Atmosphere: Thin Shield, Big Impact - A Short History of Nearly Everything

Okay, so, wow, the atmosphere, right? Like, thank goodness for it. It gives us, you know, a warmish place to hang out. Otherwise, Earth would be, like, a totally dead ice ball, averaging, I don't know, like, fifty below zero Celsius. And, you know, the atmosphere, it absorbs or deflects, like, all this crazy cosmic radiation, charged particles, UV rays... all that stuff. Basically, a thick atmosphere is, um, equivalent to, like, four and a half meters of protective concrete. Without it, all that space junk would, like, be stabbing us with tiny little daggers, you know? Even raindrops, without the atmosphere slowing them down, would knock us flat.

But, get this, the atmosphere isn't really all that much. It stretches up, like, 190 kilometers, which sounds like a lot, but if you, like, shrunk the Earth down to the size of a desk globe, the atmosphere would only be, I don't know, maybe one or two coats of paint thick.

So, for scientific reasons, they've divided the atmosphere into, like, four layers of varying thickness: the troposphere, the stratosphere, the mesosphere, and the ionosphere, which is often now called the thermosphere. Now, the troposphere is, like, the really precious part for us. It contains, you know, enough heat and oxygen to keep us alive, even though, you know, if you start climbing up into it, it quickly becomes a pretty unpleasant place to be. From the ground to its highest point, the troposphere, or the対流圏, is about sixteen kilometers thick at the equator, and less than ten or eleven kilometers thick, um, in the temperate zones where most of us live. Eighty percent of the atmosphere's mass, and practically all weather, are contained in this relatively thin, kind of wispy layer. So, there's actually not much between you and the sky, you know?

Above the troposphere is the stratosphere. When you see the top of a thundercloud spreading out into that classic anvil shape, you're actually looking at the boundary between the troposphere and the stratosphere. This invisible ceiling is called the tropopause, and it was actually discovered in, like, 1902 by a Frenchman, um, Leon-Philippe Teisserenc de Bort, in a balloon. "Pause," in this case, doesn't mean "stop," but rather "capping," you know?

Even at its highest point, the tropopause isn't really that far away. A fast elevator in a modern skyscraper could, like, get you there in, I don't know, twenty minutes. But, I wouldn't really recommend it, because, without being sealed in, that rapid ascent would probably give you severe edema in your brain and lungs. All that extra fluid in your tissues, you know, to a dangerous degree. When those observation deck doors open, almost everyone inside would be dead or dying. Even a slower climb would be pretty unpleasant. The temperature at ten kilometers altitude is, like, minus fifty-seven degrees Celsius, so you'd need oxygen, like, immediately.

After leaving the troposphere, the temperature rises pretty quickly again, to, like, four degrees Celsius, because of the ozone layer absorbing radiation. Then, in the mesosphere, the temperature plunges again to, like, minus ninety degrees Celsius. And then, in the appropriately named thermosphere, it suddenly rises to, like, over fifteen hundred degrees Celsius. And the temperature in the thermosphere can vary by, like, over five hundred degrees Celsius from day to night. But, it's important to understand that at that altitude, temperature is kind of just a theoretical concept. Temperature is really just a measure of how much molecules are moving. At sea level, the air molecules are really close together, so a molecule only has to move a tiny distance – like, a millionth of a centimeter, to be exact – before it bumps into another molecule. With trillions of molecules constantly bumping into each other, there's a lot of heat being exchanged. But up in the thermosphere, at eighty kilometers or more, the air is so thin that molecules are separated by several kilometers and barely ever touch. So, even though each molecule has a lot of kinetic energy, there's very little interaction between them, so there's not much heat transfer. Which is actually good news for satellites and spacecraft, because if there were a high frequency of heat exchange, anything orbiting up there would, like, burst into flames.

Even so, spacecraft have to be really careful in the outer atmosphere, especially when they're re-entering Earth's atmosphere. The Space Shuttle Columbia disaster in, you know, showed that, even though the atmosphere is thin, if a spacecraft enters at too steep an angle – like, more than six degrees – or too fast, it'll hit so many molecules that it generates a huge, easily ignitable force. Conversely, if the returning craft enters the atmosphere at too shallow an angle, it'll probably just bounce back into space, like a pebble skipping across water.

