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Earth's Heat Budget

 

To better understand the physical nature of the oceans; to become familiar with the consequences of differential heating of our planet, namely the winds, the currents, waves, weather, heat transport, chemical transport, vertical mixing, ventilation of gases, transport of larvae and plankton, primary productivity, fish and marine mammal migration and climate change (to name a few); to better evaluate the consequences of natural and man-made perturbations in the atmosphere, I offer this brief summary of Earth's heat budget. You might have guessed that it's pretty important...

 

 

Before I talk about the heat budget, let's be sure we understand the definition of heat. Heat is a form of energy arising from the random motion of molecules. Those molecules may be air, water, stone or any other form of matter. We measure the heat of a material by taking its temperature, but heat and temperature are not the same thing. I repeat: heat and temperature are not the same thing. Temperature is a measure of the heat of a substance; the heat is the degree of movement of the molecules.

Let's explore this a little further: As you heat a substance, it gains energy and its molecules move more rapidly. As you cool a substance, you remove energy and its molecules move more slowly. Think about your body when you've been out carving up the slopes all day or when you've been diving in the cold Pacific all day. Your fingers and everything else are hard to move. That's because the molecules in your body are at a colder than optimum temperature and the molecules that make your limbs operate are moving slower.

Let's explore this even a little further: the second law of thermodynamics roughly states that heat moves from a region of high energy to a region of low energy. Said another way, heat moves from a region of high temperature to a region of low temperature. If you heat one end of a metal rod, the heat moves down the rod from the hot end to the cold end. The heat also moves out into the surrounding air IF the surrounding air is colder. If you turn on your stove, what happens to the air surrounding the burner? Where does it go? How far does it go?

And an interesting note: the absence of any molecular motion means that heat is absent. So far, this has never occurred in nature or in a lab, but scientists have defined that state (when all molecular motions cease) as 0 degrees Kelvin.

We will come back to some of these properties of heat when we discuss the properties of water. If you are having trouble understanding them, please e-mail me or post a message on the forum for this section. Back to the Earth's heat budget...

As the word budget implies, we need to take an account of the sources and sinks; the incomes and outcomes; the money made and the money spent, so to speak, with regards to heat energy on our planet. And that's exactly what we will do.

Let's look at how solar energy arrives at our planet and its fate once it gets here. The path of a photon from the sun to the surface of the sea is not an easy one. Besides traveling through the vacuum of space to the Earth's outer atmosphere (a journey that takes about eight minutes), a photon must contend with a whole host of "road blocks" to even reach the Earth's surface. These road blocks fall into two general categories: things that absorb and things that scatter.

Absorption is a process whereby light of a particular wavelength interacts with a molecule (or group of molecules) such that it is removed from the light stream. This absorbed light may add a charge or confer vibrational energy to a molecule; it may be transformed into heat and re-radiated; or it may be "trapped" and converted to chemical energy through the process of photosynthesis. I think most of you have an intuitive feel for absorption, so we won't belabor the technical description of the process. While it may seem simple enough, we could spend half a semester just on absorption, if we were learning optics.

Scattering is a process whereby the path of a particular wavelength of light is altered by a molecule (or group of molecules). Light may be scattered forward (forward scattering) or it may be scattered backwards (backscattering).

I should mention two other phenomena of light that we should be familiar with: reflection and refraction. Reflection and refraction occur at the interface between two different media, such as air and water. As light enters the air-sea interface, some of it will be reflected, i.e. some of it will bounce off and travel back through the atmosphere. Light also slows down in water; this process of slowing down causes the light to bend downward. When looking in a pond or a glass of water with a coin pr pencil in it, this has the effect of making the object seem where it is not.

Try this simple little experiment using your flashlight again. Fill a glass with water and point the light beam at an angle towards the surface of the water. You should be able to observe part of the light reflecting off the surface and creating a beam of light on your wall or ceiling. Wiggle the glass and watch the patterns. Like all waves, light waves can interact and create interesting patterns. Now stick a pencil in the glass of water and look at it from the side. Can you see the pencil bending? That's because the light that reflects off the pencil in the water is slower than the light that reflects off the pencil above the water and you see a displaced pencil (a bent pencil) when looking at it. Cool, huh?

Now check out this web page, http://webphysics.ph.msstate.edu/jc/library/22-2b/index.html and run the refraction simulation. Play around with the angles and compare the refractive behavior of water versus glass. What is the difference between the refractive properties of water versus glass or diamond? What is happening that makes the light bend more in certain materials? Submit your answer here.

