The greenhouse effect and the 2nd law of thermodynamics
discovery of the greenhouse effect: the process he did estab lish the effect that the atmosphere had relationship between an object's temperature and. Earth's relatively stable and hospitable average temperature is the result of a phenomenon called the greenhouse effect. The presence in the atmosphere of. American Journal of Physics 67, (); barcelonatraveller.info . The greenhouse effect was apparently first recognized by Fourier (Ref.
Greenhouse effect - Wikipedia
Like most pioneering efforts, Fourier or Arrhenius did not have the last word, and we still have much to learn today, but they provided a big leap in how we understand planetary temperatures and the role of the atmosphere in radiative balance.
Fourier was one of the first to speculate that human activities could influence climate, and such topics are rather important in modern times. Notwithstanding the ancients who thought the Earth sits on the back of giant animals, it is well known that the Earth sits in the large vacuum of space.
Because there is no physical medium by which the Earth can receive sunlight, it must travel some , km in the form of electromagnetic radiation. Because the planet is also not in physical contact with anything, the only way in which it can lose heat is by radiation. All objects above absolute zero emit radiation, and the intensity of that radiation depends on the objects temperature.
Physics of the Greenhouse Effect Pt 1
The Earth is no exception, and in order to lose heat to space, it must balance the incoming solar radiation with its own infrared radiation at the top of the atmosphere. If it did not emit radiation back out to space, it would only gain heat and eventually heat up without bound. Any object that is not in radiative equilibrium will warm or cool depending on if its received more energy than it emits, or emits more than it absorbs, but the temperature change is how the object comes back to balance.
Recall that the electromagnetic spectrum is divided into several parts, such as in the infrared, visible, UV, microwave, etc. In this discussion, we will deal only with visible also shortwave radiation and infrared also called longwave radiation.
Visible radiation is more intense than infrared radiation, and is emitted by objects at higher temperatures. So far, all we need to get is that the sun preferentially emits in the visible, while the Earth loses heat preferentially in the infrared. Just to get a bit of this out of the way, the effective temperature of the Earth with radius r is: The gap between our present day comfort, and an iceball planet is due to the fact that some of the outgoing infrared radiation is not immediately sent right back to space, but is absorbed by the atmosphere, where some is radiated downward to the surface.
- The greenhouse effect and climate change
- Climate Science Glossary
- What Is the Greenhouse Effect?
This is due to the fact that we have greenhouse gases, which are transparent to incoming solar radiation, but absorb outgoing infrared radiation strongly. No longer freezing, but rather comfortable and unique to the solar system.
So what is going with this greenhouse??
The following image shows a spectra at the top of the atmosphere which shows the absorption of photons by CO2, water vapor, ozone, etc. These gases absorb the longwave IR radiation emitted by the relatively warmer surface and emit radiation to space at the colder atmospheric temperatures, leading to a net trapping of IR energy within the atmosphere the greenhouse effect. The long-term climate of our planet is governed by a balance between the incoming solar radiation, the reflected solar radiation albedo changes based on land cover changes, presence or absence of ice sheets, etc and the emitted IR.
If one increases the amount of infrared absorbing gases, then excess energy is suddenly available to drive the climate system. Now there is a popular misconception that more greenhouse gases warm the surface on the pure basis that they enhance the downward infrared emission. I want to examine what is going on in a bit more detail, because saying that the globe is warming simply because the downward IR has increased is not accurate.
Physics of the Greenhouse Effect Pt 1 | Climate Change
The below figure shows how a quick profile of the atmosphere. As we can see from the Y axis, pressure decreases with height, and so does temperature. The temperature decrease with altitude should be familiar to anyone who has ever climbed a mountain, and is known as the lapse rate.
The temperature gradient between the surface and the cold atmosphere gives rise to to atmospheric motions that contribute to vertical transport of heat, such that the air temperature decreases about 6. If the pressure of the surrounding air is reduced, then the rising air parcel will expand.
If the total amount of heat in a parcel of air is held constant no heat is added or releasedthen when the parcel expands, its temperature drops because of the inverse relationship between the volume of an air parcel and its temperature. Put simply, because of gas expansion principles the air temperature decreases with height. The atmosphere also gains heat by sensible and latent heat fluxes from the surface. The atmosphere radiates energy both upwards and downwards; the part radiated downwards is absorbed by the surface of Earth.
This leads to a higher equilibrium temperature than if the atmosphere were absent. The solar radiation spectrum for direct light at both the top of Earth's atmosphere and at sea level An ideal thermally conductive blackbody at the same distance from the Sun as Earth would have a temperature of about 5.
The atmosphere near the surface is largely opaque to thermal radiation with important exceptions for "window" bandsand most heat loss from the surface is by sensible heat and latent heat transport.
Radiative energy losses become increasingly important higher in the atmosphere, largely because of the decreasing concentration of water vapor, an important greenhouse gas. It is more realistic to think of the greenhouse effect as applying to a "surface" in the mid- tropospherewhich is effectively coupled to the surface by a lapse rate.
The simple picture also assumes a steady state, but in the real world, there are variations due to the diurnal cycle as well as the seasonal cycle and weather disturbances. The second law of thermodynamics has been stated in many ways. For us, Rudolf Clausius said it best: That's so obvious that it hardly needs a scientist to say it, we know this from our daily lives. If you put an ice-cube into your drink, the drink doesn't boil! The skeptic tells us that, because the air, including the greenhouse gasses, is cooler than the surface of the Earth, it cannot warm the Earth.
If it did, they say, that means heat would have to flow from cold to hot, in apparent violation of the second law of thermodynamics. So have climate scientists made an elementary mistake?
The skeptic is ignoring the fact that the Earth is being warmed by the sun, which makes all the difference.
To see why, consider that blanket that keeps you warm. If your skin feels cold, wrapping yourself in a blanket can make you warmer. Because your body is generating heat, and that heat is escaping from your body into the environment. When you wrap yourself in a blanket, the loss of heat is reduced, some is retained at the surface of your body, and you warm up.