You really don't need to risk going to the edge of the atmosphere to appreciate how dependent you are on the ground. Anyone who's lived in a high-altitude city knows that you don't have to get more than a few hundred meters above sea level before your body starts to feel uncomfortable. Even experienced mountaineers, even though they're fit and trained and have oxygen tanks, quickly get altitude sickness: confusion, nausea, fatigue, frostbite, hypothermia, loss of appetite, and all sorts of other malfunctions. The human body has a lot of really insistent ways of reminding its owner that it's not meant to function at high altitudes, you know?

"Even in the best circumstances," mountaineer Peter Habeler wrote about being at the top of Everest, "every step at that altitude requires an enormous act of will. You have to force yourself onward, to grab at what you can grab. You feel, all the time, an extreme sensation of fatigue." In "The Other Side of Everest," British mountaineer and filmmaker Matt Dickinson recorded an incident involving Howard Somervell, who was climbing Everest in, you know, with a British expedition. Somervell "found a piece of inflamed flesh coming away and blocking his windpipe, very nearly choking him." It took Somervell a lot of effort to cough the flesh up. It turned out he coughed up "the entire mucous membrane of his throat."

At altitudes above, like, seventy-five hundred meters – what climbers call the "death zone" – the body gets really unhappy. But, a lot of people get extremely weak, even critically ill, at only, like, forty-five hundred meters. Sensitivity has very little to do with physical fitness. Sometimes, grandmothers are perfectly fine at high altitudes, while their strong, young descendants are struggling and have to be taken down to lower elevations, you know?

As for human endurance for continuous living, the limit is about fifty-five hundred meters. Even people who are used to living at high altitudes can't really tolerate that altitude for long. Francis Ashcroft, in his book "Life at the Extremes," says that there are sulfur mines in the Andes at fifty-eight hundred meters, but the miners would rather walk down four hundred and sixty meters every night and then climb back up the next day, than live at that altitude continuously. People who live at high altitudes often gradually develop, over thousands of years, unusually large chests and lungs, which increase the concentration of oxygen-carrying red blood cells by, like, a third. But there's a limit to how much red blood cell concentration can be tolerated. If the concentration gets too high, the blood can't flow smoothly. And, at altitudes above fifty-five hundred meters, even women who are fully acclimatized can't supply enough oxygen to a developing fetus. They just give birth prematurely.

So, in, you know, Europeans started making balloon ascents for testing. And they were surprised to find that the higher they got, the colder it was. For every thousand meters you climb, the temperature drops about 1.6 degrees Celsius. Logically, it seems like you should feel warmer the closer you get to the heat source. But, part of the explanation is that you're really not getting much closer to the sun. The sun is, like, 150 million kilometers away. Moving a few hundred meters closer to it is, you know, like standing in Ohio and walking one step closer to a forest fire in Australia, hoping to smell the smoke. To answer this question, we have to go back to the problem of molecular density in the atmosphere. Sunlight activates atoms, increasing their speed. The atoms, when activated, collide with each other, releasing heat. When you feel the sun warm on your back on a summer day, you're actually feeling the sunlight activate atoms. The higher you climb, the fewer atoms there are, so they collide less often. Air is, like, really deceptive. Even at sea level, we tend to think of air as being very light, almost weightless. But actually, air has a lot of mass, and it often makes itself known. A long time ago, marine scientist Wyville Thomson wrote: "When we get up in the morning, we sometimes find that the barometer has risen an inch [two and a half centimeters], showing that about half a ton weight has been pressing quietly on us all night without our feeling any inconvenience, but rather with a sense of increased elasticity, because a smaller effort is necessary to move the body in a denser medium." You don't feel crushed under that extra half ton of pressure for the same reason that your body doesn't get crushed at the bottom of the ocean: your body is made up mostly of incompressible liquids. The liquids generate a counterforce that keeps the pressure inside and outside your body balanced, you know?

