of thousands to millions of years, is that atmospheric CO₂ levels are self-correcting: when they rise (usually due to geological activity), the rate of weathering speeds up, which sequesters the excess carbon faster.
Under normal (i.e., preindustrial) conditions, this process is slow: volcanoes emit no more than 380 megatonnes, that is 380 million tonnes, of CO₂ into the atmosphere yearly, while natural weathering sequesters about 300 megatonnes. Atmospheric carbon levels rise (and fall) very slowly under normal circumstances, since one atmospheric ppm (part per million) of CO₂ represents about 7.8 gigatonnes (7,800 megatonnes), and some is absorbed by the oceans. At a yearly net increase of 80 megatonnes, it would take over thirty millennia for atmospheric CO₂ levels to rise as much as they have since the mid-eighteenth century. Even in the absence of volcanic activity it would take 130 years for natural weathering to remove even a single year of today’s emissions.
As this article went to press in May 2023, global temperatures were sitting at a little over one degree centigrade above the preindustrial average and closer to two degrees where most people live. While this sounds like a nuisance, the effects are stark and becoming increasingly clear every year – witness the 18°C high in Warsaw on New Year’s Day of this year (five degrees above the previous record for January), or the heat wave that struck China during the summer of 2022, during which overnight lows hovered at nearly 35°C and the country’s largest river, the Yangtze, ran dry. Though the truly apocalyptic scenarios of four or five degree warming feared in the early 2010s now look much less likely as the renewable energy transition accelerates, it is nearly certain that the Earth will warm to 1.5 degrees Celsius above preindustrial levels within the next couple of decades, and probable that we will reach a cumulative two-degree benchmark by the end of this century in the absence of much stronger emissions reductions efforts.
While merely reducing emissions will prevent additional future warming and its accompanying consequences, it won’t undo the damage that’s been done, at least not on the timescales to which humans need.
It’s worth recalling that sustained warming over pre-industrial levels did not start until around 1920, and cumulative warming did not hit the half-degree mark until about the turn of this century. Indeed, even if everyone on Earth stopped burning all fossil fuels tomorrow, temperatures could continue to rise for several decades, as there is a lag between the emission of carbon dioxide and its full effect on surface temperature (though there is a counterbalancing effect from carbon sinks that makes this judgment uncertain).
And while investment in renewable energy has been skyrocketing in recent years, over 80 percent of the world’s energy consumption still comes from fossil fuels. An immediate and abrupt collapse in carbon emissions would mean returning the vast majority of the world’s population to preindustrial living standards. In addition to being politically infeasible, it’s not even clear that the benefits of such a move would outweigh its moral costs. Without fossil fuels, for example, the Haber-Bosch process for making fertilizer, without which the world’s fields would only be able to feed about half as many people as they do today, is not possible.
Reducing carbon emissions to zero tomorrow is impossible, undesirable, and insufficient. The West will likely always need to use some fossil fuels, and developing countries will need to use some fossil fuels to continue growing their economies, but we should aim for levels of atmospheric carbon that are lower than today’s, within the preindustrial range. Together this means we need to think about an additional route: as well as finding ways to provide electricity, and power cars cheaply and reliably without fossil fuels, what if we removed excess CO₂ from the atmosphere as well?
When the term ‘carbon sequestration’ comes up, most people think of trees: purchase a carbon credit when booking a flight and, more likely than not, you’ve paid someone to plant a sapling somewhere.
Unfortunately, tree planting has serious disadvantages. Most significantly, its space requirements are immense. To reduce atmospheric CO₂ (currently about 418 ppm) by 100 ppm, within striking distance of the 280 ppm found in preindustrial times, you’d need to convert 900 million hectares to mature forest (an area about 94 percent the size of mainland China and 85 percent the size of Europe).
Even if that was possible, mature forests (which sequester more carbon in their soil than in their trees) take a long time to grow, and much if not most of the land available for reforestation is held by private actors, which creates significant political difficulties.
