A stable climate is based on a long-term energy balance between the amount of energy that Earth receives from the Sun and the amount that Earth radiates back out into space. Any change in the atmosphere will ultimately result in a condition where Earth radiates the same amount of energy that it absorbs. However, the temperature, composition and circulation patterns of the air may all change to create that new equilibrium.
The Role of Carbon Dioxide
The normal level of carbon dioxide in the atmosphere during recent glacial and interglacial periods (last few hundred thousand years) has varied between approximately 180 ppm (parts per million) and 280 ppm. Since the beginning of the industrial revolution, this has risen more than 40% above the interglacial maximum, and is currently 400 ppm. Burning fossil fuels releases carbon dioxide into the atmosphere, and that amount of carbon dioxide can be measured. We (humans) are emitting more than 10 GT (giga-tons, billions of tons) of carbon into the atmosphere every year.
The Law of Conservation of Energy states that energy is never created or destroyed. This is the central pillar of all of science, and a principle that has been tested millions of times and confirmed without exception. As a consequence, for the climate of Earth to remain stable, with no changes in average temperatures, the total amount of energy absorbed by planet Earth from the Sun must be equal to the amount of energy Earth radiates back into outer space. If the energy received does not equal the energy radiated, the average temperature of Earth will necessarily rise or fall until a new equilibrium is established.
How does Earth change its equilibrium? The amount of energy radiated from Earth into outer space depends on the average temperature of the Earth. When the temperature is lower, Earth radiates less. When the temperature is higher, Earth radiates more. All objects do this.