Part I: Global Climate Basics

Although it is too strong to state that all of the critical environmental issues facing the Southwest are related to the climate, a case can be made that climate is central to many of them. Moreover, the evolution of the climate over the next several decades, both the global climate and its manifestation regionally, could well involve new stresses on already stressed ecosystems in the Southwest. In particular, the accumulation in the atmosphere of carbon dioxide (CO2) from fossil fuel burning and other sources has the potential to change the climate that we have become adapted to. Therefore, it seems useful to provide a background discussion of the Earth’s climate, how the climate of the Southwest fits into the global picture, and what we may be in for down the road. My intention here is to provide only the outline of the complex system of processes that controls the climate and its variability. This is part one of two, dealing with processes in the global climate system; the second part discusses the climate of the Southwest.

A global system

Earth’s Climate and Its Stability

Radiation—the driver

First, it is important to introduce, and to de-mystify, the term radiation, because this is what controls the global climate at the most fundamental level. Not “ionizing” radiation, the kind associated with nuclear reactions or radioactive isotopes, but radiation from the Sun and from the Earth itself: sunlight and Earth’s radiant heat.

This discussion is not meant to be comprehensive, and the subject of quantum physics is too far afield to dig into here. Suffice it to say, therefore, that it is a fundamental property of matter that it emits energy in the form of electromagnetic radiation if its temperature is greater than absolute zero.

We are all familiar with the temperature at which water freezes, 0oC (or 32oF). Absolute zero—the temperature at which matter does not emit radiation—is measured with a different temperature scale, so that 0oK (Kelvin) is about -273oC (or about -460oF). Anything warmer than this emits some radiation.

Visible wavelengths of sunlight

Further, the peak wavelength of the spectrum of radiation emitted decreases with the temperature of the matter doing the emitting. Much of the radiation emitted by our Sun, the surface of which, at several thousand degrees Kelvin, is pretty hot, is in the visible part of the spectrum. The spectrum of colors in a rainbow, or as seen using a prism, represents much of the Sun’s emitted energy. At the other extreme, even the sparsely distributed gas molecules in deep space emit radiation—not much, but it’s there, very faint and at very long wavelengths. A couple of guys at Bell Labs got a Nobel Prize some time back for measuring it.

One way to understand this is with an experiment that you can do at home, if you have an electric stove. (It’s a lot safer to make this a thought experiment by just thinking about it instead of doing it. If you do it, don’t burn yourself!) Pick one of the burners on the top, one that’s been turned off for a long time (like, overnight) and so is at room temperature. If you touch it, it will feel cool, because it’s metal and will conduct the heat away from your hand efficiently. If you put your hand just over it, you probably won’t feel anything. Then, if you turn the burner on high, you’ll feel it start to warm up, even though you’re not touching it. You can’t see the heat that you can feel—this is infrared radiation. Pretty soon, it will get so hot you won’t want your hand there any more (so move it before you burn yourself!), and before too long, you’ll start to see the burner glowing deep red. As it gets hotter and hotter it changes to orange. This is an example of how the spectrum of the radiation, which you see as the dominant color, changes with temperature. You can’t see infrared radiation (but you can take a picture of it with infrared film). You can see the deep red and the orange because they are visible radiation, like the radiation from the Sun.

Because the Sun is hotter than the Earth, its radiation (solar radiation) has shorter wavelengths (orange is shorter than red is shorter than infrared) than the Earth’s radiation (terrestrial radiation, in the infrared part of the spectrum). These two kinds of radiation interact very differently with the molecules of air in the atmosphere, and with clouds.

Conceptually, Earth’s climate is pretty simple. The Sun emits solar radiation, some of which hits the Earth. The Earth absorbs a fraction of this and heats up. It heats up to the point that the terrestrial radiation emitted by the Earth just balances the amount of energy it’s absorbing from the Sun. If something changes, the balance may take a while to achieve—the oceans, for example, store a lot of heat and would take some time to cool down, should the Sun get dimmer. But it’s this balance—the Sun heats up the Earth and the Earth emits its own radiation to cool off—that the climate always tends toward.

