What if there were no greenhouse gases?

The Sun warms Earth. Some gases in the air help keep that warmth in.

What if those gases were gone?

You have probably heard that greenhouse gases are warming the planet. But a natural amount of them has always been part of our air, and it does an important job: it keeps Earth warm enough for liquid water and for life.

So how much do they really matter? What would Earth be like with none at all? In this lab you can remove the greenhouse effect from a simple model of the planet's energy balance and see what temperature it settles at.

NASA lists Earth's greenhouse gases as carbon dioxide, methane, ozone, nitrous oxide, chlorofluorocarbons and water vapor. Together they slow the escape of infrared heat to space, keeping the surface warmer than sunlight alone would.

This lab quantifies that natural greenhouse effect. In a one-temperature energy-balance model, the greenhouse effect is represented by an effective emissivity. What is the planet's balance temperature if that emissivity is set to 1, as for a bare body with no greenhouse gases?

Sunshine comes in. Some bounces away. The rest warms the ground.

The warm ground sends heat back up. Greenhouse gases catch some of it.

Spread over the whole globe, sunlight brings about 340 watts of power to every square meter at the top of the atmosphere. About 29 percent bounces straight back to space, mostly from clouds. The rest warms the land, oceans and air.

The warm Earth glows heat back out as invisible infrared light. Greenhouse gases absorb some of it and send part back down. NASA says this natural greenhouse effect keeps Earth's average at about 15 °C, more than 30 degrees warmer than it would be without it.

NASA's energy budget puts global-mean incoming sunlight at about 340 W/m² (a quarter of the solar irradiance), with about 29 percent reflected. The model uses S = 1,361 W/m² and α = 0.30, so it absorbs S(1 − α)/4 = 238 W/m².

Balancing that against εσT⁴ with an effective emissivity of 0.612 gives 287.8 K, about 15 °C, close to the observed average. The emissivity is a teaching value chosen to match that average, not one calculated from greenhouse gas amounts.

In our pretend lab, Olly takes the greenhouse gases out of the air.

Now the heat escapes easily. Earth gets colder and colder, until it is far below freezing.

Choose 'no greenhouse effect'. Now all of Earth's glow escapes freely to space. The same sunlight comes in, but heat leaves more easily, so the planet cools until it balances again.

In the model it settles at about −19 °C. Watch the graph: the oceans hold heat, so Earth cools over years. After about 5 years it is near −15 °C, and after about 10 years it is about −18 °C, still creeping toward −19 °C.

With ε = 1 the balance temperature becomes [238.2/σ]^¼ = 254.6 K, −18.6 °C: 33.3 K below today's model value. Starting from 288 K with a 70 m ocean mixed layer, the model reaches −15.0 °C after 5 years and −18.1 °C after 10, with an e-folding time of about 2.4 years.

This is a thought experiment carried out with a model. The model holds albedo fixed, so it cannot show what ice or clouds would do as the planet cooled.

Greenhouse gases work like a blanket. A blanket keeps your warmth close.

Take the blanket away, and the warmth escapes. You get cold!

A planet warms until the heat it sends out matches the sunlight it soaks up. Greenhouse gases make it harder for heat to get out, so Earth has to be warmer to send out enough. NASA compares them to a cozy blanket around the planet.

Without them, heat escapes easily, so Earth can balance at a much colder temperature. Carbon dioxide matters a lot: NASA says that removing it would collapse the greenhouse effect and drop Earth's temperature by about 33 °C.

At balance S(1 − α)/4 = εσT⁴, so T ∝ ε^(−¼). Raising ε from 0.612 to 1 multiplies T by 0.612^¼ ≈ 0.884, from 287.8 K to 254.6 K. Physically, greenhouse gases absorb surface infrared and re-emit from colder, higher layers, reducing outgoing radiation at a given surface temperature.

NASA notes that water vapor acts as a feedback, amplifying changes started by other gases, and that removing carbon dioxide would collapse the terrestrial greenhouse effect. The model has no feedbacks: one temperature, fixed albedo, a 70 m ocean layer.

Earth has just the right amount of blanket for life. It is not too hot and not too cold.

Scientists measure the gases in the air every day to look after it.

NASA calls Earth a 'Goldilocks' planet: its natural greenhouse effect makes it not too hot and not too cold for life. A real Earth with no greenhouse gases would likely get even colder than the model says, because spreading ice would reflect more sunlight. The model keeps reflectiveness fixed, so it can't show that.

Today the opposite question matters: people have added extra greenhouse gases, mainly by burning fuels. Scientists track them carefully, and many people are working to cut what we add.

NASA's energy-budget feature gives the loss of reflective polar ice as an example of a climate feedback. Run backwards, a cooling Earth gaining ice would reflect more sunlight, so −18.6 °C is likely too warm for a real no-greenhouse Earth; that is an idea, not a model result.

The present issue is excess greenhouse gas. NOAA reports that global average carbon dioxide reached 422.8 parts per million in 2024, 50 percent above its pre-industrial level. NASA describes the two responses as mitigation, cutting emissions or strengthening sinks such as forests, and adaptation.