Silicate weathering thermostat

Why has Earth kept liquid oceans for billions of years while the Sun grew steadily brighter?

Stellar models say the young Sun was substantially fainter than today's — of the order of thirty percent fainter four billion years ago — and brightened steadily since. Take the Earth's present greenhouse and dim the Sun by that much and the calculation freezes the oceans solid. Yet the rock record shows liquid water almost as far back as there are rocks to read.

So either the models of the Sun are wrong, which is unlikely given how well they account for other stars, or the Earth's greenhouse was much stronger then and has weakened by roughly the amount the Sun strengthened. That coincidence is what needs explaining, and a one-off accident will not do: the compensation had to keep pace with a slow forcing over billions of years.

Something that adjusts to conditions rather than being fixed by them. So ask which part of the carbon cycle could be temperature-sensitive.

Carbon dioxide enters the atmosphere from volcanoes and metamorphism — gas driven off carbonate rock as it is subducted and heated. Ask what sets that rate, and the answer is tectonics: how fast plates converge, what is being subducted. It responds to the Earth's interior, not to the weather at the surface. Hold that thought, because it means degassing is the forcing in this system, not the regulator.

Now the removal side. Rain dissolves a little carbon dioxide and becomes weakly acidic, and that acid attacks rock. Here the chemistry has to be read carefully, because two kinds of rock behave completely differently. Dissolve limestone and the carbon released from the rock joins the carbon from the air in the river; when the ocean later precipitates that back as new limestone, exactly what was borrowed is returned. Over long times, carbonate weathering nets nothing.

Silicate rock is different, because it contains no carbon of its own. Weathering a calcium silicate consumes two molecules of carbon dioxide and delivers a calcium ion and two bicarbonate ions to the sea. When shells or carbonate mud precipitate from those, one carbon dioxide goes back into the water and one calcium carbonate is buried. Two in, one out: the net is one molecule of carbon dioxide taken out of the air and locked into rock for every calcium atom weathered out of a silicate. That asymmetry is the entire sink.

Now, is that sink temperature-sensitive? It has two reasons to be. Chemical reactions run faster when warm, and a warmer world evaporates and rains more, so more water passes through more rock. Both push the same way. So put the pieces together and ask what happens if carbon dioxide rises: the world warms, weathering quickens, carbon is pulled out faster than before, and the rise is arrested. If carbon dioxide falls, the world cools, weathering slows, volcanic input outruns removal, and the fall is arrested. The system does not hold a fixed temperature; it holds whatever temperature makes the weathering flux equal the degassing flux. As the Sun brightened, the temperature that satisfied that balance was reached at a lower and lower carbon dioxide level — and so the greenhouse thinned as the Sun strengthened, without anything having to coordinate the two.

Notice which part is forcing and which is feedback. Solar brightening and volcanic degassing are imposed; weathering is the response. Confusing the two would make the whole thing look like a coincidence again.

Then the question that matters most for us: how fast does the thermostat act? Divide the carbon held in the atmosphere and ocean together by the rate at which silicate weathering removes it. Both terms are known only to an order of magnitude and published values differ, so take them as such: tens of thousands of gigatonnes of carbon in the reservoir, of order a tenth of a gigatonne a year removed. The quotient is a few hundred thousand years. That is the thermostat's response time, and it is why the mechanism that has kept the oceans liquid since the Archaean is of no practical relevance to the next several thousand years of climate.

Three refinements, each of which corrects a tempting over-reading.

First, this is a negative feedback, not a guarantee. A feedback that acts over a hundred thousand years cannot prevent an excursion shorter than that; it can only end one. The Snowball Earth episodes are the instructive case. If ice covers the continents, weathering nearly stops while volcanoes keep degassing, so carbon dioxide accumulates for millions of years until the greenhouse finally overwhelms the ice. That is the thermostat working — and it worked by way of a globally glaciated planet, which is not the behaviour of a system that keeps things comfortable.

Second, the strength of the feedback is genuinely contested. The version above assumes weathering is limited by reaction kinetics, so temperature governs it. But weathering can instead be limited by supply: if erosion is slow, the rock at the surface is already leached, and no amount of warmth or rain will extract more from it. On that reading, mountain-building — which exposes fresh rock — matters as much as temperature, and it was proposed decades ago that Himalayan uplift drove the long Cenozoic cooling. Which limitation dominates, and where, is still argued, and the answer changes how strong the thermostat is.

Third, how would we know any of this is right? The clean prediction is about timescale: inject a large slug of carbon quickly, and the excess should be drawn down over roughly a hundred thousand years, not a thousand and not ten million. The Palaeocene-Eocene Thermal Maximum, around fifty-six million years ago, is the natural experiment — a rapid carbon release, an abrupt warming, and a recovery of the carbon isotope record over something like a hundred to two hundred thousand years. That is the right order. Had the recovery taken a few thousand years, some faster sink would have to exist; had it taken tens of millions, the feedback would have to be far weaker than supposed.

• Why enhanced rock weathering is being investigated as a deliberate carbon removal method, and what crushing rock does to that timescale.

Every term the collection defines is gathered in the glossary.