More promising solutions for direct-air capture are more likely to come from chemistry rather than biology. Several companies have broken ground in this field, such as Climeworks, Carbon Engineering and 1PointFive. All use a reusable sorbent, a chemical that reacts with CO₂ in the air and then releases it when energy is supplied (usually when it’s heated up). The captured, concentrated CO₂ is then pumped underground, where it is permanently trapped in geological formations in its gaseous, pressurized form, or mineralized into stable carbonates via reactions with the surrounding rock.
Sorbent-based direct-air capture is not a new idea, and is already used on space stations to moderate CO₂ levels. Like space applications, Climeworks uses an amine sorbent, which releases its captured CO₂ at a relatively low temperature (about 100°C). Unfortunately, amine-based sorbents are extraordinarily expensive – a study on the economics of amine-based sorbents published last year concluded that each tonne of CO₂ captured would incur hundreds of dollars merely in capital expenditure costs for the sorbent. Energy costs are not trivial, either: each tonne sequestered requires no less than 150 kilowatt-hours (kWh).
It is no coincidence that Climeworks operates in Iceland, because its active geology gives Climeworks access to ample carbon-free geothermal and hydro electricity at a very low cost. Even then, Climeworks currently charges €1,000 per tonne of CO₂ sequestered; its eventual goal is €600 a tonne. For comparison, the social cost of each additional tonne of CO₂ is currently thought to be somewhere around $185 (about €170 as of the time of writing), though getting an exact figure is devilishly tricky and the error bars are wide.
1PointFive and Carbon Engineering use potassium hydroxide as the sorbent, which is much cheaper than Climeworks’s amines, but the energy costs are almost as large. To regenerate potassium hydroxide, both companies use a process which includes heating a calciner (steel cylinder) up to 900°C. For Carbon Engineering, the cost of producing a concentrated stream of CO₂ was about $100-$200 a tonne as of 2018, not counting the cost of long–term sequestration.
Ultimately, solutions based on reusable sorbents suffer from a key drawback: once carbon dioxide has been absorbed in a chemical reaction, the resulting compound usually won’t give it back up in purified form unless lots of energy is added to the system. Moreover, sorbent-based processes merely produce a concentrated stream of CO₂, which must be stored (usually underground) or used.
This is easy for the first few thousand or even a million tonnes; for billions or trillions of tonnes, the logistics become nightmarish (though possible). Capturing a trillion tonnes of CO₂ (only 40 percent of humanity’s cumulative carbon emissions) via this process would require about eight times the world’s total yearly energy consumption merely to run the calciners. It could be a small useful addition to our carbon mitigation strategy, but it’s unlikely to help us roll back to a preindustrial environment.
If carbon capture with reusable sorbents is astronomically costly, at least for the time being, could we use a non-regenerating sorbent – something that absorbs CO₂ and locks it away for good?
There is a trade-off here. While we’d save the energy costs of cycling the sorbent and storing gaseous CO₂, we’d also need to produce and store truly massive amounts of sorbent. The alternatives would have to be easily available or cheaply manufactured in vast quantities; and because of the storage requirements (reaching into the trillions of tonnes) the compound would need to be non-toxic and environmentally inert. Processing the substance should require relatively little energy, and its reaction with ambient CO₂ needs to operate quickly.
The idea that silicate minerals might be able to fill this role is not, in and of itself, a new one; the earliest proposal of which I am aware is a three–paragraph letter to the editor in the 1990 issue of Nature, proposing that pressurized CO₂ be pumped into a container of water and silicates; five years later, the journal Energy published a somewhat longer outline for carbon sequestration using several intermediate steps. Neither idea went terribly far; popular activism focused on reducing emissions rather than sequestering them, and ideas published in academic journals remained mostly of academic interest.
