EARTH 002:

GAIA -- THE EARTH SYSTEM

Climate systematics

The study of climate is a study of weather patterns and the processes that produce them. For example, a climatologist might study the different weather patterns that affect San Francisco and New York. Both share a number of characteristics that are important in controlling a regions climate. They are at low elevation and near the ocean. They are at the same latitude and so receive equal inputs of solar energy each year, but their weather is quite different. New York has colder winters and hotter, more humid summers. San Francisco has very dry summers while New York has most of its yearly rainfall in the summer months. New York experiences numerous thunder storms each year while thunder and lightening are extremely rare in San Francisco. Why are they so different when they seem so similar in their geographic setting? This is a question for the climatologist.

Hurricanes illustrate another difference between climate and weather or climatologists and meteorologists. A meteorologist would study a hurricane to be able to predict its path, provide estimates of its strength and its potential to do damage. A climatologist would be more interested in the phenomena that create the hurricane and control its force and movement so as to better understand all hurricanes. She want to compare a particular storm to other hurricanes because it is by comparing many storms that the patterns needed to understand the hurricane system become apparent.

Both weather and climate are controlled by interactions among the atmosphere, the solid earth's surface, the oceans, glaciers, and the transfer of energy across space. As a result they cannot be studied without applying some of the principles of systems analysis. Climate is, therefore, a natural choice to be an example of how systems (and graphical) analysis can be applied to complex natural problems. In the next few weeks, the course will introduce basic information concerning the climate system and apply the methods of systems analysis to discover questions we might want to ask if we are better understand climate, how and why it has changed in the past and how our actions may cause it to change in the future.

For starters, climate can be reduced to a problem of energy accounting. The input of solar energy and the transfer of energy within the earth's oceans and atmosphere create weather and climate. It is a system, therefore, in which thermodynamic principles play an obvious and important role in controlling system behavior. In fact, it would be inaccurate to begin a discussion of climate without considering the energy balance that controls the heat content of the system.

This simple flow chart is extremely important. It is an over simplification, and we will add more to it later. For now let us begin by considering energy inputs and outputs. Bodies (objects) in space are either extremely cold or are constantly radiating energy to space. Nineteenth century physicists established that the radiation profile of an object like the sun or the earth is directly correlated to the temperature difference between the object and its surroundings. The graph below shows typical radiation distribution for an ideal radiator. It is important to note that hotter objects radiate both more energy and that more of the radiation they emit is shorter wavelength, higher-energy light. These facts are important to Earth's climate because both the sun and the earth act as nearly ideal radiators. Because the sun's surface is much hotter than the earth's, the sun gives off much more energy as light than the earth and the sun's light is at shorter wavelength. The sun radiates mostly in the ultraviolet and visible wavelengths. The earth because it is cooler, radiates infra-red and microwave radiation.

The energies emitted Earth and Sun are vastly different. However, because the Earth is 150 million km (93 million miles) from the sun, we only receive a very small percentage of the energy the sun emits; and a balance can still be reached.. This balance results in a nearly constant amount of energy remaining in the earth systems producing a nearly constant average global temperature. If this were not true what would happen? If the balance were distorted and more energy escaped than entered the system, of course the planet would grow cooler. But if the earth is at a stable steady state with respect to temperature, the energy systematics would change in some way to re-establish the balance and return to the original average temperature.

Two graphs, one of temperature vs. altitude and the other temperature vs. latitude indicate that the actual process is more complicated than the simple flow chart might lead us to believe.

Looking first at the energy content of the atmosphere at different altitude, one sees that temperature decreases as altitude increases up to about 12 km. Above that altitude, in the stratosphere, temperature increases. One can apply the principles of thermodynamics, especially the Second Law to discover the cause of this temperature profile. You know that heat always flows from hot to cold. Therefore, the movement of energy in the stratosphere must be from the top down and in the troposphere, the layer of atmosphere nearest the earth's surface, energy must move from the bottom up. This implies two distinct sources of heat. The stratosphere is being heated directly by the sun. Ultraviolet rays interact with ozone layer and transfer their energy to the molecules of that layer. Because the concentration of ultraviolet rays (or flux which is scientific jargon for the amount of energy moving through a system) is less at lower altitudes temperature decreases from top to bottom of the stratosphere. The stratosphere is heated almost exclusively by radiant energy from the sun.

