Earth's Atmosphere Part 6 - Comparing Titan's And Earth's Atmospheres

In this article we take a look at Titan's atmosphere and compare it to Earth's atmosphere, as well as what we just  learned about Venus and Mars. As we do this, we'll attempt to refine our question of how atmosphere and life are related to each other. Hopefully this exploration will set us up for the ultimate question (to come in the next article) of how rare or common life-supporting atmospheres on planets in our universe may be.

Titan, Saturn's largest and only substantial moon, shown below, is the only moon in our solar system that has a significant atmosphere, and it is the only object other than Earth that has stable liquid on its surface.

This is a natural colour composite (what the moon looks like to our eyes) of Titan, taken by Cassini in 2005:

Titan, slightly larger than Earth's moon, is blanketed in a thick orange haze. It has a surface gravity about one seventh that of Earth and an atmospheric pressure about 60% higher than Earth's. It's atmosphere is almost entirely composed of nitrogen, about 95%, making it much more similar compositionally to Earth than either Venus or Mars, which have atmospheres dominated by carbon dioxide and are either crushingly thick (Venus) or insubstantial (Mars). Ttitan's atmosphere also contains significant methane, and this is what makes it really interesting, because this molecule undergoes a variety of photochemical reactions in its upper atmosphere. It breaks down in sunlight and reacts to form organic compounds such as benzene, acetylene and hydrogen cyanide, smog chemicals in other words. And it is this smog that makes Titan's atmosphere appear opaque orange. There is much evidence, described in detail in my article about the evolution of Earth's atmosphere, that Earth's atmosphere once had a similar complement of organic molecules and that is how life first formed. Although Titan is a moon rather than a planet, its unique atmosphere makes it more of a "sister planet" to Earth than either Venus or Mars.

Titan is truly a world of mystery. It has a substantial atmosphere while other moons that are of similar size and internal composition, such as Ganymede, Callisto and Europa, all moons of Jupiter, have none at all. The first question to answer then is, "Where did Titan's atmosphere come from?"

The Origin of Titan's Atmosphere

Titan is composed of about half water ice and half rocky silicate-based material and it is probably differentiated into layers, as shown here in this diagram:

Titan's core may still be hot enough to support a layer of liquid water and ammonia beneath its water ice crust (1h ice is a physical state of ice just like the water ice here on Earth). This subsurface ocean, analogous to Earth's magma, is located above a slightly thicker layer of high-pressure ices. There are special phases of water ice that are actually denser than liquid water and these can form under high pressure, even in an environment that is warmed by the core. This high-ressure layer is likely composed of a combination of Ice VI and ammonia dihydrate ice. While Titan's core may have some heat, Titan is very cold. Its surface temperature is about -179°C. The presence of ammonia can keep liquid water from freezing down to a temperature of about -97°C, so this is why a liquid subsurface ocean may be possible on Titan. At the distance of around Saturn, its parent planet, from the Sun, compounds that would have been strictly gases on the rocky planets can accumulate in all three phases - solid, liquid and gas - in the material that formed planets and moons. Titan was made from about half silicates (rocky material) and half volatiles such as ices and ammonia hydrates, a much higher ratio of total volatiles than Earth.

Although it is exciting to have another nitrogen-rich atmosphere in our solar system, Titans' atmospheric nitrogen likely has a different origin than Earth's. Ammonia, which readily dissociates, might be its original source, rather than gradual outgassing which is the proposed origin of Earth's nitrogen gas. If nitrogen had originated from the outgassing of rock, one would also expect other gases from outgassing, particularly argon gas. Unlike Earth, which has about 1% argon in its atmosphere, none has been detected on Titan, with the exception of an argon isotope that is the product of radioactive decomposition of krypton.

There are a few different possibilities why only Titan, of all the Jovian and Saturnian moons, has an appreciable atmosphere. Temperatures in the disk of gases that was forming Saturn and, we assume, Titan and the other moons, would have been cold enough to allow the accretion of ammonia in solid form, whereas temperatures around Jupiter as it was accreting could have been much higher because of the higher gravitational field around this even more massive planet. Callisto and Ganymede therefore may not have accreted as much solid ammonia and their resulting atmospheres may have been too thin to withstand solar wind erosion over time. Another explanation may be that early comet impacts on Callisto and Ganymede would have been of higher energy because of Jupiter's higher gravity. These more forceful impacts could have eroded their atmospheres, while impacts on Titan would have been less energetic and actually deposited material in its atmosphere instead. However, this latter theory is weakened by the isotopic ratio of hydrogen in Titan's atmosphere. Its ratio of deuterium (²H) to hydrogen (¹H) is 1.5 times lower than that of comets, so comets are unlikely to be major contributors to its atmosphere.

