What if Earth were farther from the Sun?

The Sun shines on Earth and warms it up. Earth is just the right distance for us.

What if Earth were much farther away?

Every second, sunlight pours onto Earth and warms it. Earth glows that heat back out into space as invisible infrared light, and its temperature settles where the heat coming in matches the heat going out.

So what would happen if Earth were farther from the Sun, say as far away as Mars? Would it be a little cooler, or a lot? In this lab you can move Earth out in a model of a planet's energy balance and find out.

A planet's average temperature is set by an energy balance: absorbed sunlight in, thermal infrared out. Both sides depend on simple quantities: the sunlight arriving at the planet's distance, the fraction reflected (albedo) and how easily heat escapes (here an effective emissivity standing in for the greenhouse effect).

This lab changes only the first of these. If Earth orbited at Mars's distance, with its own air, clouds and reflectiveness unchanged, what would the balance temperature be?

Earth gets lots of sunshine. Some bounces away from clouds and ice.

The air works like a blanket that keeps some heat in. So Earth stays nice and warm.

At Earth's distance from the Sun, about 150 million kilometers, sunlight arrives with about 1,361 watts of power on each square meter facing the Sun. Spread over the whole spinning globe, that is about 340 watts per square meter, and about 30 percent of it is reflected straight back to space.

In the model, the rest warms Earth to an average of about 15 °C. Without the greenhouse effect of its air, the same model gives about −19 °C.

With S = 1,361 W/m² and albedo α = 0.30, the model absorbs S(1 − α)/4 = 238 W/m² averaged over the sphere. Balancing that against εσT⁴ with an effective emissivity of 0.612 gives 288 K, about 15 °C, close to Earth's observed average.

Setting ε = 1 (no greenhouse effect) gives 254.6 K, about −18.6 °C: a difference of about 33 K that the model attributes to the greenhouse effect.

In our pretend lab, Olly moves Earth out to where Mars is.

Now less sunshine reaches it. Earth gets colder and colder, until it is far below freezing.

Choose 'as far as Mars'. At that distance, the sunlight reaching Earth drops to about 586 watts per square meter, only 43 percent of what we get now. Keep Earth's air, clouds and ice exactly as they are, and the model settles at about −40 °C.

Watch the temperature graph as it happens. In the model, Earth does not cool all at once: the oceans hold heat, so after about 10 years it has reached about −34 °C and it takes around 20 years to get close to −40 °C.

With S = 586 W/m² the absorbed flux falls to 103 W/m² and the balance temperature to 233 K, about −40 °C, roughly 55 K below today's. The bare-rock value (ε = 1) is about −67 °C. The model's ocean response time grows from about 2.7 to about 5.2 years, so the cooling curve flattens over two decades.

This is a thought experiment carried out with a model. Earth's orbit cannot be moved, and the model holds albedo and emissivity fixed, which a real planet would not.

Sunlight spreads out as it travels. Far from the Sun, the light is weaker.

Weaker sunshine means less warming.

Light spreads out as it travels away from the Sun, like the beam of a flashlight getting wider and dimmer. At twice the distance, the same light is spread over four times the area, so each square meter gets only a quarter as much.

Mars is about one and a half times as far from the Sun as Earth, so it gets a little less than half as much sunlight. With less energy coming in, the planet only has to glow out less heat to balance, so it settles at a lower temperature.

Light obeys an inverse square law: intensity falls as 1/d². Mars's semimajor axis is 1.524 AU, and 1/1.524² ≈ 0.43, matching NASA's figures of 586 W/m² at Mars against 1,361 at Earth. At balance, T = [S(1 − α)/(4εσ)]^¼, so temperature scales as S^¼: 0.43^¼ ≈ 0.81, and 0.81 × 288 K ≈ 233 K.

The model's limits: one temperature for the whole planet, a fixed albedo and emissivity, and only a 70 m ocean layer storing heat. It has no ice, cloud or water-vapor feedbacks.

Mars really is that far from the Sun. It is much colder than Earth.

Mars has only very thin air, so it cannot keep much heat in.

Mars really does orbit at that distance, and it is colder still: its average temperature is about −60 °C. Mars has very thin air, so in the model it would be closer to the 'no greenhouse' case than to Earth.

Our model also keeps Earth's ice and clouds fixed. Shiny ice reflects sunlight, so a real Earth that froze over would reflect more and cool further. That is an idea the model does not include, because its reflectiveness never changes by itself.

Mars's measured average is about 214 K (−59 °C) and NASA lists its black-body temperature as 209.8 K, close to the model's no-greenhouse value at Mars's distance. Its thin atmosphere (about 6 mb) provides little greenhouse warming.

The model leaves out feedbacks. Ice and snow have high albedo, so a cooling Earth would likely reflect more sunlight and cool further; water vapor and clouds would change too. Those feedbacks are ideas here; the model's albedo slider lets you explore one of them by hand.