January/February 2009

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• January/February 2009
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Are you intimidated by physics? Are you mystified by global warming, spy satellites, ICBMs, ABMs, fission, and fusion? Do you think all nukes, those in bombs and those in power plants, are basically the same? Are you perplexed by claims that we are running out of fossil fuels when there are counterclaims that we are not? Are you confused by the ongoing debate over global warming, when some prestigious scientists say that the debate is over? Are you baffled, bewildered, and befuddled by physics and high technology?

If so, then you are not ready to be a world leader. World leaders must understand these issues. The moment when you are being told that a terrorist left a dirty bomb hidden in midtown Manhattan is not a good time to have to telephone your local science advisor to find out how bad the situation really is. Nor is it a good time simply to assume the worst, to decide that all government resources must now be pulled off other projects to address this new emergency. You have to know enough to act wisely, quickly, proportionately.

Maybe you did study physics, enjoyed it, maybe even majored in it, and yet even now, after you got your degree, you still don’t know the important difference between a uranium bomb and a plutonium bomb, or between ozone depletion and greenhouse warming. And when your friends ask you about spy satellites, you tell them what you read in the newspapers — because such details were never covered in your courses.

Many, if not most, important decisions today have a high-tech component. How can you lead your country into a clean-energy future if you don’t understand solar power or how coal could be converted into gasoline? How can you decide important issues about research funding, arms control treaties, threats from North Korea or Iran, spying, and surveillance, if you understand only the political issues and not the technical ones? Even if you don’t plan to be a world leader, how can you vote intelligently without understanding these issues?

Equally important to understanding the physics of modern life is unlearning the things that you may think are true but aren’t. Mark Twain is often quoted as saying,

The trouble with most folks isn’t their ignorance.
It’s knowin’ so many things that ain’t so.

Ironically, this quote isn’t even from Twain — as if to illustrate the aphorism itself. The quote is correctly attributed to Josh Bil­lings, a nineteenth-century humorist.

Don’t know the physics you need to know? Fortunately, you have found the solution, or at least the beginning of the solution. This book covers advanced physics, the stuff that world leaders need to know. I skip the math because you don’t have time (or possibly the inclination) to master it. I move right to the important issues. When you understand the underlying principles, the physics, you need never again be intimidated by high tech. And if you ever need a detailed computation, you can always simply hire a physicist.

The Greenhouse Effect

Walk into a real glass-covered greenhouse and feel the stunning warmth and humidity. Enter an automobile parked in the sun and feel the oppressive, even dangerous heat. Now walk outdoors on a spring day and feel the pleasant temperature of the earth. In all three of these cases, the warmth is a result of the greenhouse effect — the same greenhouse effect that scientists now blame for the current global warming, the same greenhouse effect that will (unfortunately) be one of the biggest issues you will have to handle during your presidency.

The greenhouse effect is real and indisputable. It happens whenever energy gets in more easily than it can get out. Think of the parked car. Sunlight streams in through the windows. Some is reflected back out, but most of it is converted into heat — heat of the seats, the steering wheel, and the air inside. Hot air rises, so if you crack open the window a bit, it escapes and cool air flows in. A tiny opening can make a big difference. To cool a car quickly, open the sunroof. To cool a house, open an upper window.

Similar physics warms the Earth. Sunlight heats the surface of the Earth and the air above it. There is no glass to keep the air in, but gravity serves the same purpose, so the heat can’t get out by convection. There is only the vacuum of space outside, so the heat can’t conduct away. In fact, the only way heat can get out is by IR — infrared heat radiation. The Earth emits IR, but air absorbs it before it can get to space; unlike sunlight, air is opaque (black) to IR. The absorption of IR from the Earth warms the air even more, and the increased air temperature in turn warms the surface. This is the blanket effect — called the greenhouse effect when caused by sunlight. Energy is reflected back, giving us more heat — as illustrated in the figure at top.

The physics of the greenhouse effect: Sunlight passes right through the atmosphere and warms the Earth, but the IR radiation emitted by the Earth is absorbed by air, and some is reflected back down. As a result, the blanket of air keeps the Earth’s surface warmer than it would otherwise be. The Earth’s atmosphere is 99% nitrogen and oxygen. Remarkably, neither of these two gases absorbs IR, so they don’t contribute to the greenhouse effect. The absorption is all done by trace gases, primarily water vapor, carbon dioxide, methane, and ozone, as well as some others. These gases are known, collectively, as the greenhouse gases. To the extent that these gases are a natural part of the atmosphere, we have a natural greenhouse effect. In fact, if not for these gases, the surface of the Earth would have an average temperature of 12°F, 20 degrees below freezing! Look again at the top figure. The surface of the Earth is receiving heat not only from the sun, but from IR emitted by the atmosphere.