But if air is in motion, like in a hurricane, or even just a strong wind, you quickly realize that air actually has a lot of mass. There are, like, 5.2 quadrillion tons of air all around us – over nine million tons per square kilometer of this planet, so, that's not nothing. When millions of tons of air are moving at, you know, fifty or sixty kilometers per hour, it's no wonder that tree branches snap and roof tiles get blown away. As Anthony Smith put it, a typical weather front can consist of, like, a billion tons of warm air piled on top of seven hundred and fifty million tons of cold air. No wonder meteorologists sometimes get excited, you know?

And, of course, the world above us is full of energy. A big thunderstorm can contain the equivalent of, you know, four days' worth of electricity for the entire United States. Under the right conditions, a thundercloud can rise to ten or fifteen kilometers altitude, containing rising and falling air currents moving at over, like, one hundred and fifty kilometers per hour. They often occur side by side, so pilots really don't want to fly through them. Inside all that chaos, the particles in the cloud acquire an electrical charge. For reasons that aren't completely understood, the lighter particles tend to take on a positive charge and get blown to the top of the cloud. The heavier particles stay at the base, accumulating a negative charge. These negatively charged particles have a strong desire to rush toward the positively charged Earth – and may whatever's in between them be, you know, safe. Lightning moves at, like, four hundred and thirty-five thousand kilometers per hour, and can heat the surrounding air to twenty-eight thousand degrees Celsius, several times hotter than the surface of the sun. At any given moment, there are, you know, eighteen hundred major thunderstorms happening around the world – that's, like, forty thousand a day on average. Lightning flashes across the planet day and night, with about a hundred bolts hitting the ground every second. The sky is really, like, a lively place, you know?

It's amazing how much of what we know about what's going on up there is relatively recent. The jet stream, which is usually located at nine to ten thousand meters altitude and can move at nearly, you know, three hundred kilometers per hour, and really affects weather systems on every continent, wasn't discovered until, you know, when pilots started flying into it. And, even now, there are a lot of atmospheric phenomena that we still know very little about. There's a type of turbulence commonly called "clear air turbulence" that occasionally causes planes to jolt violently. About, I don't know, twenty such incidents a year are serious enough to be reported. They're not related to cloud formations or anything else that can be seen with the naked eye or detected by radar. They're just little patches of turbulence in clear air. In a typical example, a plane flying from Singapore to Sydney was flying over central Australia in calm conditions when it suddenly dropped, like, ninety meters. That's enough to throw anyone not wearing a seat belt against the ceiling. Twelve people were injured, one seriously. And nobody really knows how these little pockets of chaos come to exist, you know?

The process by which air moves around in the atmosphere is the same process that makes Earth's internal machinery tick, which is called convection. Warm, moist air rises from the equatorial regions and spreads out when it hits the tropopause. As it moves away from the equator, it gradually cools and sinks. When it hits the ground, some of the sinking air flows toward low-pressure areas and heads back to the equator, completing the cycle.

In the equatorial regions, the convection process is usually pretty stable, and the weather is always pretty good. In the temperate zones, seasonal changes and regional differences are, you know, more pronounced, and there's a lack of regularity. As a result, there's a perpetual battle between high-pressure and low-pressure systems. Low-pressure systems are created by rising air, which carries water molecules into the sky, forming clouds and eventually rain. Warm air can carry more water than cold air, which is why the tropics and the rainy season have so much rain. So, low-pressure areas are usually associated with clouds and rain, while high-pressure areas are usually sunny and have good weather. When these two systems meet, you can often tell from the shape of the clouds. For example, if a rising current of moist air can't break through a more stable layer of air above it, it spreads out like smoke hitting a ceiling, forming stratus clouds: those unattractive, featureless layers that make the sky look so gloomy. In fact, if you watch someone smoking a cigarette and watch the smoke rising from the cigarette in a windless room, you'll get a pretty good idea of what's going on. At first, the smoke rises straight up – that's called laminar flow, if you want to sound, you know, intellectual. Then it spreads out, diffusing into a wavy layer. The world's largest supercomputers, used in carefully controlled measurement environments, can't accurately predict what shape those wavy lines of smoke will take, but meteorologists have to predict that motion in a constantly rotating, large, windy world. You can imagine how difficult that is, right?

