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Out in Nature

The Greenhouse Effect


The greenhouse effect is much discussed given its crucial role in determining the climate of Earth in relation to industrial gases.

In this set, we'll lay out the basic working mechanism of the greenhouse effect, see how the atmosphere is impacted by the presence of various gases, and see how the interaction between solar radiation and Earth's surface plays a crucial role.

Suppose a planet with no atmosphere orbits a star and that the planet rotates slowly compared to its orbit of the star.

How will temperature vary on its surface?

Nearly half of all energy incident from the Sun arrives in the form of visible light.

Earth’s atmosphere is practically transparent to visible light radiation (high frequency) from the Sun, neither absorbing nor blocking much of its transmission to and from Earth.

By contrast, the atmosphere absorbs infrared radiation (low frequency) with near-perfect efficiency.

Which of the following events would result in the atmosphere absorbing the greatest amount of solar radiation?

As we've seen, an atmosphere-less Earth would have serious temperature variation as we move from the sunlit side to the dark. As we'll see now, the planet would also be colder, on average.

When solar radiation arrives in the form of visible light, it hits the surface and warms it. Warm objects not only absorb energy, but radiate it as well. In fact, an object with constant temperature must radiate heat at the same rate that it absorbs. The surface of an atmosphere-less Earth would therefore radiate energy at the same rate it receives it from the Sun.

Now consider what happens to an Earth that has an atmosphere. After solar radiation passes through the atmosphere—which is transparent to visible light— it hits the Earth, warming it. However, when Earth radiates heat energy it is not in the form of visible light, but is converted to infrared (IR).

Outgoing IR from Earth warms the atmosphere, which in turn radiates heat, both down toward Earth and out into space.

How does this changes things on Earth? Earth receives visible light from the Sun at the same rate as before, since the atmosphere is transparent to visible light. However it now also receives IR radiation from the atmosphere, making its total rate of heat absorption significantly higher than it was without the atmosphere.

The Earth now radiates heat at this new higher rate, which means that its temperature is higher than before. Note that as a whole (i.e. Earth and atmosphere), the Earth still radiates heat out to space at the same rate that it did before, but the heat spends more time on Earth before it leaves as radiation up into space.

This is the essential mechanism of the greenhouse effect, to increase the heat energy of Earth by prolonging the time radiation spends trapped in the atmosphere-Earth system. Some atmospheric gases that are efficient IR absorbers (and radiators) are \(\ce{CO2}\), \(\ce{CH4}\) (methane), and \(\ce{H2O}\) (water), among many others.

What is the major cause of the greenhouse effect?

Currently, the concentration of carbon dioxide in Earth's atmosphere is about 400 parts per million. During the Pleistocene Epoch, about 150 thousand years ago, the concentration of carbon dioxide in the Earth's atmosphere was about 200 parts per million.

Based on your knowledge of the greenhouse effect, how should the average temperature on Earth during the Pleistocene Epoch compared to the temperature in modern times?

If human civilization had never developed on Earth, would there still be greenhouse warming?

One of the big environmental stories of the 80's and 90's was the depletion of ozone by so-called chlorofluorocarbons (chemicals included in aerosol products), which led to large holes in the ozone layer.

Thermal radiation from Earth consists mostly of IR radiation of wavelengths between \(8\) to \(\SI{12}{\micro\meter}.\) Below is some experimental data for the absorptive properties of ozone gas, \(\ce{O3},\) for a range of wavelengths of light.

All else being equal (the level of the other gases is constant) what effect should ozone depletion have on greenhouse warming?

Details: An absorbance of 1 means that all the radiation is absorbed, 0 means all the radiation passes through.

In this set, we investigated about the complex interplay underlying the greenhouse effect. We learned about thermal radiation and the absorptive properties of Earth's surface and the atmospheric gases. Finally, we calculated the energy budget of Earth with a mixture of physics and straightforward accounting. These investigations just scratch the surface of thermodynamics, climatology, and atmospheric physics, but the basic physics principles and accounting techniques are universal. Indeed, they underly our understanding of Earth's climate, and are the same sorts of arguments that go into more sophisticated models of our atmosphere.


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