Who sets the thermostat

In 1991 a volcano in the Philippines cooled the whole planet for about a year. Two novelists asked what happens when people do that on purpose: a Texas billionaire with a sulfur gun, and a country that has just buried its heat-wave dead. This page lets you turn the dial, and then shows what happens when someone lets go of it.

Sulfur in the sky can cool the planet within a year, but it only hides the heat while the carbon stays up there. So the first job for a world working on one shared mission isn't the spraying. It's the rules: who may start, who may stop, and who pays the people downwind.

Pick an emissions path, then set how much sulfur goes up each year and when it stops. Try each “who decides” rulebook, then write your own. Drag the year bar or press play to read any year. The bar follows you down the page.

The volcano that cooled the world

Mount Pinatubo erupted in the Philippines in June 1991. It put roughly 15 to 20 million tonnes of sulfur dioxide into the stratosphere, the calm layer of air above the weather. There the gas turned into a haze of tiny droplets that bounced a little sunlight back to space.

Over 1991 to 1993, the world's average surface temperature fell by about half a degree Celsius. Then the droplets drifted down and rained out, and the warming came back.

Set the dial to “Pinatubo” and the blue line drops by well over a degree. That's one Pinatubo's worth of sulfur, every year, for as long as you keep it up.

Takeaway.We've seen it work, once, for about a year. It lasts only as long as someone keeps putting it back.

Two hands on the dial

The roadmap fixes the air two ways: Robinson's plan on the ground and Suarez's plan in space. Both take decades. Two novels ask what happens in the years before they work, and both reach for the same quick, cheap and dangerous answer.

The two novels argue from opposite ends: a rich man acting for the countries that will drown, and a poor country acting for its own dead. Neither waits for permission. This page doesn't decide which of them was right. Try both rulebooks on the dial: the physics is the same. Only the people carrying the risk change.

Takeaway.In both books, the people who acted were the ones who couldn't wait.

What the sulfur doesn't do

It doesn't touch the carbon. The CO₂ stays in the air, and the ocean keeps soaking it up and turning more acidic. The IPCC rates this with high confidence: sulfur would not stop CO₂ from rising or the ocean acidifying that comes with it. Watch the green readout on the dial. No setting of the sulfur moves it.

It doesn't fall evenly. Cooling the planet changes where the rain goes. Model studies find sulfur in the stratosphere could weaken or shift India's summer monsoon, drying much of the country, with the size of the effect depending on where the sulfur goes up. Stopping suddenly snaps the monsoon back toward its warmed state. These are models, not measurements, and they are exactly the risk India fights in Termination Shock.

It can't simply stop. This is the termination shock that gives Stephenson's book its title. Sulfur falls out of the sky in a year or two. If the spraying ends while the carbon is still up there, the hidden warming comes back within about a decade. In one model study it came back at about 1.2 °C per decade on a high-emissions path, and under 0.4 °C per decade on a low one. Today the world warms about 0.27 °C per decade.

Try it. Pick “Roughly as now,” press “Hold 1.5 °C,” and stop the spraying in 2070. Then switch to “Fast cuts and removal” and do it again. On the fast path there's almost nothing to hide, so there's almost nothing to come back.

Takeaway.You can't stop once you've started, unless the carbon is already coming down.

The rules so far

“The great question of our time is no longer whether climate change is happening but what to do about it.”

Neal Stephenson, Termination Shock, loc 8092

Like the discount rate on What a child is worth in 2100, this is a dial someone gets to set. Here is what the real world has written down about who that is, as of October 2026.

1. 1977The ENMOD Convention opens for signature in Geneva. It bans military or other hostile use of techniques that change the environment on a large scale. It says nothing against peaceful use. It has 78 parties.
2. 2010The Convention on Biological Diversity, meeting in Nagoya, adopts decision X/33. It asks countries to allow no climate geoengineering that could affect biodiversity until there is a sound scientific basis and global, transparent, effective regulation. Small, controlled research is exempt. It's often called a moratorium, but it isn't binding, and the United States isn't a party.
3. 2022A startup, Make Sunsets, launches two balloons carrying sulfur dioxide from Baja California in April, without the Mexican government's consent.
4. 2023On 13 January, Mexico's environment ministry and science council announce that solar geoengineering experiments won't be allowed in Mexico.
5. 2024In February, at the UN Environment Assembly in Nairobi, Switzerland withdraws its resolution to set up an expert group on the technology, after countries can't agree. In March, Harvard ends its SCoPEx balloon experiment, about a decade after it was proposed, before any flight.
6. 2025In May, the UK's research agency ARIA names the projects in its £56.8 million Exploring Climate Cooling programme. A few include small outdoor tests. None releases anything into the stratosphere.

