What Is the Greenhouse Effect?
ACS Climate Science Toolkit | Narratives
Remark: “The Earth is warming and climate science shows that the warming has something to do with the greenhouse effect.”
Reply: “I know. I wish I knew more about it, but the greenhouse effect is too difficult to understand.”
Not really. There are only a few things you need to know, some of which are probably already familiar, to understand the basic idea of the greenhouse effect and why it is so important for the Earth.
You know that when you stand in sunlight, you feel warmer than when you are in shadow, so you can feel that the light (radiant energy) the sun gives off carries energy that can warm an object—you. Although you usually cannot see it, all objects give off radiant energy and you can sometimes feel this energy. For example, if there is a pot of hot water on your stove, you can feel the radiant energy it gives off without touching it. You usually call what you feel “heat,” but it is more accurate to think of it as a kind of invisible light called “infrared radiation” that warms your skin, just like the sunlight. The amount of infrared radiation energy a warmed object gives off depends on its temperature—the higher the temperature, the more energy is given off. As you know, you can easily distinguish between a warm object and a hot object by holding your hand near the objects and feeling the difference in heating effect on your skin.
These ideas are basic to understanding the energy balance between the sun and the Earth. Just as sunlight warms you, it warms the surface of the Earth as well. The Earth does not continue to get hotter and hotter as it absorbs energy from the sun, because it gives off energy to space as invisible infrared radiation. In order to come into energy balance, the amount of infrared radiation energy given off by the Earth has to be equal to the amount of energy absorbed from the sunlight. The amount of infrared radiation energy the Earth gives off depends on its temperature. The average Earth temperature required for energy balance with the sun would be a frigid –18 °C (0 °F), if there were no atmospheric greenhouse effect. The greenhouse effect has kept the Earth’s average temperature a good deal higher for billions of years, making it possible for life as we know it to evolve. Over the past several millennia the average Earth temperature has been about 15 °C (59 °F).
The figure below illustrates how greenhouse gases keep the Earth warmer than it would be without them. Energy from the sun is shown on the left where you see that part of the radiant energy from the sun passes through the atmosphere, is absorbed, and warms the Earth’s surface. The rest is reflected, largely by clouds in the atmosphere and ice and snow on the surface, and not absorbed. Energy lost by the Earth is shown at the right where the fates of infrared radiation emitted (given off) by the Earth are shown. The straight red arrow passing from the surface through the atmosphere represents the fraction of the emitted infrared radiation that passes into space through the atmosphere without change. The rest of the infrared radiation, the thick red arrow, is absorbed by the greenhouse gases and clouds in the atmosphere and then re-emitted in all directions as shown by the collection of orange arrows. This ability to absorb and re-emit infrared radiation is the critical requirement for greenhouse gases. All gases whose molecules have three or more atoms are greenhouse gases—carbon dioxide (CO2), water vapor (H2O), and methane (CH4) are important greenhouse gases that have maintained Earth’s warm temperature for billions of years.
Some of the re-emitted energy remains within the atmosphere or returns to the surface and warms the lower atmosphere and surface. The rest of the re-emitted energy leaves the atmosphere and goes into space. The outcome of this absorption-emission process by the greenhouse gases is that less energy leaves the atmosphere than is emitted by the Earth below. But to be in energy balance with the sun, the amount of energy going into space has to be equivalent to the amount that would be emitted by a surface at –18 °C. In order for this amount of energy to leave the Earth’s greenhouse atmosphere, the surface must be warmer than –18 °C. For several millennia—until the past two centuries—an average surface temperature of about 15 °C kept the sun and Earth in energy balance. That is, the same amount of infrared radiant energy was leaving the atmosphere as was being absorbed at the surface from the sun’s radiant energy.
The increased amounts of greenhouse gases human activities are adding to the atmosphere have upset the balance that has been in place since the end of the last ice age. Adding more greenhouse gases decreases the amount of infrared radiation energy leaving the atmosphere. To get the energy back in balance, the surface of the Earth has to warm up, so that it will emit more infrared energy, some of which will leave the atmosphere and compensate for the effect of the added greenhouse gases. Thus, the greenhouse effect, which is essential for creating the climate for life on Earth, is also responsible for the Earth getting warmer than it was before we started burning large amounts of fossil fuels.