Planet Building 6: Water

#scifi #climate Water is made up of two of the commonest atoms in the Universe—hydrogen and oxygen. Specifically, it is composed of two hydrogen atoms attached to an oxygen atom. For such a simple compound, it has some rather incredible properties.

It is as close to a universal solvent as any common molecule—something we do not always appreciate simply because the things dissolved by water mostly have been dissolved or (in the case of living things) have evolved membranes that keep water out. This probably played an important role in the evolution of life, and the search for extraterrestrial life is still focused on the search for liquid water.

It is both the brightest and the darkest substance on our planet—brightest in the form of snow or cloud tops (albedo 50% to 80%) and darkest in the form of deep ocean water (albedo 3% to 5%, though dark coniferous forests may be as dark.) This makes it very important in the overall albedo of Earth.

Water has a very high specific heat—which means it takes a lot of energy to change its temperature. It takes about five times as much energy to raise the temperature of a mass of water by a degree as it does to raise the temperature of the same mass of dry soil, rock or most building materials by the same amount. The result of this is that an enormous amount of energy can be stored in the oceans.

High as this specific heat is, the latent heat of water is even higher. Latent heat? This is the energy required to change phase—to evaporate water or melt ice. The melting of ice takes about 80 times the energy it takes to raise the temperature of the resulting water by 1 degree C. But this is dwarfed by the energy needed to evaporate water: 600 times the energy needed to raise the temperature by 1 degree C.

Why is this important? Because when the water condenses to form clouds, it gives that heat back to the air. But why does it condense?

Air can hold only so much water in vapor form. Further, the limiting amount is primarily controlled by temperature. Very roughly, when the temperature rises by 20 degrees F, the amount of water the air can hold doubles. When the air is at forty below (not a totally unreasonable temperature for cloud tops) it can hold about 3% as much water vapor as it can hold at freezing. At 68 degrees, it can hold about 3.8 times as much as it can at freezing. And at 80 degrees, it can hold almost 6 times as much as it can at freezing.

We’ve all seen condensation on iced drinks or cold windowpanes, when the air is holding more water vapor than would be possible at the temperature of the glass. If the air itself cools, this condensation will occur on dust particles in the air, and the result will be fog.

But when air goes up, whether it is rising because of buoyancy or because it is rising over terrain or a colder air mass, it expands and cools. The result is fog above the ground—a cloud. And as the water condenses to form a cloud, it releases the same latent energy to the air as it took to evaporate the water in the first place.

The result is a transfer of energy from the solar-heated surface or ocean to the higher parts of the atmosphere. We think of thunderstorms in terms of rain, lightning and tornadoes but they are in fact one of the most effective means of transferring energy from the Earth’s surface to the upper troposphere.

The same is true of frontal storms, but here the transfer is not only from the surface to higher in the atmosphere, but horizontal as well as vertical—from the equatorial regions to the poles.

Hurricanes deserve a post to themselves, but they also transfer energy from the ocean to high in the atmosphere. In fact, any time it rains, we are seeing a side effect of transfer of energy.

What would happen without this transfer of energy by latent heat? The winds would have to blow much harder to transfer the same amount of energy, and there would be much more contrast in temperature between poles and equator, and between the surface and the same height in the atmosphere. But the albedo and the greenhouse effect of water vapor would also change, so the overall effect on temperature would be hard to calculate and would depend very much on the albedo of the rock making up the surface of the planet.

Planet Building 5, #Greenhouse Effect

#scifi If we want a planet on which human beings can live, we’d better have a suitable temperature—not only one at which human beings are comfortable, but one at which water can exist in all three phases, allowing for temperature variations from equator to pole and through the height of the atmosphere. So what determines the temperature of a planet?

The starting point is what is called the equilibrium temperature of a planet. This is based on the idea that the planet is receiving energy from the sun at the same rate it is radiating it out to space.

Any object that is not at absolute zero temperature radiates energy, and at what we consider comfortable temperatures, most of that energy is in what we call the thermal infrared portion of the spectrum. We can’t see it, but we can feel it. On a very cold day, we may say that the ground and sky are radiating cold, but that is only because our skin is at a higher temperature and is radiating energy away faster than it is receiving it back.