In 2007, however, the Dutch press began entertaining a rather more sensational idea: the Netherlands’s, and perhaps the world’s, carbon emissions could be effectively and cheaply offset by spreading huge amounts of ground olivine rock – a commonly found, mostly worthless silicate rock composed mainly of forsterite, Mg₂SiO₄ – onto the shores of the North Sea, producing mile after aesthetically intriguing mile of green sand beaches as a side effect. The author of the proposal, Olaf Schuiling, envisioned repurposing thousands of tankers and trucks to ship ground rock from mines in Norway, covering the coast of the North Sea with shimmering golden-green sand and saving the human race from the consequences of the Industrial Revolution.
It seemed too good to be true – so in 2009 the geoscientists Suzanne Hangx and Chris Spiers published a rebuttal. While it was true that ground forsterite has significant sequestration potential on paper (each tonne of forsterite ultimately sequestering 1.25 tonnes of CO₂), Hangx and Spiers concluded that the logistics of Schuiling’s proposal would make the project an unworkable boondoggle.
Start with transport requirements. For the past two decades, the Netherlands has emitted about 170 megatonnes of CO₂ a year on average; each year, around 136 megatonnes of olivine would be needed to sequester Dutch emissions in full. The nearest major olivine mine, Gusdal, is located in Norway, around a thousand kilometers away. Transporting the required olivine by sea with the most commonly-used cargo ship (the $150 million Handysize vessel, with a capacity of about 25 kilotonnes) for example, would require over 100 trips a week – five percent of the world’s Handysize fleet – further clogging some of the world’s busiest waters for shipping. And that’s just for the Netherlands, which is only responsible for about 0.5 percent of the world’s carbon emissions.
Then there’s the environmental angle. While forsterite on its own is harmless, olivine usually contains trace amounts of other minerals and heavy metals, most prominently nickel, whose effect on marine life, while understudied, is known to be less than benign.
But the real Achilles heels of the Schuiling proposal were matters of physics. The rate of rock weathering is, to a first approximation, a function of three variables: the concentration of CO₂ in the water, the ambient temperature, and (most importantly by far) particle size. While CO₂ concentration in surface ocean water is about the same everywhere, temperature is not: sequestration by forsterite is about three times faster at 25°C (the approximate water temperature off the coast of Miami) than at 15°C (the average in the North Sea). But there’s another problem: olivine needs to be extremely small to weather effectively. Hangx and Spiers estimated that olivine particles 300 microns in diameter (the average size of a grain of beach sand) would take about 144 years to finish half their potential sequestration, and seven centuries to react completely. Bring out the saplings.
But what if the problems with Schuiling’s idea were in the execution, not the concept? The Intergovernmental Panel on Climate Change (or IPCC), the world’s most authoritative body on the problem, takes the climate and atmosphere of 1750 – when the atmosphere was about 280 ppm CO₂ – as its starting point. What would it take to return to this point?
Since that time, humanity has pumped a little over two trillion tonnes of CO₂ into the atmosphere, which would require about 1.6 trillion tonnes of raw olivine to sequester. You can imagine this as a cube measuring about eight kilometers or five miles on each side. Luckily for us, sources of high-quality olivine are fairly common, bordering on ubiquitous; and because it’s not (yet) very economically valuable, most deposits haven’t been thoroughly mapped. Assuming we’re simply trying to speed up natural processes, the end destination for the olivine will likely be the ocean.
Rock weathering takes place only where the rock is exposed to the elements; a gigantic pile of olivine is only as good as its surface area, and the only way to increase surface area is to break the rock into smaller particles. If you halve the size of your particles, the surface area available is doubled at worst, and you sequester carbon at least twice as quickly (the exact proportion will depend on how many cracks and crevices there are in the breakage – the more jagged the particles, the more surface area and the faster sequestration proceeds). To get back to preindustrial concentrations on a time scale of decades, we’d want to process a lot of olivine and break it down into very small particles – not sand, which (with diameters in the hundreds of microns) is too large, but silt (with diameters in the 10-50 micron range).