The heating of troposphere is quite different. Radiation, conduction and convection are all important to the process, and the heating by the sun is indirect. The earth's surface must be heated first and then it transfers the radiant energy it has absorbed to the atmospheric gases above it. Before continuing, take a moment to use the Second Law to convince yourself that this must be true. Where would the troposphere be hottest if it were heated directly by the sun? Well, you ask is it possible that the lower atmosphere is not heated by the sun at all? Perhaps it is energy escaping the earth's interior that warms the atmosphere Didn't Lord Kelvin show that heat is escaping? Yes, but this cannot account for the energy differences in the atmosphere. Because the earth is round, heat should move outward from the core relatively smoothly in all directions if minor irregularities like volcanoes are ignored. As a result what should be true about the temperature distribution relative to latitude if it were the earth's heat that warmed the lower atmosphere? (Email your ideas.) We can also measure the amounts of heat escaping the earth. Nearly 1000 times more energy reaches the earth's surface from space than reaches it from the interior. So no, the earth's surface is the source of heat for the lower atmosphere, but the original source of the heat must be the sun.

The heating process begins when the earth's surface -- rock, soil, vegetation, or water -- absorbs radiant energy emitted by the sun. The amount of energy absorbed depends on several factors. Color is important. Clouds and snow because they are white are very reflective and redirect most of the energy that strike the back into space. Rocks and vegetation are usually fairly dark and so absorb most of the energy. Water absorbs energy well when the light enters the water at a nearly perpendicular angle, but becomes more reflective as the angle of incidence, the angle at which light strikes the surface increases. Angle of incidence also affects the energy that reaches an area of the surface as well. When the light strikes at an angle, the rays are spread over a larger area and so each square inch of surface gets less than if the light was striking at a 90° angle. You can demonstrate this with a flashlight if you shine it directly on a piece of paper and then gradually tilt the paper to increase the angle of incidence. You will see the paper become dimmer as the light is spread over a bigger area. It's just another example of the First Law

Angle of incidence is always measured as the larger of the two angles created by the ray and the earth's surface. Because angle B is smaller than A, more energy is delivered to the surface at B.

As the sunlight strikes the earth's surface, its temperature rises and it becomes warmer than the air around it. Heat is then transferred to the air in contact with the surface by conduction, and as the air is heated, its density decreases. (Make sure you can explain why this happens.) The less dense warm air begins to rise starting a convection current with cooler, denser air moving down to the surface to replace the rising warmer air. The rising air will transfer some of its energy to the higher atmosphere by mixing with it as it rises. This causes the rising air to cool. The net result is a general decrease in temperature with altitude.

Two other processes also act to create this temperature profile. One is radiation. The infra-red and microwave radiation that is emitted by the earth is partially absorbed by CO2, H2O, and methane in the atmosphere. Just as the effect in the ozone layer was greatest nearest the source of radiation (the sun), the effect of atmospheric heating by earth's radiant energy is most effective near the earth's surface. Higher in the atmosphere, there are fewer rays and molecules are available and so the heating is less effective.

The third process is called adiabatic heating and requires that we digress briefly to consider air pressure. Air pressure is the force exerted by air molecules on an object. This force can be described as a result of the weight of all the molecules in the air above the object. On average, a column of air stretching from the top of the atmosphere to sea level and having an area of one square inch will weigh 14.7 pounds. If you go higher in the atmosphere, because their are fewer molecules above you, the air pressure decreases. Now imagine a volume air that is rising held together by an invisible balloon that is so stretchable that it exerts no force on the air within it. Because the external pressure decreases as the air rises, the volume we are considering expands. Make sense? Now think back to the ideas about potential energy and entropy, and reversible and irreversible change. Can the air be compressed again if it were brought sown to lower elevation? Yes. The change is, therefore, reversible. And, which tendency caused the air to expand? Sure, increasing entropy. If that is true, what had to happen to potential energy for this to be a reversible change? It had to increase which means that energy had to be stored, transformed from an active to a potential form. The active form that is stored is heat and because heat is stored, the temperature of the gas decreases.

(Ask yourself does this make sense on a molecular level? If there are only two molecules in the balloon, when do they have lowest potential energy, and what happens to potential energy as those two molecules move farther and farther apart?)

This cooling process that involves no net change or loss of energy is called adiabatic cooling. Air heated when it is compressed, the reversed process, is adiabatic heating, meaning heating without energy input. If only adiabatic cooling affected the change of temperature with altitude, there would be a decrease in temperature equal to about 10° C per km. However, convective mixing and radiant heating also affect the temperature profile as does evaporation and condensation of water. As a result the profile is not fixed. Temperature usually decreases between 5 and 10° C/km.

Reading question:  Explain why Recife, Brazil that lies near the equator should on average be warmer than Philadelphia and why on the early days of summer, Philadelphia might be warmer than Recife.

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