The greenhouse effect is one of the fundamental facts of atmospheric science. It is real; that fact is beyond dispute. Without it, the entire surface of the ocean would be frozen solid. Life — at least the kind that depends on liquid water and warmth — could not survive. We owe our existence to the greenhouse effect.

So why are we worried about it?

The physics of the greenhouse effect, with cloud reflection and atmospheric leakage included. The answer is that some of the heat radiation leaks out through the atmosphere, because there is not enough water vapor, carbon dioxide, and other gases to absorb all of the IR. Think of the atmosphere as a leaky blanket. This more accurate picture is shown in the bottom figure.

This figure shows two additional subtle but important effects. Not all of the sunlight reaches the surface; some is reflected by clouds. In addition, not all of the IR emitted by the Earth is absorbed by the atmosphere; some leaks through directly to space. Increase the clouds, and it will get cooler. Plug the IR leaks, and the Earth will get warmer.

We are currently doing just that, plugging the leak — not on purpose, but inadvertently. We are making the atmosphere into a better blanket — by pumping in carbon dioxide and other greenhouse gases. That’s the reason we are worried about the greenhouse effect. Remember, the basic greenhouse effect is real, responsible for the comfortable warmth of a spring day and the possibility of life on Earth. If we make the greenhouse effect stronger, the surface temperature of the Earth will rise. The IPCC estimates that the current rate of carbon dioxide injection will do a good job of plugging the leaking IR, and that will cause a rise in temperature somewhere between 3ºF and 10°F during your lifetime.

Carbon Dioxide

Carbon dioxide is created whenever carbon is burned. As its name suggests, a molecule of carbon dioxide consists of one atom of carbon and two (that’s the di-) of oxygen, giving it the chemical symbol CO2. Burn carbon, and you release both energy and CO2. We can separate the carbon dioxide back into its components, but only by putting back in the energy we took out. If we have used the energy — for example, to make electricity — we are stuck with the CO2.

Carbon dioxide is a tiny constituent of the atmosphere — only 0.038% — but it is enormously important for life. This trace gas is the primary source of our sustenance. Virtually all of the carbon in plants, the source of our food, comes from this tiny amount in the air. Plants use energy from sunlight to combine CO2 with water to manufacture hydrocarbons such as sugar and starch, in a process called photosynthesis. These hydrocarbons are the building blocks of our food and fuel. Photosynthesis also releases oxygen into the atmosphere. When we breathe in oxygen and combine it with food, we get back the energy that the plants absorbed from sunlight.

When we breathe in oxygen and combine it with food, we get back the energy that the plants absorbed from sunlight.

Scientists traditionally refer to 0.038% as 380 parts per million, abbreviated 380 ppm. The figure at the right shows how this level has changed over the past millennium. The amount of carbon dioxide was pretty constant from AD 800 until the late 1800s, at a level of 280 ppm. In the last century it has shot up to 380 ppm — an increase of 36%. If we continue to burn fossil fuels, we expect the carbon dioxide to keep rising.

Carbon dioxide in the atmosphere during the past 1,200 years. The sudden 36% rise in the recent past is due primarily to the burning of fossil fuels. It’s the recent rise that concerns people. Other measurements (not shown) tell us that the carbon dioxide level now is higher it has been at any time in the last 20 million years. That fact is not disputed; it is astonishing but not surprising. The carbon dioxide comes from human activity, including the burning of fossil fuels and the destruction of enormous regions of forest, primarily in South America and Africa. The latter cannot continue long, even if not stopped by conservationists, because we will run out of forests. In contrast, we will not run out of fossil fuels — at least not coal — for centuries. If we do nothing to stop it, the increase in carbon dioxide is expected to continue.

Until 2006, the United States was the biggest source of the carbon dioxide increase, contributing about 25% of the yearly additions. In 2006, China surpassed the United States, and its contribution continues to grow. China is building the equivalent of 50 to 70 new gigawatt (very large) coal-burning plants every year. Just one gigawatt coal plant burns a ton of coal every 10 seconds. Add in two oxygens from the atmosphere to make CO2, and that means 3 tons of carbon dioxide every 10 seconds, for each plant. World total power production is about 1000 gigawatts.