What would it take to start making a serious dent in atmospheric CO₂? Say we shot for 80 gigatonnes of olivine a year, locking away 100 gigatonnes of the stuff when fully weathered. Unlike many proposals for carbon sequestration, olivine intervention is not contingent on undiscovered or nascent technology. Let’s take a look at the process through the lens of an increasingly small grain of rock.
Our particle of olivine would begin its journey on a morning much like every day of the past hundreds of millions of years; it is part of a large deposit in the hills of Suluwesi, a fifteen-minute drive from the coast. (Indonesia is particularly well-suited for processing due to its vast expanse of shallow, tropical seas, but the ubiquity of olivine formations means that sequestration could happen in any number of places.)
This particular morning, however, is different. A mining worker has drilled a hole into the exposed surface of the formation, inserted a blasting cap, and – with a loud bang – smashed another fraction of the rock into pieces small enough to be carried by an excavator. The largest excavators in common use, which cost a bit under two million dollars each, can load about 70 tonnes at a time – a small, but important, fraction of the 220 megatonnes or so the world would need to process that day. Each of several hundred excavators takes no more than a minute or so to load up, complete a full trip to the haul truck, and come back to the front lines. It’s probably cheapest to run it, and the rest of the mining equipment, on diesel; even though it guzzles nearly 200 liters (50 gallons) an hour, the rock it carries will repay its five-tonne-a-day CO₂ footprint tens of thousands of times over.
Our grain of olivine (now part of a chunk the size of a briefcase) is off on a quick trip to the main processing facility in one of about a few thousand haul trucks (each costing nearly five million dollars and carrying up to 400 tonnes at a time), where it’s subjected to a thorough pummeling until it’s reached pebble size. Then it’s off to a succession of rock mills to grind it down to the minuscule size needed for it to weather quickly.
It’s a good idea, at this point, to talk a bit about the main costs involved in such an immense proposal. As a rule of thumb, the smaller you want your end particles to be, the more expensive it is to get them there. Once a suitable olivine formation has been located, quarrying rock out of the formation is cheap. Even in high-income countries like Australia or Canada where mine workers make top-notch salaries, the cost of quarrying rock and crushing it down to gravel size is generally on the order of two to three dollars a tonne, and it requires very little energy. Since reversing global warming would entail the biggest quarrying operation in history, we might well expect costs to drop further.
Depending on the deposit, haul trucks might prove unnecessary; it may be most cost-effective to have the crusher and mills follow the front lines. The wonderful thing about paying people to mill rocks is that we don’t have to know for sure from our armchair; the engineers tasked with keeping expenses to a minimum will figure it out as they go.
What is quite certain is that the vast majority of that expense, both financially and in terms of energy, comes not from mining or crushing but from milling the crushed rock down to particle size. Hangx and Spiers (the olivine skeptics above) estimated milling costs for end particles of various sizes; while sand-sized grains (300 microns across) required around eight kWh of energy per tonne of olivine processed, grains with a diameter of 37 microns were projected to need nearly three times as much energy input, and ten-micron grains a whopping 174 kWh per tonne. Since wholesale electricity prices worldwide are about 15 cents per kWh, that implies an energy cost of around $26 per tonne of olivine, or about $20 per tonne sequestered – at least $1.2 trillion a year, in other words, and a ten percent increase in the world’s electricity consumption. Can we do any better?