The high isotopic ratio of lighter nitrogen (¹⁴N) to heavier nitrogen (¹⁵N) suggests that, even though Titan's atmosphere is fairly dense, much of it has been lost over time. Lighter isotopes are a bit more easily stripped away by solar radiation through Jeans escape, as discussed in the previous article. The young Sun would have had about 70% of today's solar output but it would have had a much higher output of X-ray and ultraviolet photons. Much of Titan's nitrogen loss could have occurred very soon after it formed, within about 50 million years of accretion.

Composition of Titan's Atmosphere

Titan's upper atmosphere contains many layers of haze as shown in this ultraviolet image:

Titan has atmospheric layers analogous to those of Earth, described in a previous article. The purplish upper layers are composed almost entirely of nitrogen and hydrogen (0.1% - a trace gas in Titan's atmosphere). Methane condenses out of Titan's atmosphere above 32 km. This is Titan's tropopause, analogous to Earth's tropopause, above which water vapour condenses out.  Below this, the abundance of methane increases to a maximum of about 5% between 8 km altitude and the surface. Titan's troposphere, extending from the tropopause down to the surface, is where a methane convection cycle occurs, analogous to the water cycle bound within Earth's tropopause. This graphic of Titan's atmosphere and its sometimes-called "methanologic cycle gives you an idea of how it works:

Above the layer of methane weather, a relatively small amount of stratospheric methane exists and it is in this layer that sunlight breaks methane molecules apart. The products react with each other to create various organic molecules. The opaque orange haze may come from tarlike organic precipitates from these reactions, called tholins. Tholins are abundant on the surfaces of icy bodies in the solar system, giving them a reddish brown appearance. The outer edge of the orange haze marks the edge of Titan's stratosphere.

If you look at the curving yellow line in the graphic above, you can see that Titan's surface is actually much colder than its upper atmosphere. The reason for this is that the orange haze creates an anti-greenhouse effect by reflecting what little sunlight Titan receives (about 1% of what Earth receives) back into space. Methane on the other hand is a very potent greenhouse gas, so it has a warming effect on Titan's surface. Without atmospheric methane, Titan's surface would be far colder than -179°C. Liquid methane rains down from methane clouds creating seasonal lakes and even seas. Its water-hydrocarbon ice surface, perhaps coated in tholins, is as hard as rock. Its dim surface, seen by the Huygens probe after it landed in 2005, looks much like this artist's rendition:

There is evidence that Titan's ice rock is weathered by methane just like stony Earth rock is weathered by water.

The Dynamics of Titan's Atmosphere

There are a variety of ways in which Titan should be continuously losing its atmosphere. A robust and continuous replenishment of methane must counteract this.

Titan has no magnetic field of its own but it is sometimes enveloped in Saturn's powerful magnetosphere as it orbits the planet. This should be protective but there is a problem. The difference between Saturn's rotational period (10.7 hours) and Titan's orbital period (16 days) means that Saturn's magnetized plasma strikes Titan at a speed of 100 km/s. That kind of buffeting likely speeds up its atmospheric loss rather than protecting it from solar wind.

Solar radiation should have converted all of Titan's methane into more complex hydrocarbons within about 50 millions years of its formation, based on the expected rates of various reactions in this cold environment. This means that methane must be continuously replenished and most researchers believe that the source is cryovolcanic activity. Cryovolcanoes, like volcanoes on Earth, release thermal energy from the mantle to the surface. Most of the thermal energy within Titan's mantle may come from tidal flexing, as the moon is continuously stretched and deformed by Saturn's gravity. Cryovolcanic activity may also simply be pressure release from higher-pressure liquid ammonium sulphate escaping up through the lower pressure ice layer above it . Mysteriously, Titan doesn't appear to have very many of these volcanoes. In fact, none have yet been definitively identified. One researcher, Jeffrey Moore, proposes that methane in Titan's atmosphere might not come from cyrovolcanic activity at all, but from slow diffusion out of a cold stiff interior. In this case Titan could still have a liquid subsurface ocean, and internal radioactivity could be sufficient to heat it enough to stay liquid at -97°C. Unlike Venus and Mars, Titan also shows evidence of active tectonic movement in a north-south direction. It doesn't appear to undergo subduction, like plates on Earth do, but areas of plate compression and stretching are evident. The resulting ridges are also likely the result of tidal flexing.

