Tag Archive: greenhouse gasses

Thursday morning I happened to catch NPR’s Talk of the Nation on whether the extraordinary tornadoes this year had any link to global change. This is not a simple question, but it got me thinking about one aspect of climate change that I haven’t seen discussed much.

Weather is driven by the fact that some parts of the earth-ocean-atmosphere system receive more radiant energy than they emit to space, and other parts radiate more to space than they receive. Energy is transported from regions of excess to regions of deficit by the atmosphere and the oceans, and the result is what we call weather and ocean currents. This is one of the fundamentals of atmospheric science.

What has received less attention is that there are two types of energy imbalance that the oceans and atmosphere must balance. The first is the equator-to-pole imbalance. This is what drives ocean currents and the huge horizontal eddies that we call frontal storms and anticyclones, and the great northward and southward excursions if the jet stream. Individual years may vary greatly, both in time and in space, while the total energy transport stays about the same. A difference in the apportioning between atmosphere and ocean could make a huge difference, and this is the basis for concern that a change in the so-called conveyer belt of the oceans could be a major climatic switch.

But there is a second imbalance, and this is between the surface, which absorbs energy from the shortwave (visible) radiation of the sun, and the upper atmosphere, which radiates longwave (infrared) energy to space and back to the surface. The surface transfers energy to the air near the ground, both in the form of moisture and direct heating, and this energy must be transferred vertically to the upper atmosphere where it is radiated away—largely by water vapor, clouds, carbon dioxide and methane. Certainly some of this vertical transport is accomplished by the great horizontal eddies, via fronts—sloping surfaces where warm air moves up over cold air. But some, especially in summer, is due more directly to warm air rising with very little large-scale horizontal temperature gradient. This process produces the more violent storms—thunderstorms (which produce not only thunder but lighting, hail and tornadoes) and hurricanes.

Horizontal gradients are certainly important as feedback processes. If the Arctic sea ice continues to melt, the Arctic Ocean will absorb much more solar energy, the summer gradient of temperature will decrease, and summer storms of the large-eddy type will decrease. But the direct effect of carbon dioxide and other greenhouse gasses is to increase the vertical energy gradient, and thus the amount of energy that must be transported vertically by the atmosphere.

In current climate models this vertical energy transport is parameterized—that is, it is based on statistics taken from the present-day climate. Why? Because it happens largely through processes of cloud physics that are just too small-scale to be included directly in the models. But any time you hear climate scientists talking about a tipping point, they are really talking about a change that may change the statistics on which those parameterizations are based. If that happens, change may be much larger, and in a totally unexpected direction, than the models predict.

Could the changes be in a direction as to be opposite the prediction, so there is no real change? Possibly, just as it is possible that one storm could put things back the way they were before an earlier one struck. I am more concerned that attempts at modeling past changes we know occurred, like the glacial-interglacial transitions, generally underestimate, not overestimate, the change actually observed.

The Greenhouse Carol

(To the tune of “Auld Lang Syne”)

Should present climate be forgot,
And ne’er again be seen?
Should glaciers melt and oceans rise
Just because our house is green?
Because our house is green my friends,
Because our house is green,
We’ll sit and swelter in the sun
Because our house is green.

Should deserts spread across the land
While hurricanes grow cruel
From cows and swamps and growing rice,
And from burning fossil fuel?
From burning fossil fuel, my friends,
From burning fossil fuel,
We’ll all dehydrate in the sun
From burning fossil fuel.

Should the I T C Z go away,
And the savannahs return?
Should glaciers melt and cities drown
Because the jungles burn?
Because the jungles burn, my friends,
Because the jungles burn,
We’ll parboil in the tropic sun
Because the jungles burn.

(This was actually written 22 years ago, but it’s as true as ever.)

This is an excellent DVD for getting across the idea that the inner workings of the earth, while at times disastrous, are essential for life.

