Last time I promised you a thermostat with no thermostat in it.
Here are its three moving parts: rain, rock, volcanoes.
Rain falls with a little carbon dioxide dissolved in it, which makes it faintly acid. That weak acid eats at silicate rock — the ordinary granite-and-basalt stuff continents are made of — and carries calcium and magnesium off in solution. Rivers deliver them to the sea, where they meet dissolved carbonate and settle as limestone. Tens of millions of years later that seafloor slides down a trench, gets cooked, and hands the carbon dioxide back to the sky through a volcano.
Nothing in the loop is stored. A carbon atom enters the air, waits a few hundred thousand years, and leaves as stone. The amount holds; the atoms do not.
Why that is a thermostat and not merely a circuit
Rock dissolves faster when it is warm, and faster when it is wet.
That sentence is the whole mechanism. In 1981 James Walker, Paul Hays and James Kasting wrote the paper that assembled it, and its title is the argument: A negative feedback mechanism for the long-term stabilization of Earth’s surface temperature.
Follow it round. Carbon dioxide warms the planet. A warmer, wetter planet weathers its rock faster. Faster weathering buries carbon dioxide faster. The gas doing the warming is removed at a rate that climbs with the warmth — while the volcanoes refill the air at their own indifferent pace.
The reason to think it has been running is that we are here to ask. The sun has brightened by something like a third since the Earth formed; on the physics alone the early ocean should have been ice, and the rock record says it was water, back to 3.8 billion years ago. Sagan and Mullen posed that in 1972. Walker’s answer: carbon dioxide stood high when the sun was faint and fell as the sun brightened, because the loop drew it down. He was careful to add that his own version got only about halfway there.
The number that actually defines it
Everyone stops at Earth has a thermostat. The interesting part is its clock, and the clock is not a property of the chemistry at all.
Walker’s paper puts the volcanic and metamorphic supply at 3.3 × 10<sup>14</sup> grams of CO₂ a year, and notes in passing that this is enough to double all the carbon in the ocean and atmosphere in four hundred thousand years. That aside is the answer. The response time is the size of the tank over the rate of the tap. A thermostat cannot act faster than its own plumbing.
Thirty-four years later Colbourn, Ridgwell and Lenton put the loop inside a full Earth-system model and measured the recovery. Silicate weathering pulls an excess of carbon dioxide down with an e-folding time of about 240,000 years — anywhere from 170,000 to 380,000. A back-of-envelope from 1981 and a model ensemble from 2015, sharing almost no assumptions, land on the same few hundred thousand years, because both are weighing the same reservoir against the same flux.
Some of the planet is not listening
The live argument is not whether the loop exists. It is how much of the land is wired into it.
Weathering runs into two different limits. Where erosion strips the surface fast — mountains, young steep ground — fresh mineral is always exposed, and the rate is set by chemistry, which is to say by heat and rain. Where erosion is slow, the rock is weathered to depth already and the rate is set by how fast new material arrives from below. Warm that ground and little happens; there is not much left to dissolve.
West, Galy and Bickle showed in 2005 that both regimes occur in the field, and drew the conclusion that matters: only the fast-eroding fraction responds to climate at all. The thermostat’s strength is diluted by however much of the world’s weathering happens on ground that isn’t listening — and a change in climate moves the border between the two.
What the lag is for
No. 116’s reactor is steerable because its feedback is slow: eighty milliseconds is a delay a person can work inside. This is the same structure with the decimal point moved fifteen places, and the delay does the opposite job.
The archive’s other self-repairing atmospheric loop, the ozone layer of No. 29, resets in hours. No. 86’s atmospheric carbon-14 sits between a cosmic-ray factory and a deep-ocean graveyard, but neither of those rates cares what the level is — that is a balance, not a thermostat. What makes this one a thermostat is that the drain widens as the tank fills.
What makes it useless is only how slowly it widens. It is the most successful control system on this planet, it has held an ocean liquid through the entire brightening of the sun, and it has never once run fast enough for anything with a heartbeat to watch.
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One loop I’m watching
Next: a machine that gets harder to start the longer it has been off, and hardest of all a few hours after you stopped it — a negative feedback carrying so much delay that the correction comes back with the wrong sign, and a control room learns to wait it out. Next time.