Titan: Organic Chemistry → Life?

Some researchers think that Titan's atmospheric methane could at least in part come from living organisms. Astrobiologist Chris McKay suggests that methanogenic life could exist in Titan's methane lakes. These cold-world organisms would take in hydrogen gas instead of oxygen, react it with acetylene instead of glucose and release methane as a waste gas rather than carbon dioxide. There is much debate about whether this kind of biochemistry is feasible. There is a possibility too that life might exist within Titan's ammonia-water subsurface ocean, with energy supplied by tidal friction and radioactive decay heat. There is also some speculation that while Titan is now a very cold environment, it could be much warmer billions of years from now as the Sun evolves into a red giant. The Sun's evolution would be gradual enough to supply a relatively stable warm environment with surface temperatures at around -70°C, perhaps lasting long enough for microorganisms to evolve. By then, much of Titan's anti-greenhouse haze will be depleted as the Sun's ultraviolet output decreases and Titan's methane greenhouse effect takes over as a primary warming mechanism. It may not be methane lakes that populate this warmer world. It will likely sport a liquid ammonia water surface instead.

Regardless of whether life exists on Titan, its mix of complex organic molecules is very exciting to researchers interested in how life made its appearance on early Earth. Scientist David Grinspoon believes that Titan and Earth are examples of bodies with surfaces and atmospheres that are, or have the potential to be, intimately interconnected with life. These kinds of atmospheres have an active cycling of a compound between gas, liquid and solid states that tends to maintain equilibrium and they provide opportunities for chemical interactions with other compounds. There is an active ongoing chemistry in other words, in which life has an opportunity to evolve right along with its abiotic environment. Although Titan does not have any appreciable carbon dioxide in its atmosphere and this gas was likely both abundant and played an important role in the prebiotic chemistry and the formation of life on early Earth, Titan does have a carbon source for complex organic molecules to form and that is methane. Using this molecule, Titan maintains a chemical balance analogous to Earth's.

From Titan to Extrasolar Planets

Life on planets in other solar systems may be expected to be associated with environments that have similar active cycling. This is the basis of the Gaia Hypothesis, and we will be taking a closer look at how scientists can use hypotheses like this one to focus their search for other planets in the universe that could support life.

Titan provides us with yet another laboratory in which to study how initial formation conditions determine what kind of atmosphere develops, what kinds of factors are important in determining how atmospheres evolve, and how composition and environment determine atmospheric activity. Venus and Mars seem to be worlds that have failed to achieve an equilibrium state as their atmospheres now move unchecked toward extremes. Titan, on the other hand, offers us an exciting example of how a world under physical circumstances greatly different from those on Earth can develop atmospheric cycling mechanisms and achieve a state of equilibrium. It is perhaps these kinds of mechanisms that provide conditions stable enough for life to evolve.

If you would like to know more in general about Venus, Mars, Earth or Titan, please take a look at these previous Scientific Explorer articles:

Earth
Mars
Venus
Titan (as well as Saturn and some of its other moons)

Next, we will find out what scientists are looking for as they search for distant planets that could harbour life.

Earth's Atmosphere Part 7 - Are There Any Other Earths Out There?

In this article we continue to examine the complex and essential relationship between atmosphere and life as we expand upon what we know into new territory - extrasolar palents.

This is an artist's image of an overhead view of planets in systems that NASA's Kepler observatory just discovered. All the coloured planets have been verified. The grey ones have not.

In January 2012, NASA's Kepler mission, shown below in an artist's rendering, announced that it just found 11 new solar systems containing at total of 26 confirmed planets, ranging in size form 1.5 times the diameter of Earth to larger than Jupiter.

This is just the latest data adding to the 729 known extrasolar planets (in 594 planetary systems) listed in the Extrasolar Planets Encyclopedia. There are a lot of planets out there in the cosmos. There are two questions to answer. We want to know which ones sustain life of some kind. Titan's strange atmosphere gives us tantalizing clues and makes this question is challenging to define: What kinds of different atmospheres could harbor life, and what would those different kinds of life chemistries look like? The second question is more human-centric. Are there any other planets out there that have oxygen-based atmospheres and possibly life just like us?