This carbon dioxide is being dumped into the atmosphere, where it is plugging the leaky greenhouse blanket. On that basis alone, we expect that the temperature should have risen slightly over the past century. To calculate just how big a rise it should have caused, we have to consider some other effects. The atmosphere is sufficiently complicated that the computation is best done with a computer — a big computer.

Calculating Greenhouse Warming

The computer programs used to estimate global warming are very similar to the computer programs used to predict weather. They are very good, but they are limited in their ability to get the details right. The real complication comes from the complexity of the Earth and the intricacy of the flow of heat, air, and water. There are mountains and valleys, oceans and glaciers, snow and foliage. Energy is transferred not only by conduction and radiation, but also by transport — ocean currents and trade winds. Those can be modeled; more difficult is transport on the small scale: thunderstorms and hurricanes and dust storms. Worst of all is cloud cover. Clouds are highly variable, and they can cool or warm, depending on their thickness and altitude and the time of day. Heat is transferred not only vertically but horizontally, in ways we don’t fully understand.

Everything is made more complicated by the response of the Earth to warming. A little carbon dioxide added to the atmosphere plugs the infrared leak and should certainly warm the Earth, provided nothing else happens. But other things do happen. Heating the oceans causes more water vapor to evaporate. Water vapor is also a greenhouse gas, so the temperature goes up even more. That’s an example of positive feedback; you get more warming than you might have expected. Estimates vary, but calculations indicate that current water vapor feedback should approximately double the warming effect of carbon dioxide. On the other hand, more water vapor might increase cloud cover, which reflects sunlight and reduces the heating. That’s negative feedback.

Why do I say increased water vapor might increase cloud cover?

Amazingly, our poor understanding of cloud formation is responsible for the largest uncertainty in climate calculations. Clouds are complicated. They are patchy, they affect each other, their reflectance depends on their altitude and thickness, and they move. Sometimes they even lead to rain. All this is far too complicated for physicists to be able to calculate, even using the biggest and the best computers, so we resort to approximations and empirical relations from past experience. As a result, we wind up with huge uncertainties. That’s why we can’t be 100% certain that carbon dioxide increases the temperature. It is largely the uncertainty in the behavior of clouds that led the IPCC to conclude that there is a 10% chance that humans are not responsible for global warming. In this scenario, cloud cover is canceling the carbon dioxide effect, and the warming is due to an unknown natural effect, perhaps a continuation of our exit from the Little Ice Age. On the other hand, for most people, a 90% certainty that humans are responsible is high enough to demand action.

Another Danger: Acid Oceans

The increase in atmospheric carbon dioxide leads to another potential problem — one that worries some people more than global warming. About half of the carbon dioxide released into the atmosphere dissolves into the surface water of the oceans, and that makes the oceans slightly more acidic. We measure the strength of acids in units called pH, with lower pH meaning more acidic. The best estimate is that the pH of the oceans has already decreased by about 0.1 as a result of fossil fuel burning. If the carbon dioxide in the atmosphere doubles (and this is expected to happen by the middle of this century) the pH of the ocean surface waters will drop by about 0.23. By 2100, the total drop will be between 0.3 and 0.5, assuming that we burn fossil fuels at the expected (not treaty-regulated) pace. These numbers are far more certain than the predicted values for the temperature change.

Is such a pH increase bad? In fact, there is quite a bit of variability in the pH of the oceans right now — about plus or minus 0.1 pH for different locations. The expected increase in the acidity of ocean water is not as severe as acid rain, which has a pH lowered by 2 full units. In fact, right now the oceans are actually a bit alkaline (the opposite of acid), so the net effect will be to make the water slightly less alkaline and more neutral. But whether we call it acidification or neutralization, the specific concern is from the fact that dissolved carbon dioxide interferes with the formation of external skeletons and shells in many organisms, from plankton and algae to corals. A pH change of 0.2 or greater is likely to trigger noticeable changes in ocean life, and most people think such changes are unlikely to be good.

In a broader sense, the worry is that we are indeed now significantly changing the chemistry of the oceans. The pH of a liquid is very important in determining the rate of chemical reactions. We are experimenting with the oceans in a way that cannot be undone in any conceivable way in the foreseeable future.