In the simplest calculation, using the relevant formulas from math and physics along with satellite observations of how much sunlight the Earth absorbs, it’s possible to calculate what the temperature of the Earth should be. The satellites tell us that the Earth absorbs about 70% of the sunlight that it intercepts. This, combined with the fact that it is the disk of the Earth that is doing the intercepting of solar radiation while it is the entire Earth’s surface doing the emitting of terrestrial radiation, suggests that the Earth’s averaged surface temperature should be about -18oC (or just about 0oF).

Needless to say, this is nonsense. Even taking into account the cold polar regions, this is far, far too cold—observations of the Earth’s averaged surface temperature tell us that it’s more like 14oC (or 59oF—a rather cool room temperature).

Figure 1: Click for full size

The problem with this simple calculation is that it ignores the Earth’s atmosphere. Except for clouds, the atmosphere is almost transparent to solar radiation, but is only semi-transparent to terrestrial radiation. Thus (ignoring clouds for the moment), sunlight passes through the atmosphere and warms the Earth’s surface, which radiates its heat away. Some of this is absorbed by the atmosphere and some passes through to space. The part that’s absorbed by the atmosphere heats the atmosphere up, so the atmosphere radiates its own heat away, but in both directions, up and down, as depicted schematically in Figure 1. So now the Earth’s surface is absorbing radiation (that is, it’s receiving energy in the form of radiation) from both the sun and the atmosphere, and it maintains the 14o C averaged surface temperature as a result. This phenomenon—the extra heating of the Earth’s surface by the atmosphere—is part of what we call the Greenhouse Effect.

Of course, describing the Earth’s climate with one “Magic Number”—14oC—misses a lot. Nonetheless, a considerable amount of effort has gone into trying to understand how this “Magic Number” might change with the increase of the strength of the Greenhouse Effect due to increased amounts of CO2 in the atmosphere. This effort is understood in scientific circles to relate to providing a baseline from which more detailed work can grow. In the media, however, this effort has provided the grist for sound bites and no little amount of controversy.

Hot equator, cold poles

Perhaps the most obvious feature of Earth’s climate that the “Magic Number” approach misses is spatial variations, in particular the substantial difference between the climates of the tropics and the polar regions. The Earth is nearly spherical, and the angle of the incoming sunlight is more oblique in the polar regions (that is, the sun is lower in the sky—all day long—in the polar regions). So there is less sunlight absorbed in the Arctic and in Antarctica. So it’s colder. Because it’s colder, there is snow and ice on the ground, which makes the ground whiter than the tropical rain forests, and this tends to reflect sunlight, meaning that even less is absorbed and it is even colder. The percentage of sunlight reflected, instead of being absorbed by the Earth, is called the albedo. Snow and ice have high albedos, sometimes as high as 75%.

As with this example of snow and ice in the polar regions, the albedo is linked to the climate itself. This is why, in the simple calculation that gave the wrong answer above, we had to use satellite observations of the amount of absorbed sunlight. To calculate that would require knowing what the climate is doing, and even so it would be extremely complicated. Computer models of climate have become increasingly sophisticated, and complicated, in their attempts to include all of these relevant processes. It is these complicated feedback loops—in this case, cold temperatures making snow, which reflects sunlight, which makes it colder—that makes the study of climate both so difficult and so interesting.

The fact that the tropics absorb much more sunlight than the polar regions, and so are much hotter, leads to another important factor in climate: the winds. In a global sense, the winds are simply nature’s way of trying to adjust the temperature to be even. Molecular diffusion (the process that makes the handle of the skillet hot even though it’s not over the flame) isn’t fast enough, and so the air starts moving around. Moving air is wind. Of course, once the air starts moving, lots of other factors come into play, such as the Earth’s rotation and the characteristics of the surface (from fields to forests to mountains).

The humidity is also an important part of this, because water vapor in the atmosphere can condense when it’s moved around, making clouds. When water condenses, it releases energy in the form of heat, and this can create even more motion—another feedback loop. This is more easily understood if we first discuss a second feature of how the atmosphere varies spatially.

