Radiative forcing is what happens when the amount of energy that enters the Earth’s atmosphere is different from the amount of energy that leaves it. Energy travels in the form of radiation: solar radiation entering the atmosphere from the sun, and infrared radiation exiting as heat. If more radiation is entering Earth than leaving—as is happening today—then the atmosphere will warm up. This is called radiative forcing because the difference in energy can force changes in the Earth’s climate.
Heat in, heat out
Sunlight is always shining on half of the Earth’s surface. Some of this sunlight (about 30 percent) is reflected back to space. The rest is absorbed by the planet. But as with any warm object sitting in cold surroundings—and space is a very cold place—some energy from Earth is always radiating back out into space as heat.
Radiative forcing measures how much energy has come in from the sun, compared to how much has left, over a period of time. The analysis needed to pin down this exact number is very complicated. Many factors, including clouds, polar ice, and the physical properties of gases in the atmosphere, have an effect on this balancing act, and each has its own level of uncertainty and its own difficulties in being precisely measured. However, we do know that today, more heat is coming in than going out.