Energy costs for milling olivine at various grain sizes.
|Final Grain Size (microns)||Energy cost per tonne milled (in kilowatt-hours)||Cost to mill one tonne at $0.10/kWh||Energy cost of milling 80 gigatonnes of olivine yearly, in terawatt-hours||Approximately equivalent to the yearly energy consumption of:|
|1000||4.94||$0.49||395.2||Maryland or Romania|
|300||8.17||$0.82||653.6||Algeria or New Jersey|
|100||13.4||$1.34||1,072||Pakistan or Ohio|
|10||173.71||$17.37||13,896||Half the total energy consumption of the United States|
We probably can; it matters a lot, it turns out, what kind of rock mill you use. For example, while Hangx and Spiers assumed the use of a stirred media detritor (SMD) mill for the ten-micron silt, other researchers showed that a wet-attrition miller (WAM), working on equal amounts of rock and water, could achieve an average particle size of under four-microns for an all-inclusive energy cost of 61 kWh ($9.15) per tonne of rock – about $7.32 per sequestered tonne of CO₂, or around $732 billion a year in energy costs.
And the largest rock mills are large indeed; the biggest on the market can process tens of thousands of tonnes a day. It should be clear by now that capital expenditures, while not irrelevant, are small compared to the cost of energy. Though there’s no way to know for sure until and unless the sequestration industry reaches maturity, a reasonable upper estimate for capital investment is about $1.60 per tonne of CO₂ sequestered, giving a total cost per sequestered tonne of no more than nine dollars. The resulting bill of $900 billion per year might sound gargantuan – but it’s worth remembering that the world economy is a hundred-trillion-dollar-a-year behemoth, and each tonne of carbon dioxide not sequestered is more than 20 times as costly.
Upon its exit from the mill, our particle, now just five to ten microns in diameter, finds itself in a fine slurry, half water by mass. Silicates usually find their way down to the ocean via rivers, so we’ll have to build our own. Thankfully, the water requirements are not high in the grand scheme of things. 80 gigatonnes of rock a year will need about 2300 cubic meters of water a second; split across dozens of mines worldwide, water requirements can easily be met by drawing from rivers or, in a pinch, desalinating ocean water.
The slurry is pumped into a large concrete pipe (since it’s flowing downhill, energy costs are minimal), and our particle of magnesium silicate comes to rest on the ocean floor of the Java Sea, where it reacts with dissolved carbon dioxide and locks it away as magnesium bicarbonate within a few years. (Because the Java Sea is shallow, it is constantly replenished with atmospheric CO₂ from rainwater and ocean currents. Carbon in the deep ocean is cycled at a far slower pace.)
While there are a handful of trace minerals in most olivine formations, especially nickel and iron, the ecological costs are local and pale in comparison to the global ecological costs of global warming and ocean acidification.
The proposal I have outlined is – for now – a carefully estimated thought experiment drawn from existing data, not an empirical report on costs from the front lines of a hundred-gigatonne-a-year sequestration megaproject. It is not unreasonable for readers to regard some of my numbers as a little rough around the edges. But even if energy costs are off by a factor of two, the conclusion seems inescapable: one promising, comparably cheap, and relatively easy pathway to reversing global warming and returning to the cooler, more stable world of the second millennium is by deploying Earth’s natural sequestration process at industrial scale, using technology we already have, at a cost that might feasibly clock in at less than one percent of the world’s GDP.
There is a larger point to be made, however, about the political reality of climate change. Traditionally, solving the problem of climate change has been conceived of as a problem of decarbonization. Processes that traditionally burned coal and oil must be switched to cleaner sources of energy; where this is difficult, countries and individuals must learn to make do without, by eating less meat, taking the metro to work, or consuming less electricity. It would be unfair to say that this approach has failed entirely; total emissions in almost all developed countries are trending downward due to the falling costs of renewable energy and improvements in energy efficiency.
But decarbonizing the entire world economy is another matter entirely. It is harsh – but probably fair – to say that it is likely a political nonstarter without major technological breakthroughs that may not happen as quickly as we need them. Some sectors are harder to decarbonize than others, and many of the sectors most difficult to decarbonize are precisely those that have done the most to drive the industrialization of middle-income countries and lifted hundreds of millions of people out of poverty. Take construction, for example: ten percent of the world’s carbon emissions come from iron, steel, and cement, and the chemistry of their manufacture makes avoiding fossil fuels difficult, at least for now. The production of fertilizer for the world’s farms is similarly constrained by basic chemistry, and despite heroic decreases in the costs of renewable electricity and electric vehicles, the developing world’s existing power plants and trucks will hang on for some time – if for no other reason than that there is not yet enough manufacturing capacity to provide enough replacements, and building that capacity will require additional carbon emissions in the short run.