In fact, the amount of energy an object radiates is directly proportional to the fourth power of the absolute temperature. For a planet, then, the total energy radiated is proportional to the surface area of the planet times the absolute temperature multiplied by itself four times.

But the energy received from the sun is proportional to the luminosity of the planet’s sun divided by the square of the planet’s distance from the sun times the cross sectional area of the planet. For spherical planets, the cross sectional area is one fourth of the surface area. A fraction of that radiation will be reflected out to space and can be ignored. This fraction is called the albedo, or reflectivity of the planet.

When all of these things are put together, we can get an equation for what is called the equilibrium temperature of the planet. The table shows this temperature (in degrees Farenheit) for three planets of the solar system: Venus, Earth, and Mars. It also shows their real surface temperatures.

What happened? These planets have atmospheres! The equilibrium temperature is the average temperature of the radiating layer, which is generally somewhere above the surface, in the colder part of the atmosphere. How high depends on the wavelength. In visible wavelengths, the outgoing radiation is from the visible surface—partly cloud tops (usually colder than the ground) and partly ground or ocean. Far more important is the fact that some of the gasses in our atmosphere—notably carbon dioxide, water vapor, and methane–are opaque in large parts of the thermal infrared. Most of the energy at temperatures we are comfortable at is radiated in the thermal infrared.

This is what is meant by the greenhouse effect, which actually has very little to do with why a greenhouse stays warm, especially during the day. The difference between the Earth’s equilibrium temperature and the actual temperature is a measure of the greenhouse effect with the current amounts of carbon dioxide, water vapor, methane and a few other radiatively active gasses in the atmosphere.

We are adding carbon dioxide to the atmosphere at an unprecedented rate. Further, the warming of the oceans is adding water vapor, a second important greenhouse gas, and agriculture, permafrost thawing and possibly the thawing of methane clathrates is adding methane as well. All of this means that the radiation the Earth is returning to space is coming from higher and higher in the atmosphere, and since the rate at which the air cools with elevation is unlikely to undergo major change, the surface gets hotter.

What does all this have to do with planet building?  It suggests that the solar constant, proportional the luminosity of the star around which the fictional planet is circling divided by the square of the distance between the planet and its sun, had better be about the same as ours if the atmosphere and albedo are similar. If we make this assumption, we can say some things about the way the sun will look and the length of the year.

Exchange Teleports (Homecoming Glossary)

EXCHANGE TELEPORTS: #scifi Teleports in which energy and momentum are balanced by exchanging two equal masses.

Boosting (Homecoming Glossary)

BOOSTING: #scifi Interstellar teleportation or communication takes more energy than most people can comfortably spare, and the amount of energy needed tends to increase with the distance. Boosting means the person is using an external source of energy. This may be direct or (more often) through a computer link.

Why Planets have Seasons

Why do planets have seasons?

The chance that a planet will rotate in exactly the same plane as its orbit is pretty remote.  So is the chance that its orbit will be exactly circular. (I have a planet in the sequel to Homecoming that is seasonless because its equator is in its orbital plane, but that’s fiction.)

The direction of the axis of rotation, the planet’s pole, plays a large role in its weather, and especially in the planet’s seasons.  Seasons on Earth are controlled primarily by the tilt of the earth’s equator relative to the plane of its orbit.  This tilt for the earth is 23 degrees and 27 minutes.

An increase in the tilt would enhance the contrast between the seasons, but reduce the difference in the total annual solar heating between the equator and the poles.  (If the tilt is greater than about 60 degrees, the poles actually get more annual average incoming energy than the equator, as is the case on Uranus.)

In contrast, if the equator were closer to being in the same plane as the planet’s orbit, there would be less difference in the seasons and more in the difference of the average annual energy budget between the equator and the poles.

