The Thermostat You Can Break

Two models sit below. The first has no carbon in it at all: one planet, one temperature, and a sun you control with a slider. That is enough to produce the strangest fact about Snowball Earth, which is that the planet can have two different climates at the same brightness of sunlight and remembers which one it arrived in.

The second model adds carbon dioxide and lets the planet regulate itself. Turn the weathering up far enough and the regulation fails in a specific way: the planet stops settling anywhere and starts going round a loop instead. That loop is the claim in the paper this page accompanies.

Model 1: one planet, one temperature, one sun

The whole model is a balance sheet. Sunlight arrives, some fraction is bounced straight back to space (that fraction is the albedo), and the rest heats the surface. Infrared radiation leaves, faster when the surface is warmer. The climate sits wherever the incoming and outgoing curves cross.

The albedo depends on temperature, because a cold planet grows ice and ice is bright. That one dependence bends the incoming curve into an S shape, and an S can cross a straight line in three places rather than one. Two of those crossings are stable climates you could live on. The middle crossing is a knife edge: nudge it either way and it runs away from you.

Start at the Sturtian value of 1281 W/m² and press Dim the sun. Nothing dramatic happens for a long while. The planet cools a little, grows a little more ice, cools slightly faster because of it. Then the warm branch simply runs out of curve, there is nowhere left to stand, and the surface falls about seventy kelvin in a single step.

Now brighten the sun back to where you started. It does not come back. You have to push the sun well past its original value before the frozen branch runs out in its turn and the planet jumps the other way. Two thresholds rather than one: that gap is what a physicist means by hysteresis and what everyone else means by saying a system remembers its history.

Model 2: now let the carbon move

Carbon dioxide is not really a slider on Earth. It is a balance between volcanoes putting carbon into the air and the chemical weathering of silicate rock taking it out. Weathering speeds up when the planet is warm and wet, which is what makes it a thermostat. Heat the planet and it draws its own carbon dioxide back down.

Two things break that thermostat. Freezing the planet switches weathering off almost completely, because rain does not fall on ice. And burying a tropical continent under fresh basalt multiplies the weathering rate at any given temperature, because young volcanic rock dissolves far faster than old granite. The second slider below is that multiplier. The paper's proposal is that the Franklin basalts pushed it past the point where any stable climate exists.

Leave weatherability at 1 and the planet finds a temperature and keeps it. That is the ordinary behaviour of the carbonate-silicate thermostat, and it is why Earth has had liquid water on its surface for four billion years. Now drag the slider upward. Somewhere past three and a half, the balance point slides onto the unstable middle branch where nothing can rest, and the planet has no option left except to circle.

Watch what the outgassing slider does and does not do. Turning volcanoes down stretches the frozen half of every cycle, because the frozen half is nothing but the time volcanoes need to refill an atmosphere that has no weathering to empty it. Turning weatherability up shortens the warm half and barely touches the frozen one. There is a floor on how fast this planet can flicker, and volcanoes set it.