So full decarbonization on the short time scale envisioned by many activists would not primarily be a matter of Americans buying Teslas or Swedes foregoing cheap flights to Barcelona. Its costs would, on the contrary, be borne by the world’s broad majority: the Indian farmer facing a tenfold jump in the cost of fertilizer; the Vietnamese factory worker out of a job for lack of sufficient electricity; the slum-dweller of Karachi or Lagos whose dreams of a clean, modern apartment are dashed; a Chinese freight truck driver reduced to destitution. If carbon emissions stopped tomorrow, or even five years from now, the human toll would be immense, quite possibly entailing the largest famine in history – and while developed countries might pull through, devastated supply chains would ensure crippling economic depression. And because past emissions stick around for a long time, the Earth wouldn’t return to the cooler, more stable climate of the twentieth century for many millennia.
Complete decarbonisation will be difficult for a long time. Complete and rapid decarbonisation would entail the death of half the planet. And in the long run, we should aim not just for net zero, but net negative emissions, to return us to the safer climate of the preindustrial era. All of these militate towards not just reducing the carbon we produce, but sucking it out of the air too.
Seen in this light, fixing global warming is a simple matter of applying energy to an unfavorable material situation, much as the refrigerator eliminated age-old problems of food spoilage and the electric streetcar the problem of manure-covered cities. Indeed, while the energy costs of processing rock are huge in absolute terms, the sequestration return on energy invested is so forgiving that the process itself doesn’t even have to decarbonize. Even if we made a mildly pessimistic assumption of 100 kWh of processing energy per tonne of rock (80 kWh per tonne of processed CO₂), we’d find that we could power the whole thing on fossil fuels without too much trouble; even a coal plant would only produce emissions equal to eight percent of the sequestered carbon, and a modern natural gas plant four percent.
But even this underscores the massive importance of cheap, abundant energy – the universal currency. As we’ve seen above, the fixed capital costs of an industrial-scale weathering program are small – the vast majority of expense, for now, is energy. With the ballpark figures per tonne sequestered of $1.60 in capital costs and 80 kWh of energy, total yearly costs for a 100-gigatonne-a-year sequestration program are estimated below.
Climate change is a difficult problem – but one in the same category as the problems of pumping water out of mines, refining aluminum from its ores, or ensuring the world’s fields have enough nitrogen to feed its people. What once seemed impossible, with sufficiently abundant energy and a bit of ingenuity, could become a bargain; problems of coordination and cost-sharing can be ignored if those costs are low enough.
Suppose, for example, that the developing world refused to contribute, arguing very reasonably that domestic economic development was a better use of their public funding than direct-air capture. At 15-cents-a-kWh, the project would cost about $1,200 per person in the developed world per year; but at four or five cents, yearly per person costs drop to a few hundred dollars. There is still a risk that voters, especially the elderly who increasingly dominate electoral politics in wealthy countries, might balk at the tax bill. Even then, however, there is good precedent for using long-term bonds to finance the response to existential threats. If retirees in the rich world have no interest in footing the bill, why make them? Their grandchildren gladly will.
For that matter, sufficiently cheap sequestration might not even need to be paid for primarily by governments. Plenty of private-sector institutions have good reason to protect investments in low-lying coastal areas, after all, and the sort of people who enjoy skiing in Aspen or Zermatt tend to be disproportionately well-heeled. Globally, more than $600 billion dollars is already spent on climate mitigation and adaptation. Since climate change will not be reversed overnight, even with a mature sequestration program in place, there will still be a place for investments in levees and crop management. The point is that a large-scale sequestration effort would be well within range of what the world already spends on adapting to climate change.
