Hail Mary Science
Light, heat and climate

Global warming potential comparator

Not all greenhouse gases warm the planet equally. The comparator shows the global warming potential (GWP) of four reference gases, using approximate values from the IPCC reports.

Key formula

CO₂ equivalent = mass of gas × GWP

Try it

How to use the simulator

Not all greenhouse gases warm the planet equally. The global warming potential (GWP) compares how much heat a ton of a gas traps with a ton of CO₂ over a set period, usually 20 or 100 years. The comparator shows the figures for four reference gases, based on the values published by the IPCC.

Controls

  • The bars show each gas's 20-year GWP, relative to CO₂ (which is 1 by definition).

Things to try

  1. Compare methane over 20 years (about 80 times CO₂) with its 100-year value (about 28). The number falls because methane leaves the atmosphere in roughly a decade.
  2. Look at SF₆: it's thousands of times stronger than CO₂, but it's emitted in such small amounts that its overall effect is smaller.
  3. Consider why the novel picks methane and not SF₆: to warm quickly and be able to undo it later, you want a potent gas that doesn't linger for centuries.

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What GWP measures

Global warming potential compares how much energy a ton of a gas traps with a ton of CO₂ over a set period, usually 20 or 100 years. CO₂ is 1 by definition. If a gas has a GWP of 28, emitting one ton is equivalent to emitting 28 tons of CO₂.

The result depends on two things: how effectively the gas absorbs infrared at wavelengths where the atmosphere is otherwise transparent, and how long it stays there before breaking down.

Why the time horizon matters

Methane is very potent but short-lived: most of it is gone in about 12 years. That is why its 20-year GWP (about 80) is much higher than its 100-year value (about 28–30). CO₂, by contrast, lingers for centuries, so its effect builds up.

Sulfur hexafluoride (SF₆), used as an insulator in electrical equipment, is the most potent regulated gas: more than 20,000 times CO₂, with a lifetime of thousands of years. It is emitted in such small amounts that its overall contribution is modest.

Worked example

Example: a cow's methane emissions

  1. A dairy cow emits on the order of 100 kg of methane a year.
  2. Over 100 years (GWP ≈ 28): 0.1 t · 28 ≈ 2.8 t of CO₂ equivalent a year, roughly what a car emits driving 15,000–20,000 km.
  3. Over 20 years (GWP ≈ 80): 0.1 t · 80 ≈ 8 t of CO₂ equivalent. The same gas looks almost three times worse depending on the horizon you pick.
  4. That is why cutting methane is one of the fastest ways to slow warming in the short term.

In Project Hail Mary

The novel considers warming Earth on purpose to offset the cooling, choosing a potent gas that won't linger for centuries. The comparator helps explain why methane fits better than the others.

This tool appears in:Chapter 14Erid

Frequently asked questions

Is water vapor a greenhouse gas?

Yes, the most important one in the natural greenhouse effect. But its amount depends on temperature: we don't control it directly; it responds to warming caused by other gases and amplifies it.

Why is CO₂ the main problem if its GWP is the lowest?

Volume. We emit about 37 billion tons of fossil CO₂ a year, tens of times more than any other gas, and it lingers for centuries.

What is geoengineering?

Deliberate large-scale modification of the climate, for example reflecting sunlight with aerosols or pulling CO₂ out of the air. It is studied as a possible complement to cutting emissions, not a replacement, and carries significant risks.

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