Condensed water vapor: a cloud


Everyone knows that warm air rises. This is why hot-air balloons fly, why candle flames look the way they do, and why thunderstorms work. But if this is so, why do people drive up to the mountains in the summer to cool off? Well, everyone also knows that the temperature of the atmosphere is lower the higher up you measure it. So what happened to all that rising hot air?

Simply put, it expands and cools off. If you compress air, it heats up, and if you do something to let it expand, it cools off. Pumping up a bicycle tire with a hand pump, and then feeling the base of the pump where the air goes into the tube (or valve) will prove that compression heats. (Yes, there’s friction heating, too; but mostly, the hot bicycle pump is because of compressing the air.) And, if you don’t mind the extra work, you can prove the expansion cooling part by letting the air out of the tire and feeling how it’s cooler than the ambient air—although to really prove this, you need a thermometer, because the expanding air will be moving and that makes it feel cooler, too.

Condensed, frozen water vapor: snow

Anyway, the atmospheric pressure is simply the weight of the column of air above where the pressure is being measured. At sea level, a one-inch square column of air weighs about 15 pounds. The pressure decreases with altitude, because there’s less air above where you are. So, as air rises, it expands, and when it expands, it cools off. Near the surface, below bases of clouds, the free atmosphere cools at a rate of about 10o C per kilometer of altitude (or about 5.5o F per thousand feet). Higher up, where clouds are condensing and releasing heat, the cooling rate is only about two-thirds of this. Of course, if you drive up into the mountains, you’re not in the free atmosphere, you’re stuck to the surface instead, so it doesn’t cool off quite this quickly.

Altitude is one of the most important factors when considering the climate of the Southwest. Not only does it control the temperature, it is the primary reason there is any water at all.


Find yourself a world map somewhere and trace the latitude (37o N) of the Arizona/Utah and New Mexico/Colorado state lines around the world. Except for the maritime longitudes—the Atlantic and Pacific Oceans, and the Mediterranean Sea—and except for a few areas such as the U.S. east of the Mississippi, the Himalayas of north of India, and Eastern China, this is a pretty dry latitude circle: it includes not only the US Southwest but also the Middle East, Iran, Afghanistan, and the deserts of Tibet. Moreover, the maritime longitudes are quite dry as well: the classification “Mediterranean Climate” describes a mild, dry region (such as Spain), so it is no accident that the Southwest is arid.

But it is not completely arid (officially, it’s classified as “semi-arid”). As noted in the previous section, altitude is a strong influence on the Southwest’s water resources, because the precipitation—both rainfall and snow—tends to be strongly linked to mountains. This is not only because it’s colder up there, but also because when air flows over a mountain, it gets lifted up, cooled, and the water vapor can then condense into clouds and make precipitation. The popular explanation for this is that warmer air can “hold” more water vapor than cooler air. This is not a precise description of the phenomenon, let alone an explanation of it, but it is sufficient for our purposes here.

(This makes two popular descriptions of processes in atmospheric science that are misnomers. The “Greenhouse Effect” really isn’t; and warm air does not “hold” more water vapor than cool air. I can’t fix this stuff, but I’ll try to be honest about it.)

Water, in its three phases, plays a variety of roles in the climate. As noted above, snow and ice (the solid phase) affect the amount of sunlight the Earth absorbs. The vapor phase functions to redistribute water from its origins in surface evaporation, especially from the ocean surface, across the planet. When water vapor condenses, it releases heat and affects the atmospheric circulation and, as rain and snow, provides liquid water resources for life itself. This complex behavior makes water by far the most important substance in the climate system, and the fact that it is essential for life emphasizes this importance.

Another role played by water vapor is another complicated feedback process in the climate system. Like CO2, water vapor contributes to Earth’s Greenhouse Effect. As a natural greenhouse gas, water vapor in the atmosphere will change as the climate changes, in all probability. If atmospheric water vapor increases as the climate warms up, it will cause the climate to warm up some more.

And clouds are water, in liquid drops and ice crystals. Clouds have their own Greenhouse Effect; and, like snow on the ground, they also reflect sunlight. Because of these competing effects on climate, they deserve their nickname as the “wild cards” of the climate system.

With this background, it’s possible to illustrate some aspects of the Four Corners Climate with examples.

Clouds do affect climate

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