How long it would take to reach preindustrial CO₂ levels, though, would depend on a number of variables, including how quickly processing can scale up, the rate at which emissions fall (or don’t), and whether or not there will be some processes that simply cannot be decarbonized affordably. I wrote a short Python script to calculate how many years it would take to reach preindustrial levels given a number of variables. The following graphs illustrate a range of potential scenarios, based on rates of sequestration and decarbonization, with the following assumptions:
- At year zero – when sequestration begins – there are 2.7 trillion tonnes of CO₂ to be sequestered. Humanity emits 60 gigatonnes that year, and 0.5 gigatonnes are sequestered.
- The amount sequestered doubles every 18 months until the target is reached.
Note that, given the world’s extensive olivine deposits, there is no strong reason to believe that 100 gigatonnes a year is a hard upper limit as opposed to 200 gigatonnes or even 400 gigatonnes. There may be political upper limits in the form of the willingness of governments and citizens to finance the venture quickly, which is one reason why very long-term bonds may be the best way to go, since the faster sequestration can get going, the less damage global warming will do. If we assume there is no upper limit to the rate of sequestration, then the trip back to the preindustrial atmosphere depends on the growth rate of mining and processing capacity.
The following graph shows the time to 280 ppm for a range of potential doubling times, with 500 megatonnes sequestered the first year, assuming a relatively slow decarbonization rate of 0.5 gigatonnes yearly. As usual with exponential growth, even small increases in the growth rate can yield significant savings in time and ecological damage.
Time to 280 ppm, assuming 500 megatonnes were sequestered in the first year, given a range of doubling times
|Doubling time||Time to 280 ppm||Notes|
|10 years||99 years|
|5 years||53 years|
|4 years||44 years|
|3 years||34 years|
|2.7 years||31 years||The growth rate of photovoltaics|
|2 years||24 years||Approximately the current growth rate of utility-scale batteries|
|18 months||19 years|
|12 months||13 years||The current growth rate of electric cars|
|9 months||10 years||The growth rate of lithium mining in the mid-2010s|
The path forward should be clear. Cleaning up the atmosphere will pay for itself in the long term, so capital to do so should be made freely available until the atmosphere returns to preindustrial CO₂ concentrations. While there may be concerns about fiscal irresponsibility, the debt that’s needed will be cheaper if it’s paid off over a long period of time.
If this seems too good to be true, it’s worth recalling that many institutions and governments will be selling bonds as a form of insurance policy; interest rates have been declining for centuries and are unlikely to rise very much over the course of the coming century, given the world’s rising number of retirees. So long as the yearly economic damage to an institution or country from unmitigated climate change would be higher than the cost of coupon payments, its sequestration bonds should have no trouble finding buyers. Indeed, I suspect the only reason this hasn’t happened yet is that investors know, deep down, that tree planting isn’t going to cut it. While decarbonization remains necessary, it is neither sufficient nor the best immediate approach in all cases: resources should be spent where they will do the most good.
Is spreading olivine silt in the ocean a complete panacea for our environmental ills? No – no single thing can be. Even after CO₂ levels return to preindustrial ranges, some measure of lasting damage will remain, and climate change isn’t the only environmental change the planet faces – it’s just the most significant. Ground silicates on their own won’t do anything about plastic pollution, deforestation, invasive species or fertilizer runoff; other solutions must be sought for these, and there is no true substitute for not polluting in the first place.
In an ideal universe, we’d have instituted a worldwide carbon tax in the mid-twentieth century, or at least traveled back in time to warn our forefathers about the long-term costs of oil and coal. But while we can’t hop in a time machine to the 1950s, we may be able to turn the clock back on the atmosphere and undo some of the damage done since then. It may, at least, be worth trying.