In general the oceans, which hold heat and release it slowly, will be most influenced by the average annual energy budget, while the land areas are more influenced by the seasonal energy input.  Thus the progression of the seasons will be strongly influenced both by the distribution of land and water on the planet and by the inclination of its axis.

Water, by the way, is not a rare substance in the universe.  It is the simplest possible chemical compound of the most abundant element in the universe, hydrogen, and the third most abundant, oxygen.  (The second most abundant, helium, does not usually form chemical compounds and can be ignored for this purpose.)

Water is going to be lacking on a real planet only if it has been removed at some point in the planet’s history.  In our system, the near lack of water on Mercury and its scarcity on Venus is probably due to the fact that water, being relatively volatile, was blown out of the inner Solar system by the tantrums of the early sun.  Mars also seems short of free water, but that is likely due to its small size and correspondingly weak gravitational field and ice is still a possibility.

Getting water in liquid state is largely a function of planetary temperature and atmospheric pressure. Making the planet the right size (gravity) and putting the planet at the right distance from its sun are probably the critical factors here.

Solstices are defined as the dates when the planet’s axis points most nearly directly at the planet’s sun. In one hemisphere the sun will be at its highest in the sky and the days will be at their longest. In the opposite hemisphere the sun will be at its lowest and the days will be shortest. Close to the poles, the sun may appear to be above or below the horizon for 24 hours at a time. Summer and winter are defined as starting at the solstices.

Equinoxes are defined as the dates when the planet’s axis is at right angles to the line between the planet and its sun. At this time the difference in the radiation received at the pole and at the equator is at its maximum, but there is no difference between the northern and southern hemispheres. Spring and fall are defined as starting at the equinoxes.

There is another way of getting seasons on a planet, minor but perceptible on Earth, but possibly major on a planet whose equator is close to the plane of its orbit.  This is the ellipticity, or elongatedness, of its orbit.  The ellipticity of the Earth’s orbit is only about 0.016 at the present, though it has been as high as 0.06 in the past.  But ellipticity influences both the energy received from a planet’s sun, with maximum energy coming at perihelion (planet closest to sun) and the length of the seasons, since the planet moves fastest in its orbit when it is closest to its sun.  Seasons controlled by the orbital eccentricity differ from those due to axial tilt in several ways.

Firstly, pure tilt-controlled seasons are opposite in the two hemispheres, while pure perihelion-controlled seasons are the same in the northern and southern hemispheres.

Secondly, pure tilt-controlled seasons are equal in length–the summer half-year is the same length as the winter half-year, counting the equinoxes (when the sun is 90 degrees from its midsummer position relative to the stars) as the dividing points.  Perihelion-controlled seasons have a long winter and a short summer.

Thirdly, the length of time the sun is above the horizon each day varies markedly over a tilt-controlled seasonal cycle, as does the direction of sunrise and sunset at medium to high latitudes.  Tilt-controlled seasons at the equator have a double cycle, with the most incoming energy at the equinoxes.  In pure perihelion-controlled seasons, there is no change in the apparent path of the sun in the sky, and the equatorial seasons follow those nearer the poles.

Fourthly, in pure tilt-controlled seasons, there is no change in the apparent size of the sun in the sky.  In perihelion-controlled seasons, the sun appears larger in summer and smaller in winter.

Most real planets have both kinds of seasonality at once, with one dominating.  On Earth, tilt dominates, but summer is slightly longer than winter because Earth is at perihelion around January 4.  (This is why the number of days between the spring and fall equinoxes is slightly more than the number between the fall and spring equinoxes—or more simply, why February is the shortest month, while July and August are the only two adjacent months with 31 days.)

Since tilt, perihelion date and eccentricity are the variables that can change in a multi-body system (and if fact do change slightly over a few thousand to hundreds of thousands of years on Earth, just due to the other planets in our system) the seasons in a multiple star system are likely to be rather variable and not really conducive to earthly life.  Even on Earth, the slight variations in orbital elements produced by the other planets are thought to be enough to account for the alternation between ice ages and interglacials.

The Four Horsemen of the Apocalypse

Domestication is a mutual process—the plants and animals domesticated historically have met us halfway.