As we have learned so far, atmospheres have complex and evolving chemistries and in order to understand them we must have some idea of how they form. There are still many unanswered questions about the atmospheres within our own solar system and as we work toward answering them, we can simultaneously refine what kinds of markers we need to look for when we look at atmospheres on extrasolar planets. This is an excellent example of a tenant central to scientific research: Our answers are only as good as the questions we ask.

We already have some basic guidelines to go on:

An atmosphere depends on how far away from its star it formed. Heavier gases like carbon dioxide, oxygen and nitrogen should, in general, be more abundant on closer planets and light gases like hydrogen and helium should be more abundant on more distant planets.

It depends on the gravity of the planet.

It depends on the planet's magnetosphere (and this depends on the planet's interior composition and perhaps other factors such as collision history).

The shape of the planet's orbit is as well as how fast it revolves is important - is it in a geostationary orbit where only one side faces its star?

Does it have an axial tilt - does its atmosphere experience seasons, in other words?

We have two basic challenges to consider:

We don't yet fully understand how atmospheres work (but we have some general ideas).

We are just now able to "see" distant planets. Astrophysicists are still refining the technology needed to analyze their atmospheres.

Hot Jupiters

The first extrasolar planet found orbiting a Sun-like star is 51 Pegasi b, discovered in 1995. This massive Jupiter-sized planet is 51 light years away orbiting very close to its star.

As early as 2001, the Hubble Space Telescope, shown below, was able to detect an atmosphere on a Jupiter-sized planet, named HD 209458, 150 light years away.

This is an artist's concept of what it might look like:

Its spectrometer analyzed light from the planet's star filtered through the planet's atmosphere as it transited it, finding it to be rich in sodium. It is a gas giant orbiting its star even closer than Mercury orbits the Sun, with atmospheric temperature of 1100°C. They believe huge monsters like these can hold onto their atmosphere even at this close range with their intense gravitational field. Tiny Mercury would never be able to hold onto an atmosphere. An international team of scientists is now using Hubble to search for other hot Jupiters focusing on the atmospheric compositions of these planets. This Hubble video describes how they do it:



As they orbit so close to their stars, they all tend to have a rotation synchronous to their orbit so one side always faces the star. It will be very interesting to figure out what kinds of atmospheric dynamics these planets have.

Another hot Jupiter, called XO-2b, was recently found by scientists using the land-based Smithsonian Astrophysical Observatory telescope. This planet, like the other one is about the size of Jupiter and orbits very close to its star. Potassium has been detected in its hot atmosphere.

These giant planets tend to be "puffy." Intense heat from their close-by star and internal heat tend to inflate their atmospheres. They are at most slightly more massive than Jupiter but with much greater diameters. These are the easiest planets for us to see. We can zero in on them because they are large fast orbiting bodies ? they tend to induce a noticeable wobble in the orbits of their stars. Their close orbits also mean we have a good chance of observing them as they transit their star, and we have a opportunity to analyze their atmospheres as they do so.

Many hot Jupiters have been discovered so far. Scientists now estimate that about 7% of the stars in the Milky Way alone should have hot Jupiters orbiting them. This has led scientists to wonder if systems with these planets reduce the likelihood of earth-like planets. Would these giants, orbiting so close to their star knock smaller Earth-sized terrestrial planets from their orbits, or, more likely, hog up all the gas and dust and prevent their formation altogether?

At first, physicists were confused about how such massive planets could form so close to their star. The discovery of hot Jupiters is a good example of how theories evolve as new information comes in. Scientists had to reconsider their theories of planetary formation. They now believe that these giants formed much further out, past the frostline, the orbital distance at which ices don't sublimate away into space. Here, these planets quickly accumulated light volatile elements and migrated inward since. The inward migration may tend to occur in protoplanetary disks that were extra-thick with accreting gases. A Jupiter-sized planet would experience viscous drag making it spiral inward. Computer models show that, with extra-dense gas clouds, there would enough raw material for terrestrial planets to form as well, and in most types of modeled systems, Earth-size planets could exist in stable Earth-like orbits along with a hot Jupiter. There is a catch though. Scientists believe that Jupiter's gravity have defected water-rich asteroids to collide with young Earth and contribute much of our ocean's water. Without Jupiter (in its distant orbit), Earth may have been a much more parched desert-like planet. This makes the formation of life less likely but not impossible. The likelihood of a wet "Earth" within a system containing a hot Jupiter is difficult to calculate and, so far, unknown.