The DVD actually has two programs, both originally shown on the Discovery channel: Inside Planet Earth and Amazing Earth. The graphics are intriguing, though some are repeated a bit too often. The actual camera work is excellent.

My only objection was that at times the narration could be misleading. True, we have been in an ice age for the last 40 million years. But most of the evolution of mammals – and certainly of humans – has taken place during that period. We are adapted to an ice age in the broad sense. My concern is that many people will take “ice age” to mean the periods like 20 thousand years ago, when ice sheets covered much of North America and Europe.

Over all, I found this a good program if a bit sensationalist – and this is my field, so I am aware of shortcomings.

Know how a contrail forms?

No, it has very little to do with particles produced by a jet engine (or a propeller engine, for that matter.) The culprit is water vapor.

Burning any hydrocarbon fuel, no matter how cleanly, produces two gasses: carbon dioxide and water vapor. If the fuel is dirty or combustion is incomplete or very hot, other things may be produced—sulfur compounds, nitrogen oxides, carbon monoxide, particulates—but the energy we get out of burning hydrocarbons comes from combining oxygen from the air with the hydrogen and carbon that make up the bulk of the fuel.

An oxygen atom plus two hydrogen atoms is a molecule of water. A carbon atom plus two oxygen atoms is a molecule of carbon dioxide. The definition of clean combustion is combustion in which only these two compounds are produced.

Fuels vary in their ratio of carbon to hydrogen. Coal has about equal quantities of each, and when cleanly burned produces more than three times its weight in carbon dioxide and somewhat less than its weight in water. Gasoline has about 2 atoms of hydrogen to one of carbon, and produces a little less carbon dioxide but more than its weight in water. Straight hydrogen does not produce carbon dioxide, but produces a whopping nine times its weight in water.

In most climates, we can ignore the water, at least near the ground. But the air can only hold so much water, and the amount it can hold decreases rapidly with temperature. What’s more, the limit on how much it can hold differs depending on whether ice is present. It is perfectly possible for air to have more moisture than it could hold if ice were present, but not enough that moisture condenses out in cloud droplets. In fact, this is very common at high elevations.

Further, cloud droplets can have a temperature well below freezing, but still be liquid droplets. They can be triggered into freezing by ice nuclei, the most effective nucleus being a sliver of ice. This is how wing icing on airplanes occurs, by flying through what are called supercooled clouds—clouds of liquid water drops at subfreezing temperatures..

At very low temperatures, below about –40, an ice nucleus is not necessary, as a droplet will freeze spontaneously.

Now imagine an airplane flying in air with a temperature below –40 (true of most commercial flights today) with a moisture content not high enough for cloud formation, but high enough that ice crystals can grow. The engine exhaust contains large concentrations of water vapor—enough to cause condensation of droplets just behind the plane. Since the temperature is below –40 these droplets will freeze very rapidly. Once they are frozen they gather in water from the air around them. The result is a contrail that is not only visible, but grows.

Most areas don’t have ground temperatures below –40 very often—but here in Fairbanks, Alaska, we do. Automobiles leave contrails in these conditions. More, many of the pollutant particles we spew into the air act as ice nuclei at temperatures a little warmer than –40, so the combined persistent contrails—ice fog—can occur well above –40. It’s fog made of ice particles, rather than water droplets. It’s densest just behind each vehicle, making it hard to see the tail lights of the car ahead.

Growth from vapor makes well-formed crystals that produce optical effects like sun dogs, halos, and ice pillars. We have those, but not in ice fog. Ice fog particles are basically frozen droplets, and while they are crystalline, they do not generally have the clearly defined facets necessary for them to act as prisms. So ice fog looks just like fog.

Sad to say, my photos of ice fog all seem to be slides that have not been digitized. Does anyone have a good photo of ice fog or contrails I could put on this page?

On the Eighth day of Christmas my true-love gave to me,
Eight cars polluting,
Seven blizzards raging,
Six aurorae swirling,
Five solar flares.
Four chickadees,
Three mammoths
Two ptarmigan
And a spruce hen in a spruce tree.

