The Greenhouse Effect: Earth's Energy Balance
See how greenhouse gases keep the Earth warm, using the energy that flows in and out. Sunlight arrives, some of it bounces straight back, and the rest warms the ground. The warm ground gives off infrared, and gases in the air absorb some of it and send part of it back down, so the ground ends up warmer than it would be without them. Arrows show each flow in watts per square metre, drawn to one scale, and a graph follows the surface temperature over 25 years. Set the carbon dioxide level (180 to 1,200 ppm), how much sunlight is reflected, whether the water vapour feedback is on, and whether the air starts as it was before industry or as it is today. For instance, with the starting settings the surface is 14.0 °C at 280 ppm, and without any greenhouse gases it would be about −18.6 °C. It is a simple model with one layer of air, so its numbers are a guide, not a forecast.
Preview — sign up free to change the numbers yourself
What you can adjust
- Carbon dioxide level
- 180 – 1200 ppm
- Share of sunlight reflected
- 0.1 – 0.5
- Water vapour feedback
- 0 – 1
- Starting air
- 0 – 1
The amount of carbon dioxide in the air, in parts per million. It was about 280 before industry and is about 420 today. The run jumps the air to this level at the start and then follows the surface as it warms or cools. Each doubling traps about 3.7 W/m² more.
The share of the incoming sunlight that bounces straight back to space from clouds, ice and bright ground. For the Earth it is about 0.30. A higher share leaves less sunlight to warm the surface, so the surface ends up colder.
Warmer air holds more water vapour, which is itself a greenhouse gas, so it adds to any change. It is on to start with, which roughly doubles the response in this simple model. Switch it off to see the carbon dioxide on its own.
The air the surface has settled into before the run starts. Choose pre-industrial air (280 ppm) or today's (420 ppm). The carbon dioxide level above is then applied at the start, so choosing the same level in both leaves nothing to change.
About this simulation
What it shows
Sunlight arrives, some of it bounces straight back, and the rest warms the ground. The warm ground gives off infrared, and the gases in the air absorb some of it and send part of it back down. Arrows show each flow in watts per square metre, drawn to one scale, and a graph follows the surface temperature over 25 years.
The key idea
Greenhouse gases absorb infrared from the ground and send part of it back down, so the ground has to be warmer to get rid of all the energy it receives. At the end, the infrared leaving to space equals the sunlight absorbed.
Try this
Run the defaults: 14.0 °C at 280 ppm warms about 1.2 °C at 420 ppm. Set Water vapour feedback to Off for the carbon dioxide alone, about 0.66 °C. Set reflected sunlight to 0.40 for a colder surface.
A simple model: one layer of air, averages over the whole Earth and year, an ocean layer 100 m deep that sets the settling time (about 4 years), and the carbon dioxide change applied all at once at the start. The water vapour feedback, on to start with, is a single factor of 1.8 on the change; switching it off shows the carbon dioxide alone. The real climate has further feedbacks, and clouds, ice and the deep ocean, that this one-layer model leaves out, so the numbers are a guide, not a forecast. Without any greenhouse gases the same model gives about −18.6 °C. The 25 years are played in 10 seconds. On narrow screens the picture, graph and table are stacked, and the labels under the arrows are staggered if they would not fit.