Challenges of Searching for Planets Like Earth

The discovery of hot Jupiters suggests to us that planetary atmospheres are not at all uncommon in the universe. Gas giants simply tend to form out of accreting debris within a certain range from a forming star. This leaves the question of how likely a terrestrial planet atmosphere is, and how many would exist in some kind of equilibrium state like Earth's, rather than the atmospheres of Venus and Mars.

Scientists are approaching these questions from two directions. Life seems highly unlikely in the extreme environment of a gas giant, but what about smaller terrestrial planets more like Earth? This means improving our ability to locate and identify smaller planets and analyze their atmospheres, and that requires better telescopes. Second, astrobiologists are exploring what kinds of environments life could inhabit, in an attempt to focus the search. We have life on Earth to use as a template and we can use our knowledge of biochemistry to extrapolate from that to conditions different from Earth where different kinds of molecules could arrange into structures complex enough to carry out life functions. At the same time new kinds of life are continuously being discovered in environments right here on Earth that were previously thought too extreme for allow the biochemistry of life to work.

Scientists are refining their set of hypothetical conditions that would be necessary for a planet to exist that could support life. And they are building a list of the basic ingredients that life requires. As we explore these two lists, you will may it striking how critical our knowledge of Earth's atmosphere and of atmospheric dynamics in general are to constructing such lists.

A Recipe for Life

95% of all life on Earth is built from only 6 elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. These elements are the building blocks of biological molecules from which complex (living) structures form. Carbon is especially important because it forms flexible covalent bonds with many other elements and it is the organic basis upon which energy transfer can occur.

Life on Earth requires water as a solvent in which biological molecules can form and grow in complexity.

Life requires an input of energy to sustain metabolic reactions. Solar energy is the basis of almost all Earth life, but electrical energy (lightning) turned a mixture of gases into amino acids, life building blocks. Chemical energy is used on Earth by chemotroph bacteria as well as more complex organisms around hydrothermal vents. These organisms use hydrogen sulphide, sulphur, iron hydrogen or ammonia as their sole energy source. The recent discovery of complex life forms sustained only by chemical energy opened up a whole new possibility for extraterrestrial life. Similar organisms could evolve in subterranean seas on moons such as Europa or ice-covered planets elsewhere in the cosmos.

These elements are abundant in the universe and should exist within every protoplanetary disk, which supplies the raw material of planets. Therefore, many experts suspect that life on other planets would likely be based on similar organic chemistry. Some have speculated on a few possible variations on this theme. Silicon forms bonds similar to carbon and could replace carbon in a silicon-based biochemistry. Earth and other terrestrial planets in our system are very rich in silicon compared to carbon (by a factor of 925:1) and yet carbon seems to be much more successful as a basis for Earth life. Silicon doesn't bond with as wide a variety of other elements as carbon does and, at least with an oxygen respiring organism, the waste product analogous to carbon dioxide would be silicon dioxide, sand in other words rather than gas. Still, there is some speculation that silicon could form the basis of biochemical structures that would be far more stable in very high temperatures where carbon-based structures would dissociate. This 3-minute excerpt from the BBC documentary "Cosmic Safari" explores what silicon-based life might look like:



Life forms using ammonia rather than water as a solvent have also been suggested. Like water, it dissolves many other chemicals, it forms compounds without being either too stable or too reactive and it's abundant. Life on Titan, if it exists, could be based on such chemistry.

Finally, if life is defined as fundamentally self-replicating reaction, it is possible that such a life form could even exist within the intense plasma of a star. This speculation, which seems really out there like an episode from Star Trek, stretches our ideas about how we define life.

Life-friendly Planets - The Goldilocks Zone

The goldilocks zone, or habitable zone as it is often called, is a theoretical orbital zone around a star where conditions are neither too hot nor too cold for liquid water to exist on a planetary surface. Its location and range depends on various factors such as how luminous the parent star is and how massive the planet is, as it must have enough gravity to hold onto water vapour. This zone is extrapolated from water-based life on Earth, the only life we know exists. Planets in this zone offer us the best chance of finding life that is based on biochemistry similar to ours. However, many researchers find it too constrictive, considering that life based on liquid methane as a solvent or silicon as a chemical backbone could exist in theory. If these potential biochemistries are considered, the goldilocks zone must be significantly widened accordingly. The potential for life in subsurface oceans, brought to light by the discovery of deep see thermal vent organisms, also widens the goldilocks zone considerably.

