Why is the atmosphere near the Earths surface warmed from below by the surface?

To understand the basics of the Greenhouse Effect and why it is so important for the Earth, there are only a few things you need to know, some of which are already probably familiar.

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.

The Physics and Astronomy Department has decided that, starting with the admission cycle for the 2022-23 Academic Year (i.e., Fall 2022 start of the PhD or MS programs), GRE General and Physics subject test scores are no longer required, will no longer be accepted, and will no longer be considered in the evaluation of graduate applications. Please, read the full statement here.

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