We’ve talked about the need to transfer energy from the surface of a planet to high in the atmosphere, where greenhouse gasses can radiate it away. But solar energy is not absorbed uniformly over the face of a round planet, or uniformly in time if that planet is rotating. This means energy must also be transferred horizontally, and water plays an important role here, as well.

Since we’re close to the equinox, let’s look first at how incoming energy is distributed then. The sun at this time of year is almost directly over the equator, and on the horizon at the poles. This means that the incoming energy is at a maximum—in both time and space—at the equator, and zero at the poles.  Unless the poles are at absolute zero (- 460 degrees F) energy must be transferred from the equator to the poles.

Remember that liquid water can store an enormous amount of energy as heat, and about half of that transfer is actually carried out by the ocean currents. When you add the rotation of the planet, we have currents like the Gulf Stream and winds in temperate latitudes generally blowing from west to east. The result is that west coasts are generally warmer than east coasts—southern France, for instance, is at the same latitude as Maine. This will carry over to any planet with oceans, and because oceans are so good at storing energy, it is true at any time of year.

The atmosphere also carries energy from the equator to the poles. In fact, this transfer of energy is what drives the whole weather system. The eddies we call cyclones and anticyclones, the lows and highs on the weather map, carry energy from the regions of surplus near the equator to the regions that radiate energy away near the poles.

Some of this energy is carried by warm air moving poleward while cold air moves equatorward.  But a large part is carried by latent heat, just as in pure vertical transport. As the warm, moist air moves poleward, it tends to ride over colder air coming equatorward, and as it is lifted and cools, water condenses. Thus the latent heat added by evaporation over the tropical oceans is released to the air at much higher latitudes.

What about other times of year, such as the solstices?

The solstices are defined as the dates when one or the other of the planet’s poles are pointed most nearly directly at the sun. This affects day length as well as the angle at which the sun falls on the surface, with the surprising result that for the earth’s axial tilt (23 degrees 27 minutes) the maximum incoming energy at the top of the atmosphere is at the summer pole, and there is very little variation of incoming energy with latitude in the summer hemisphere.

Ice at the Earth’s poles keeps surface temperature low and the low angle of the sun means more energy is absorbed in the atmosphere, but high midsummer temperatures at the pole would be quite possible on a world without water. If the tilt were increased even farther, to around 70 degrees, the annual average incoming radiation at the poles would actually exceed that at the equator.

Another important way in which water affects temperature is its large capacity for storing heat, which means that in summer water is generally cooler than land. This leads to coastal climates being cooler in summer than those inland. It also leads to the summer monsoons, when hot air rising over the continents sucks in the cooler, moister air from the nearby oceans.

This is reversed in the winter hemisphere, where the water is usually warmer than land. Cold air flows out from the continents, often triggering storms when it moves over the warm, moist oceans.

Because the energy input near the equator is only a little less than during the equinoxes, while that of latitudes above the polar circles is zero, a great deal of energy must be transferred from the equator to the pole of the winter hemisphere. Thus the violence of winter storms, as energy is transferred both upward and toward the winter pole.

Finally, the daily cycle is hardly noticeable in water temperature, but it has a large effect on land temperature. The result is local winds from colder to warmer temperature near the surface—the sea breeze in the daytime and the land breeze at night. In some cases these moist, cool winds from the sea trigger thunderstorms when they move over the warm land.

All of these types of weather will be found on any rotating planet with oceans of liquid water. Don’t think they are “earth-only” effects in building a planet.

#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.

#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.

MIRROR: A planet on which life is just beginning to invade the land surface, and is still confined to the wave zone. Very warm. It is highly unusual in that life forms exist which utilize both left-handed and right-handed proteins. The atmosphere is denser than Earth’s and has a much higher carbon dioxide content, but does not actually require space suits.

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.


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