Scientists may need to expand their assumptions about where life could exist even further. For example, microbes called Deinococcus radiodurana are frequently found in highly radioactive nuclear waste. Our DNA is very vulnerable to radiation, but these organisms survive by having multiple copies of DNA and rapid DNA repair mechanisms. Such life forms would have no problem on a highly radiated planet surface as long as it had the right ambient temperature and an energy source. Many more extremophile organisms are being discovered every day in environments that we think should be unlivable.  Astrobiology Web provides an excellent list of such organisms.

Meanwhile, NASA, using the Kepler Space telescope, shown below, announced in December 2011, to a great deal of excitement, the first confirmed discovery of an alien goldilocks planet.

This is the focal plane of the Kepler telescope, containing a total of 95 megapixels.

The planet, called Kepler-22b, located 600 light years away, orbits a Sun much like our own. It is about 21/2 times larger than Earth with similar surface temperatures. It might look like this:

We don't yet know if it has surface water, however, just that it could. Its mass still unknown. Once researchers can determine its mass, they can determine the planet's density and make some predictions about its composition, how much rock versus how much water exists on it for example. Planets with low masses and long orbits like this one, and any other Earth-like planets we may discover, make it very difficult to determine mass because their radial velocity signal is very small. Whether Kepler-22b could support life depends critically on what kind of atmosphere is has, if any. Its surface temperature is estimated to be about 22°C on average but this estimate depends on what kind of atmosphere the planet has. If Earth had no atmosphere, for example, its surface temperature would be about -20°C. The greenhouse effect from atmospheric carbon dioxide, water vapour and methane warms it up by trapping some of the Sun's thermal radiation. No one knows yet if the planet is tidally locked or not either. If it is, this could make it inhospitable to life as we know it.

I mentioned Gliese 581g as the first discovered candidate as a goldilocks planet in my article on the goldilocks zone. There was a great deal of excitement in the science community when this planet was announced in 2010. The planet was detected using radial velocity measurements. Since then, several research teams have been unable to confirm its existence. As mentioned, radial velocity measurements of small mass planets tend to be very weak. They are difficult to verify statistically above background noise. To make these measurements, a series of observations of the electromagnetic spectrum emitted by a star are made. If periodic variations in the spectra occur, they may indicate the radial velocity of the star being altered by a planet's mass as it orbits it. If the data is plotted, the curve will indicate the mass of the planet. Apple even has a new Exoplanet app plotting this curve, for the iPhone, iPad and iPod touch, if you're so inclined. Planets a few times greater than Earth mass and smaller and orbiting Earth-distance or further from their star make these measurements extremely challenging. They are often so weak they get lost in background noise. This may be what happened with Gliese 581g; it may not exist after all. Kepler-22b, on the other hand, has been confirmed by the data from the Spitzer Space Telescope, an infrared space observatory shown prior to its 2003 launch below.

As well, Kepler-22b has been observed long enough to confirm at least three transits (it has an orbital period or year of 290 days), further supporting its existence.

Extasolar planets are, for the most part, simply too far away to observe directly. Fewer than 5% of the planets discovered so far have been directly observed, and those that have are especially large hot planets that emit enough infrared radiation to be directly detected. Everything we know about smaller more Earth-like planets, we must measure indirectly and this makes the atmospheric study of these planets very challenging. The James Webb Space telescope, shown below, scheduled for launch in 2014 is a large infrared-optimized telescope that will succeed both the Hubble and Spitzer space telescopes.

Although it was originally designed to study extremely distant (and old) galaxies, it also will be able to search for and study extrasolar planets in a new way. It may be equipped with a star shade that will work like putting your thumb in front of the Sun. It should be possible then to see a nearby orbiting planet within 1 AU (distance from the Sun to Earth) and further out. It may be able not only to directly image a terrestrial Earth-size planet but also distinguish seasonal changes in its atmosphere due to colour changes and determine its rotation. We may actually be able to see the atmosphere of a distant Earth-like planet!

