Six places to park carbon, ranked by the lease

Ask anyone selling carbon removal how much their method stores and you'll get a crisp number in tonnes. Ask how long it stays stored and the room goes quiet. That second number is the one that matters, and it is a materials question, which happens to be my whole job. Every material has a service life — a bridge deck, a gasket, a tonne of carbon. So here are six storage media I find quietly wonderful, ranked not by capacity or cost but by lease length: how long, roughly, the carbon stays put.

1. Basalt, freshly carbonated

Inject CO2 dissolved in water into basalt and the gas stops being a gas: it reacts with the calcium and magnesium silicates and becomes carbonate minerals, plain rock. A much-cited field trial in Iceland reported that most of the injected CO2 had mineralized within a couple of years — faster than almost anyone predicted. The lease is geological. That carbonate is stable for as long as the rock is, which is the only term long enough to stop negotiating with the atmosphere every few decades.

2. Olivine ground into sand

Olivine is a magnesium silicate, and weathering it is a lovely bit of chemistry: the rock consumes CO2 and gives you magnesium carbonate plus silica. Spread finely crushed olivine on farmland or beaches and rain does the work. In theory a tonne of the stuff can bind on the order of a tonne of CO2, depending on its magnesium content. The honest catch: measured field weathering rates are slower and more contested than the lab numbers, and rock sitting in a quarry bag has stored exactly nothing.

3. Peat, kept wet and left alone

Waterlogged ground has no oxygen at depth, so dead plants don't fully decay — they just stack up, a millimeter or so a year, for millennia. Peatlands are often cited as covering only a few percent of land while holding something like a third of the world's soil carbon; treat both figures as ballparks. The permanence clause here is written in water: keep the water table high and the carbon sits for thousands of years, drain it and oxidation hands it back to the sky within decades.

4. Biochar

Char biomass in low oxygen and you rearrange its carbon into fused aromatic rings — structurally the difference between a salad and charcoal briquette. Soil microbes that would dismantle a cornstalk in a season find those rings nearly indigestible, which is why charred carbon is routinely described as persisting for centuries, with real variation by feedstock and soil. I like biochar because the mechanism is visible in the material: charring raises the activation energy of decay, and chemistry is stubborn about activation energies.

5. Structural timber

Dry wood is roughly half carbon by mass, so a mass-timber building is a carbon vault with a fire rating and a lease of decades — the life of the structure, give or take. What earns it a spot above that is the renewal option: harvest, build, regrow the forest, and you can re-sign the lease with a new crop of trees. It's not permanent storage; it's a rolling subscription, which is weaker than basalt but far better than burning the feedstock outright.

6. Old concrete, recarbonating

Cement making is often credited with around eight percent of global CO2 emissions — the accounting varies, but everyone agrees it's embarrassingly large. The consolation prize: concrete slowly absorbs CO2 back. Carbon dioxide diffuses in, reacts with calcium compounds, and forms calcium carbonate right in the pore structure; crushed demolition waste, with all its fresh surfaces, does this faster. Estimates of how much gets reabsorbed range from a few percent to something like a third depending on assumptions. Concrete is the only item on this list atoning for its own manufacture.

My opinion, from someone who thinks about service lives professionally: a tonne held for thirty years is not a tonne held for ten thousand, and carbon accounting that ignores lease length is a spec sheet with the time column deleted. Permafrost is the cautionary tale on the other side — carbon stored for tens of thousands of years is currently renegotiating its terms. Ask about the lease first. The tonnage is the easy part.