What we do know is that the sun's uneven distribution of heat creates different pressures across the planet. Air can't tolerate that, so it rushes around, trying to achieve a balance everywhere. Wind is just one way that air tries to achieve that balance. Air always flows from high-pressure areas to low-pressure areas, which makes sense. The bigger the difference in pressure, the faster the wind.

Oh, by the way, wind speed, like, most accumulative things, increases exponentially. So, a wind blowing at three hundred kilometers per hour isn't, you know, ten times stronger than a wind blowing at thirty kilometers per hour. It's one hundred times stronger. So, it's a lot more destructive. Accelerating millions of tons of air to that degree can generate huge amounts of energy. A tropical hurricane releases, you know, the same amount of energy in twenty-four hours as a rich, medium-sized country like Britain or France uses in a year.

The atmosphere's drive to achieve balance was first discovered by Edmond Halley, and was articulated in, you know, by his British compatriot George Hadley. Hadley noticed that rising and falling air columns tend to create "cells" - since then referred to as Hadley cells. Hadley was a lawyer by profession, but he had a keen interest in weather, and he proposed the relationship between circulation, the Earth's rotation, and the apparent deflection of the air. It turns out that deflection creates the trade winds. However, it was Gustave-Gaspard de Coriolis, an engineering professor at the École Polytechnique in Paris, who worked out the details of these interactions. So, we call it the Coriolis effect. Coriolis also invented the water cooler, which is still known as the Coriolis cooler. The Earth rotates at about, you know, sixteen hundred and seventy-five kilometers per hour at the equator. As you move toward the poles, that speed slows down considerably. If you think about it, the reason is self-evident. If you're at the equator, the Earth has to rotate you over a pretty long distance, like, about forty thousand kilometers, to get you back to where you started. But if you're at the North Pole, you only have to walk a few meters to complete a circle. However, in either case, it takes you twenty-four hours to get back to where you started. So, the closer you are to the equator, the faster you'll have to rotate, right?

So, why does an object moving in a straight line in the air perpendicular to the direction of Earth's rotation seem to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, as long as it travels a considerable distance? The Coriolis effect says that it's because the Earth is rotating underneath, you know? A common way to understand this is to imagine standing in the middle of a large stadium and throwing a ball to someone standing on the edge. By the time the ball reaches the edge, the person has moved forward and the ball has flown past them. From their perspective, the ball appears to have curved around them. That's the Coriolis effect. That effect makes weather systems curl, and makes hurricanes move like spinning tops. The Coriolis effect also explains why the navy has to adjust the direction of its cannons to the left or right during shelling. Otherwise, a shell fired at a target twenty-five kilometers away will miss by, like, ninety meters.

Considering how important weather is to, like, almost everyone, in both practical and psychological ways, it's really amazing that meteorology didn't become a science until the nineteenth century. Accurate readings depend on, you know, a glass tube with a very uniform inner diameter, which turns out to be pretty difficult to make. The first person to solve this problem was Daniel Gabriel Fahrenheit, who made a very accurate thermometer in, you know,. However, for some unknown reason, he set the freezing point on his thermometer at thirty-two degrees and the boiling point at two hundred and twelve degrees. That quirky scale made things inconvenient for some people right from the start. In, you know, Anders Celsius, a Swedish astronomer, proposed another scale. To prove that inventors rarely get things completely right, Celsius put the boiling point at zero degrees and the freezing point at one hundred degrees on his scale. But that scale was quickly reversed, so it's all good.

The person most often seen as the father of modern meteorology is Luke Howard, an English pharmacist who became famous in, you know,. Howard's main contribution was to give names to the types of clouds. He was an active and respected member of the Linnaean Society. He used the Linnaean principles in his new system, but he chose the little-known Askesian Society as the forum to announce his new classification scheme. It's hoped Howard's statements were taken seriously. Howard's acolytes are, like, oddly silent about this point.