The Ozone Hole

I include a few paragraphs here on the ozone hole problem because it is often confused with the greenhouse effect. You need to know the difference. Both the ozone and greenhouse problems have to do with pollution in the atmosphere and with the absorption of radiation invisible to human eyes. Other than that, the two problems are quite different. In fact, in many ways the ozone story is happier.

Sunlight consists of visible light, infrared heat radiation (IR), and ultraviolet light, or UV. Unlike IR, ultraviolet light plays no important role in the greenhouse effect, but it is the key player in the ozone problem. UV is also called black light and is used for Halloween displays because it is invisible to humans but can make some chemicals glow brightly. It is also the component of sunlight that does the most damage to your skin, causing sunburn and possibly cancer. UV light is so potent at killing bacteria that a black light is frequently used as a germicidal lamp for sterilization.

Ozone is a very strong absorber of UV radiation from the sun.

UV is dangerous because the individual photons carry much greater energy than do those of visible or IR light. In your skin, these photons can break apart DNA and cause mutations. In the atmosphere, they break up O2 molecules into two individual oxygen atoms. These atoms attach themselves to nonbroken O2 molecules to make O3, also known as ozone. Ozone is a very strong absorber of UV radiation from the sun. This is another example of positive feedback: the air absorbs a little UV, and that creates a chemical (ozone) that absorbs even more. Most of the ozone is created between the altitudes of 40,000 and 60,000 feet, a region known as the ozone layer. The net result is good for us. The UV is absorbed in the upper atmosphere, and we are spared most of these deadly rays.

The ozone hole, 1981–99. Antarctica is prominent, and the southern tip of South America is seen in the upper right. Darker gray indicates ozone depletion. Without sunlight to create it, there is no ozone. That means the ozone layer is absent over the South Pole during its long sunless winter. When the sun finally rises (once every year), the ozone layer forms. For decades scientists have been studying this ozone cycle using UV sensors in Antarctica. In the 1970s, they noticed that the amount of ozone formed was decreasing every year. This decrease became known as the ozone hole. The figure above shows a NASA plot of the growing ozone hole.

Was this ozone decrease natural or caused by man? Would the hole spread to the entire globe or be restricted to Antarctica? Nobody knew, although some people thought the hole might be due to a pollutant introduced into the atmosphere by humans. In fact, that is what it turned out to be. A chemical called Freon was in widespread use at the time — in refrigerators and air conditioners, and as a cleaning agent. Freon and its relatives contained the elements chlorine, fluorine, and carbon. For that reason they are called chlorofluorocarbons, or CFCs. CFCs are highly stable; they don’t decompose readily, so when they are leaked into the atmosphere from defunct refrigerators and air conditioners, they stay there a long time. CFCs are carried by winds and storms and eventually reach the ozone layer, where they are hit by ultraviolet light. The energetic UV photons break the CFCs into their constituents of chlorine, fluorine, and carbon atoms. It turns out that chlorine and fluorine are very effective at converting ozone back to ordinary oxygen, O2. They are catalysts: they trigger the change but remain unchanged themselves, so they can keep acting over and over. Discarded refrigerators were, in effect, destroying the ozone layer.

The biggest effect happened to be over Antarctica. Nobody knew why, until atmospheric scientists realized that certain crystals of nitric acid formed there in the early spring, and on the surface of those crystals the chlorine and fluorine were far more effective at destroying the ozone.

Nobody knew for sure whether the destruction of ozone would continue until it reached more populated areas, but the world was sufficiently worried that it outlawed the use of CFCs in a treaty called the Montreal Protocol. This agreement has been an outstanding international success. CFC production has dropped dramatically, and as a result, we expect the problem not to grow. The existing CFCs will remain in the atmosphere for a long time, but the situation has stabilized. However, the size of the remaining hole has been of continuing concern to citizens in Australia, since distortions in the shape of the UV window sometimes extend to the southern parts of their continent.

CFCs had also been used as a propellant for aerosol cans, for everything from shaving cream to insect repellent. It has been replaced for that purpose with other gases, including nitrous oxide. Some people still boycott aerosol products because they don’t realize that the new ones are no longer dangerous to the ozone layer.

Because atmospheric chemistry is so complex, we don’t know for sure whether the ozone hole would ever have extended beyond the Antarctic region. Some people say that the real lesson from the ozone experience is that we can affect the atmosphere with human pollution, and that the effects are sometimes larger than we calculate. The success of the Montreal Protocol shows that international treaties can, in principle, be effective in stopping global pollution.

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