Light transmitting through the atmosphere and the oceans must contend with the absorption and scattering properties of water, particles, gases, dissolved substances, and microscopic organisms. Each of these components act to alter the course of a photon, or even transform it into something else.

First of all, sunlight entering the outermost atmosphere is absorbed by gases, like ozone, water vapor, clouds, and atmospheric dust. Adding up the sources of absorption as presented here, we can see that about 19% of the incoming solar radiation is absorbed before it hits the the Earth's surface.

Backscattering of sunlight also contributes to losses through the atmosphere. (Why isn't forward scattering included here?) Clouds, air, dust, haze and even the surface of the ocean (or land) cause backscatter. About 30% of the incoming solar radiation is reduced by backscattering. Note that clouds alone contribute 20% of the backscattering. Thus, clouds are a very important regulator of solar radiation.

Once sunlight hits the ocean surface or land, some of it is reflected (about 4%), and the rest is either transmitted through the water where it is eventually absorbed or absorbed by land. On the whole, a little more than half (51%) of the sunlight that hits the outer atmosphere makes it to the Earth's surface and most of that heat is absorbed by the oceans. Think about it: 71% of the surface of the Earth is covered with water. So the odds are better than 3:1 that a photon will end up in the ocean.

With me so far? Half the heat from the sun is absorbed at the Earth's surface (which includes the oceans). Put that heat in the SOURCE ledger. Now let's look at how heat leaves our planet, the SINK ledger.

The figure above lists three ways that heat can leave Earth's surface: 1) as long-wave radiation; 2) as sensible heat; and 3) as latent heat. What the heck is that stuff?

Recall that the electromagnetic spectrum consists of short wavelengths of radiation and long wavelengths of radiation. When a material absorbs heat in the form of short-wave radiation, it releases that heat as long-wave radiation. This might not seem intuitively obvious at all but just commit it to memory. The result is that the short wavelengths of light that make it through the Earth's atmosphere are absorbed and re-radiated as long wavelengths of light. This is a very important process because it's the principle on which the greenhouse effect is based. (Another way to understand this is to know that the hotter a body the shorter the wavelengths it emits; thus the sun emits much shorter wavelengths of radiation than the Earth.)

Sensible heat is the heat that is transferred from one molecule to the next as they bounce off of each other. Think of the atmosphere as a giant pool table with balls (molecules) bouncing off of each other. Every time two molecules (or balls) collide they interact and exchange energy.

The third sink for heat is latent heat. Latent heat involves the heat that is required to change the physical state of a substance, such as the conversion of ice to liquid water or gaseous water vapor. Changing the state of water from a solid to a liquid to a gas requires much more energy (heat) than the energy needed to raise its temperature. We'll learn more about latent heat when we study the properties of water.

Now, answer this question: of the radiation that reaches the Earth's surface, what percentage is released to outer space? You had better say all of it! What if the Earth's surface was releasing more heat than incoming? What if it was releasing less?

Let's take a look at a couple of the processes that affect Earth's heat budget and try to gain some understanding of the effect of natural or human processes. First, let's consider the effects of a volcanic eruption on the Earth's heat budget. When a volcano such as Mount Pinatubo erupts, it releases tremendous amount of dust into the atmosphere. What process would increases in dust affect and what would be the resultant change in Earth's temperature? You should be able to see that increases in dust would increase the absorption of light before it hits the Earth's surface. Dust also could reflect some radiation back into space. So the effect of dust is to act like an umbrella and prevent solar radiation from heating our planet. The result is a cooling of our atmosphere. During Mt. Pinatubo's eruption in 1991, the average temperature of our planet was about 1 degree cooler.

Let's look at another human creation: increases in greenhouse gases, such as carbon dioxide (CO2). Take a few moments to see where carbon dioxide plays a role in the Earth's heat budget. According to the figure, carbon dioxide can limit both incoming and outgoing radiation. But this figure only tells half the story. Carbon dioxide traps long-wavelength (infrared) radiation better than short-wavelength radiation. The effect is exactly analogous to a greenhouse. Ever left your car in the sun and came back to find the temperature inside the car is greater than the temperature outside? You have experienced the greenhouse effect. Glass and carbon dioxide trap long-wave radiation. As a result, the high-energy short-wave radiation makes it through the Earth's atmosphere but the long-wave radiation emitted by the Earth's surface doesn't. The result: heating of our planet. There is a great deal of circumstantial evidence that our planet is heating as a direct result of increases in the atmospheric concentrations of carbon dioxide.

   
   
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