Howard classified clouds into three types: layer clouds, which he called stratus; fluffy clouds, which he called cumulus. He added a fourth name later, calling a rain-producing cloud nimbus. The beauty of Howard's system is that these basic elements can be freely combined to describe every shape and size of cloud floating in the sky: stratocumulus, cirrostratus, cumulonimbus, etc. The system was an instant success, and not just in England. Goethe liked the system so much that he wrote four poems dedicated to Howard.

Over the years, Howard's system has been expanded considerably, culminating in the, you know, encyclopedic International Cloud Atlas. Interestingly, the types of clouds identified after Howard's death – such as mammatus, pileus, velum, spissatus, floccus, and mediocris – are said to be not accepted outside the meteorological community, and not accepted by many people inside it. By the way, the first and much thinner edition of that atlas divided clouds into ten basic types. The cloud type listed in ninth place, cumulus cumulonimbus, is the fullest and most cushion-like. The expression "cloud nine" seems to come from this, you know.

Despite their menacing appearance, anvil-shaped thunderclouds are actually mild and insubstantial. A fluffy cumulus cloud in the summer contains only a hundred to a hundred and fifty liters of water – "about enough to fill a bathtub", as James Trefil says. If you want to know how insubstantial a cloud is, you can walk in the fog. Fog is just a cloud that hasn't really decided to get going. In Trefil's words again: "If you walk one hundred yards in an ordinary fog, you'll only come into contact with about half a cubic inch of water – not enough to take a good sip." So, clouds aren't really big reservoirs of water. At any given time, only about 0.035% of the fresh water on Earth is floating above our heads, you know?

The fate of a water molecule varies greatly, depending on where it lands. If it lands in fertile soil, it'll be absorbed by plants or re-evaporate directly in a few hours or days. However, if it gets into groundwater, it may not see the sun again for many years, or even many thousands of years if it flows really deep. If you look at a lake, you're seeing a pile of molecules that have been there for, on average, ten years. It's thought that water molecules stay in the ocean for more like a hundred years. In general, after a rain shower, about sixty percent of the water molecules will return to the atmosphere within a day or two. Once evaporated, they spend about a week in the sky, and then fall again as rain.

Evaporation is a really fast process. It's easy to measure based on the fate of a puddle of water on a summer day. Without a continuous supply of water, even a huge thing like the Mediterranean Sea would dry up in a thousand years. That happened less than six million years ago, causing what scientists call the Messinian salinity crisis, because continental movement blocked the Strait of Gibraltar. As the Mediterranean Sea dried up, the evaporated water vapor fell as freshwater rain into other seas, slightly lowering their salinity. This actually diluted the seas just enough to allow ice to form over a larger area. The expansion of the ice fields bounced more of the sun's heat back, pushing Earth into an ice age. At least in theory, you know?

As far as we know, one thing is certain: even the slightest change in Earth's dynamics can have unimaginable consequences. And we may well have been born out of an event like that.

The ocean is the real engine of surface activity on Earth. In fact, meteorologists increasingly see the ocean and the atmosphere as a single system, so, a few more words here. Water is really good at storing and transporting heat – huge amounts of heat. The Gulf Stream delivers as much heat to Europe every day as the world produces in coal in ten years. That's why winters in Britain and Ireland are milder than winters in Canada and Russia. Water heats up very slowly, so even on the hottest days, the water in lakes and swimming pools is still cool. For that reason, it often happens that a season has already begun from an astronomical point of view, but it doesn't really feel like that season. So, spring in the Northern Hemisphere starts in March, but it doesn't feel like spring in most places until April, you know?

Seawater isn't a uniform whole. There are differences in the temperature, salinity, depth, and density of seawater in different regions. And these differences have a huge impact on how seawater transports heat, which in turn affects the climate. For example, the Atlantic Ocean is saltier than the Pacific Ocean, which is a good thing. The saltier the seawater, the greater the density, and denser seawater sinks. If the Atlantic Ocean's currents didn't have to carry that extra salt, they'd push on into the Arctic, warming the Arctic, but Europe would completely lose that valuable heat, you know? The main carrier of heat on Earth is what's called thermohaline circulation. It originates in slow currents deep in the ocean. The name "thermohaline circulation" seems to mean different things to different people. Karl Wunsch of MIT published an article called "What Is the Thermohaline Circulation?" He argued that the name at least expresses seven different phenomena in major journals. What's happening here is, after surface seawater reaches the area near Europe, the density increases. It sinks into the depths and slowly returns to the Southern Hemisphere. When it reaches Antarctica, it meets the Antarctic Circumpolar Current and is pushed on into the Pacific Ocean. The process is very slow – it takes seawater fifteen hundred years to flow from the North Atlantic to the central Pacific. But the amount of heat and water transported is pretty substantial, and the impact on the climate is huge, you know?