We and our domesticates have entered a kind of symbiosis—both we and they benefit, at least in numbers.

Plant and animal domestication was the first step toward civilization.

There are only two ways of increasing agricultural yield: Increase the amount of food produced per acre, or increase the amount of land farmed.

Once domestication occurred, we were locked into a positive feedback loop between food production and population. But a positive feedback loop is inherently limited and unstable. Are we approaching a crash?

I’ve been taking a Teaching Company course on DVD for the last couple of weeks, and I have to say it’s one of the best I’ve taken so far. I’ve always been interested in the process of domestication, especially since it became clear that the early agriculturists were generally less healthy than their hunter-gatherer ancestors. How did wolves become dogs? Who first thought of riding a horse? Did riding come before or after driving? And are cats really domesticated, or did they domesticate us?

The course is “Understanding the Human Factor: Life and Its Impact” by Professor Gary A. Sojka, but it’s really about human impact. I can’t say it answered all of my questions, or even asked them, but it did a good job of summarizing our current state of understanding, and of steering a middle course between “domestication is a sin and all domesticated animals should be returned to the wild” (most would not survive, and we probably wouldn’t, either) and “animals have no feelings and were put on this world solely for our use.” There are fewer moral problems with domesticated plants and microbes, though even here there are quandaries. How dangerous are monocultures, for instance? Or reliance on a small number of closely related varieties? (Think the Irish potato famine.)

If I have an argument with Professor Sojka, it is that he is too optimistic about the future. This may be appropriate for a college course, but I don’t feel enough sense of urgency. Yes, some people—a small minority even in the West—are beginning to think about long-term sustainability. (The politicians aren’t, by and large.) But the major problem—a population that is rapidly outstripping the carrying capacity of our planet (if it hasn’t done so already)—has become a taboo subject for serious discussion.  “The demographic transition will take care of it.” But will that happen soon enough?

Historically, our population has been kept in check by the Four Horsemen of the Apocalypse. Famine. War. Disease. Death by wild beasts—today, accidental death of all kinds. All of these are premature deaths—death by old age simply is not mentioned. Today, we tend to regard such deaths—those of the young—as particularly tragic. We fight them in every way we can—and in many ways, we’ve succeeded. What we’ve forgotten is that every person born dies eventually, and to reach sustainability we have to reduce the number of people being born until it balances the number who die. Otherwise the four horsemen will eventually increase the death rate to match the birth rate—or more.

Food and energy both rely on sunlight—the sunlight that falls on the earth today and the sunlight that fell hundreds of million years ago, and is now stored in fossil fuels. I group food and energy for several reasons. Fertilizer. Biofuels. Pesticides. Transportation. Pumping water to where it is needed for crops, in some cases pumping down water that has been in storage since the ice age. All of the advances that have allowed us to hold back that horseman ultimately rely on those fossil fuels and fossil water, or plan to replace them with agricultural products. And fossil fuels are becoming increasingly risky to exploit—look at the BP oil spill.

But an increase in agricultural output to match the increase in population means more efficiency—which we are obtaining today largely through fossil fuels—or more land in agricultural production. There is only so much land suitable for agriculture, especially if we want to keep the ecosystem services we depend on going. And one of the oldest causes for war is the desire for more land.

Disease? In part that ties back to our methods of food production, as well. Certainly much antibiotic resistance can be linked to the widespread use of antibiotics in animals, and many diseases that started out in animals have crossed over to human beings. I find it interesting that all of the great world religions, many of them very pro-natalist, trace their origins to early city dwellers. Disease can spread rapidly among city-dwellers. In fact until the last century or two, urban areas were dependent on immigration from the countryside to maintain their populations. Having many children was important to these early city-dwellers—most of their children would die before having children themselves. That’s not true today, thanks largely to public health improvements—but the mindset and the religious imperative remain.

All living things—plants, animals, and human beings—are driven to reproduce. In our case, that deep-seated drive is reinforced by religious and social pressures. We claim we have a right, even a duty, to reproduce. But do we? Not in nature. Nature says the “right” to reproduce must be earned. It’s a lesson I hope we can learn before it is enforced by the Four Horsemen.

