Hail Mary Science
Light, heat and climate

Global cooling simulator

Move the slider to lower the global average temperature and the panel describes the matching scenario, based on what we know about past climate. The key benchmark is the last ice age, when Earth was only about 6 °C colder than today.

Key formula

T_equilibrium = [ (1 − A) · S / (4σ) ]^(1/4) ≈ 255 K · greenhouse effect ≈ +33 K

Try it

How to use the simulator

This slider connects a drop in global average temperature with its rough consequences, based on what we know about past climate. The key benchmark is the last glacial maximum, about 20,000 years ago, when Earth was only about 5°C colder than today and ice covered much of Europe and North America.

Controls

  • Temperature drop: 0 to 20°C in half-degree steps. The panel changes color and describes the matching scenario.

Things to try

  1. Set 5°C and compare it with the last ice age, which happened without any outside threat.
  2. Set 10 to 15°C, the range the novel describes, and read the scenario.
  3. Think about timescales: on Earth, shifts this large have taken thousands of years. In the novel they unfold over decades, which is what makes adapting impossible.

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Earth's energy balance

Earth receives energy from the Sun and sends it back to space as infrared. Reflecting 30% of sunlight (its albedo) and with no atmosphere, its average temperature would be about 255 K, −18 °C. The real average is about 15 °C: the 33-degree difference is the natural greenhouse effect of water vapor, CO₂ and other gases.

Any change in the energy arriving, the energy reflected or the energy trapped by the atmosphere shifts that balance. That is why a few degrees of average temperature mean a completely different climate.

What the past teaches us

About 20,000 years ago, at the last glacial maximum, the average temperature was about 6 °C lower. Ice sheets kilometers thick covered Canada and northern Europe, and sea level was about 120 meters below today's.

Around 700 million years ago Earth may have frozen over almost entirely, in what is called "snowball Earth". Once ice reflects most of the light, cooling reinforces itself and is very hard to escape.

Worked example

Example: where −18 °C comes from

  1. The solar constant is S ≈ 1,361 W/m². Earth intercepts light with a disk but spreads it over a sphere, four times larger: 1,361 / 4 ≈ 340 W/m².
  2. It reflects 30%, so it absorbs 0.7 · 340 ≈ 238 W/m².
  3. Setting that equal to black-body emission: T = (238 / 5.67·10⁻⁸)^(1/4) ≈ 255 K, about −18 °C.
  4. Move the slider to 5–6 °C to see an ice-age scenario, and to 15 °C for the one the novel describes.

In Project Hail Mary

In the novel, the Sun's dimming threatens to lower Earth's temperature by 10 to 15 °C within a few decades. The simulator describes what a change of that size would mean.

This tool appears in:Chapter 4The Hail Mary Ship and the Suicide Mission

Frequently asked questions

How much has today's climate change warmed the planet?

About 1.2–1.3 °C compared with pre-industrial times. It sounds small, but it is a quarter of the gap between today and an ice age, and it happened in just over a century.

What is albedo?

The fraction of light a surface reflects. Fresh snow reflects up to 90%; the ocean, under 10%. Earth's average is about 30%.

Why do ice ages come and go?

Mainly because of Milankovitch cycles: slow changes in Earth's orbit and tilt that redistribute sunlight between seasons and latitudes, amplified by ice and CO₂.

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