How do scientists actually figure out how long it takes a drop of water to get from one ocean to another? Scientists can measure the mixtures in the water and calculate how long it's been since it was last in the air. By comparing measurements at different depths and in different locations, they can map the water's route pretty accurately.

Thermohaline circulation not only transports heat, but also stirs food as the ocean currents rise and fall, making larger areas of the sea suitable for fish and other marine animals to live in. Unfortunately, thermohaline circulation also seems sensitive to changes in the surrounding environment. Computer simulations show that even a slight dilution of ocean salinity can disrupt the cycle disastrously, you know?

The ocean also does us another big favor. It absorbs a lot of carbon and has a way of storing carbon safely. The sun is burning about twenty-five percent brighter than it was when the solar system was formed. So, the Earth should be much hotter than it is now, right? Actually, as British geologist Aubrey Manning says, "This vast change should have brought about absolute catastrophe on the earth, but our world has seemed hardly affected."

So, what's keeping the planet stable and cool? Life. When carbon in the air falls as rainwater, trillions of tiny marine organisms capture it and use it (with other things) to make their tiny shells. Those organisms trap the carbon in their shells, preventing it from re-evaporating into the atmosphere, where it would dangerously form a greenhouse gas. Eventually, all the tiny organisms die and sink to the bottom of the sea. They're compressed into limestone. The amount of carbon those organisms accumulate over time is amazing. A cubic centimeter of Dover chalk contains over a thousand liters of compressed carbon dioxide. Otherwise, all that carbon dioxide wouldn't do us any good. In general, the carbon trapped in Earth's rocks is about two thousand times the amount in the atmosphere. Much of that limestone will eventually become material for volcanoes. The limestone will come back to the atmosphere and fall back to Earth as rain. So, the whole process is called the long-term carbon cycle. The process takes a very long time to complete, about five hundred thousand years for an ordinary carbon atom. Unless other factors interfere, it helps keep the climate stable, you know?

Unfortunately, humans are disrupting the cycle by releasing huge amounts of extra carbon into the atmosphere, regardless of whether the foraminifera are ready. It's estimated that we've emitted about a hundred billion extra tons of carbon into the air since. And that number is increasing by about seven billion tons per year. That's actually not that much, in general. Nature puts about two hundred billion tons of carbon dioxide into the atmosphere every year, which is about thirty times more than our cars and factories emit. But if you look at our smoggy cities or the Grand Canyon, or sometimes even the White Cliffs of Dover, you'll see how much of a difference our participation has made, you know?

From very old ice samples, we know that the "natural" concentration of carbon dioxide in the atmosphere – that is, before our industrial activities made things worse – was about two hundred and eighty parts per million. By the time lab people started paying attention to this problem, that number had risen to three hundred and fifteen parts per million. Today, that number is already over three hundred and sixty parts per million, and it's still climbing by about a quarter of a percent per year. It's predicted that that number will reach about five hundred and sixty parts per million by the end of this century.

So far, the oceans and forests on Earth have managed to save us from our self-destruction. But, as Peter Cox of the British Meteorological Office says, "There is a critical line. At that point, the natural biosphere can no longer mitigate the effects of the carbon dioxide we're emitting on ourselves, and actually starts to amplify them." There's a fear that global warming will quickly worsen. Because they can't adapt, many trees and other plants will die, releasing the carbon they've stored and making the problem even worse. This kind of cycle has occasionally happened in the distant past, even without human involvement. But even in those cases, nature is still capable of creating miracles. Almost certainly, the carbon cycle will eventually recover and restore Earth to a stable and beautiful environment. The last time this happened, it only took sixty thousand years.