There are rules against using the sky as a weapon, a soft request not to tamper with it, and one country's no. There is no rule for who may turn it on, who may turn it off, or who pays when the rain moves.

Takeaway.There is no rule yet for who may set the thermostat.

What one mission could start now

The roadmap treats the century as one shared mission: get the air under control, give every child safety and choices, and build a fairer economy. Say the world took that mission on together. Nothing on this page says it should spray. But a mission could start several things in the next few years, before anyone reaches for the dial. Here is one way to explore them.

1. Keep cutting, and pay for removal

   Sulfur buys time and removes nothing. The work that shrinks the hidden warming is the work that shrinks the carbon. Switch the dial to “Fast cuts and removal” and the sulfur has almost nothing left to do.

   Where it could break.If sulfur looks cheap, it can become the excuse to slow the cuts. That's the path where the termination shock grows every year.

2. Study it in the open

   Models, lab work and small tests that release nothing into the stratosphere, like the UK's programme, with the results published for everyone. The monsoon numbers above come from models; better models are the cheapest thing on this list.

   Where it could break.SCoPEx was halted in Sweden in 2021 after the Saami Council objected, and it never flew. Open has to mean open to the people under the sky, not just to other scientists.

3. Write the stop rule before the start rule

   The dial shows why. A sudden stop is the dangerous moment, and in both novels the people who could force a stop were the ones who weren't asked. A rule that any spraying winds down over years, never overnight, makes the stop part of the plan instead of the disaster. Pick “A treaty of many” and stop in 2060 to see the difference.

   Where it could break.A rule binds only the people who sign it. Make Sunsets launched from Mexico before Mexico had a rule.

4. Set up the monsoon fund before the first flight

   Whoever cools the planet cools it for everyone, but the rain may move for some people more than others. A monsoon fund would pay farmers downwind if the rains fail while sulfur is up, paid for by the countries that gain from the cooling or by whoever sprays. Write in “Your rules” who pays, and see which story you land in.

   There's a real model to start from. At the climate talks in Sharm el-Sheikh in 2022, countries agreed to a fund for loss and damage from climate change, and on the first day of the Dubai talks, 30 November 2023, it became operational. A monsoon fund would be a narrower cousin, set up in advance for a risk people chose to take.

   Where it could break.Pledges aren't payments, and a failed monsoon has many causes. The fund needs its trigger written down, a rainfall measure agreed before anyone sprays, or every bad year becomes an argument.

5. Seat the people downwind first

   In Ministry, India acts because no one else would. In Termination Shock, India fights because no one asked. A mission that gave India and the other monsoon countries a seat, and a stop button, before any decision would remove the reason for both.

   Where it could break.A seat for everyone can turn into a veto for anyone. Then the decision goes back to whoever has a gun or an air force.

Takeaway.The near-term work isn't the spraying. It's the cuts, the open science, the stop rule, and the fund.

A bridge, not a destination

Suarez's answer to the same heat is to go up: build industry in orbit and beam the power home to pull carbon out of the air. A character in Critical Mass who has spent a fortune on solar and wind admits it won't be enough on the ground alone.

“It won’t be enough to save us. Even the ruins left behind by bankrupt fossil fuel companies continue to pollute.”

Daniel Suarez, Critical Mass, loc 727 (excerpt)

Stephenson's novel answers in one line.

“We gotta terraform Earth before we get distracted by Mars is my philosophy.”

Neal Stephenson, Termination Shock, loc 3634

The page doesn't have to choose. Sulfur makes sense only as a bridge, held while the carbon comes down, and both the roadmap's ground plan and its space plan are ways of bringing it down. Run the roadmap's year bar and watch its CO₂ gauge fall after mid-century. That's the shore the bridge has to reach. Earth is a starship too shows the other half: how much energy the ship can carry while it gets there.

Stephenson's book ends on a smaller, harder job.

“I’m going to work on carbon capture.”

Neal Stephenson, Termination Shock, loc 8973

Run the removal numbers in the roadmap's energy calculator →

Takeaway.Sulfur can turn down the fever. Only removal treats the infection.

Termination Shock is a novel by Neal Stephenson; The Ministry for the Future is a novel by Kim Stanley Robinson; Critical Mass is a novel by Daniel Suarez. Quotes are given word for word with their Kindle locations. This is an unofficial reading, not affiliated with or endorsed by any of the authors. The sulfur gun and India's spraying are fiction; Pinatubo and the treaties are real. The temperature lines are an illustrative model of published estimates, not a forecast. The page doesn't argue for or against putting sulfur in the sky; it shows the mechanism and the missing rules.