Why Do We Have Weather?

Why do we have weather?

It all goes back to the fact that the heating and cooling of the atmosphere is not uniform. Some volumes of atmosphere get more energy than they lose, and must get rid of the excess. Others lose more energy than they get, and must somehow get more energy. Weather and ocean currents are Earth’s way of moving energy from places that receive more energy than they lose to those that lose more energy than they gain.

We are changing the heating and cooling of the atmosphere in a number of ways. Consequently, the details of how the atmosphere and oceans move energy around–and thus the weather–must also change.

The problem is not simple. Much of the energy gain and loss of the atmosphere is itself due to the same weather that moves energy around. But the chances that all the changes will balance out naturally are about like expecting a tornado to put things back the way they were before a hurricane hit.

Please note that when I talk about heating and cooling of the atmosphere I am not talking about the replacement of warm air by cold or vice versa. Rather, I am following a mass of air, often called a parcel of air, and looking at the changes in the energy within it. So what are the processes that change the energy content of the air?

If the air is in contact with a surface, be it ground, water, or snow, energy will flow from the surface to the air if the surface is warmer, or from the air to the surface if the surface is colder than the air. The surface is most likely to be warmer than the air if it is daytime, summer, and the surface is fairly dark. If it is winter, night, or the surface is very light in color (snow, desert sand) the surface may be cooler than the air. Further, land temperatures are strongly affected by the season and time of day. Water temperatures are affected mostly by season and by ocean currents. The presence of clouds moderates the cooling or heating of the land.

Energy, in the form of latent heat, also flows when water vapor evaporates or condenses. Thus the evaporation of water from the oceans adds energy to the air near the surface, while the formation of dew removes energy from the air. The condensation of clouds does not change the energy content of the air, but it changes latent heat into a rise (or a reduction of the drop) in temperature. This is a very important way of transferring energy from the surface to cloud height.

How can air gain or lose energy when it is away from a surface? This is where the so-called greenhouse gasses become important. Air is transparent to the part of the light spectrum we see, which is where the sun puts out most of its energy. This is important for us, as our eyes wouldn’t be much use otherwise. But there is a large part of the light spectrum we do not see. The most important part for weather and climate is what is called the thermal infrared. This is the part of the light spectrum, the colors invisible to our eyes, given off by objects at what we call room temperature. Although the major components of the atmosphere (oxygen and nitrogen) are transparent in the infrared, certain gasses emit and absorb very strongly in these colors. In particular water vapor, carbon dioxide, methane and ozone are able to absorb energy given off by the earth’s surface and emit energy to space. The emitted energy depends very strongly on the temperature of the gas. Thus the rate of energy loss at 100 degrees F is over 6 times that at 0 degrees F. Cloud droplets also emit and absorb energy in the thermal infrared.

Over all, then, the air gains energy near the surface, at low latitudes, in summer and during the day. It loses energy at high elevation, at night, near the poles and in winter. Although ocean currents succeed in moving a considerable amount of the energy gained from the sun near the equator to the polar regions, the vertical energy transport and about half of the horizontal transport is left to the atmosphere—hence weather.

We have been changing the way in which the earth absorbs energy from the sun at least since the origins of agriculture, since cleared fields normally absorb energy differently from forests. But over the last hundred years or so we have also dramatically changed the greenhouse gasses in the atmosphere. Carbon dioxide and methane have increased as our growing population has relied more on fossil fuel burning and also needed larger areas given to agriculture. Warmer air can hold more water vapor, so water in the atmosphere has increased as well. It is even possible that the particles we have put into the atmosphere have made clouds more stable, which could also increase the water vapor in the air and hence the greenhouse effect.

There is no question that we have changed the energy losses and gains of the atmosphere, or that these losses and gains are what drives the entire weather system. The disagreements among climate scientists are not in whether a change will occur, it is in the details of what that change will be. But expecting no change, when we are changing the processes that drive the whole climate system, is extremely unreasonable.