We are just beginning to understand the complex interrelationship between life and atmosphere. Simultaneously, we are beginning to grasp how atmospheres develop, evolve and behave on various moons and planets within our solar system. We are becoming increasingly sophisticated in our understanding of extrasolar atmospheric possibilities, and how life based on different chemistries could exist in them. Yet it sometimes seems as if we have a lot of un-connectable puzzle pieces to work with. Ongoing research in many different fields is making headway toward a more singular understanding of the connections between atmosphere and life. An understanding of how Earth's atmosphere, and atmospheres in general, works is crucial in helping us ask the right questions, in keeping ourselves pointed in the right direction in other words, as we make our way through this murky and sometimes confusing uncharted territory. As our technology improves, we will undoubtedly make significant headway in the next few years.

A Philosophical (and personal) Note

All of this research centers around a basic and profound question. "Are we alone?" Our basic conceptions about where we fit into this unfathomably vast universe are now being challenged. Maybe it is a good time to pause for some introspection as we approach a critical point at which we may soon discover atmospheric traces of alien life. Are we ready to look over the fence into the backyard of our mysterious neighbour? We've recently made giant advances in science and technology and there is a seductive aura around them that is easy to focus on, so much that we risk losing a larger perspective. I suspect that where we are at as humans will be indicated by our approach: When we find another "Earth" (I personally think it is inevitable) will we pounce on it with our heavy guns of reason and acquisition or will we approach it with more delicate and respectful hands of an ecologist, taking the time and energy to gently unfold and ponder over this new mysterious world's secrets?

If you would to explore this further there are many recently published books to choose from. I recommend one in particular: "Lonely Planets - the natural philosophy of alien life" published in 2004. Astronomer David Grinspoon resumes where Carl Sagan left off exploring the possibility of alien life in this intelligent and thoughtful book.

Earth's Atmosphere Part 8 - How To Care For Earth's Atmosphere

This article focuses on the relationship between carbon dioxide emissions and global warming. It is a more in-depth scientific discussion than the one offered in my article on Alberta's Oil Sands.

It's so easy to take our air for granted. It seems as if it's always been there and our atmosphere is so gigantic it is difficult to imagine that the activity of humans could threaten it in any way. We see the giant smoke stacks of industry and tend to think that this grey billowing pollution will eventually dilute out and disappear.

But we've learned that Earth's atmosphere is much like the air in a terrarium. There is nowhere for pollution to disappear. Earth is an almost completely closed and very complex system that maintains a remarkably constant complement of various gases and a narrow range of surface temperatures, a perfect environment for us, and all life, to flourish.

We know that it wasn't always this way. When Earth was young its atmosphere was toxic and extreme. We would quickly die in it, and yet this poisonous atmosphere is the very environment in which the building blocks of life were formed and from which the first simple living organisms came to be. Earth was battered by water-bearing meteors and as a result we have a plentitude of surface liquid water, which sustains all life, not only as a solvent for life, but as an essential sink for carbon dioxide as well. Life itself changed Earth's atmosphere. Plants evolved and, through photosynthesis, plant life gradually enriched Earth's atmosphere with oxygen, a highly reactive gas from which a whole new efficient biochemistry evolved, giving rise to animals and eventually to us. We are intimately interconnected with our atmosphere. We have co-evolved along with it. As we start to look past Earth to other atmospheres on bodies both within our solar system and further away on distant extrasolar planets, we wonder if these events unfolded elsewhere in the universe.  Are we really the only remarkable Earth?

We've learned that this seemingly simple question is not easy to answer because we do not yet have a complete understanding of our own atmosphere to use a benchmark. Answering this question relies on a series of educated guesses and a number of simultaneously different approaches.

Through the preceding articles in the atmosphere series, we've taken a bit of a journey and now we come back home, hopefully more informed and appreciative of Earth's atmosphere. An essential first step in caring for it is to understand how various human activities impact it. This is a broad and expansive topic. Here I focus on one of the most controversial aspects of man's environmental impact - the connection between carbon dioxide emissions and global warming.

CARBON DIOXIDE AS POLLUTANT

We tend to think of smog and exhaust from factories and from our cars as air pollution, and it is, but pollutants can also be invisible. We can easily see the fine particulate matter within smog and exhaust but not pollutants such as carbon dioxide. It exists naturally at a very small concentration in the atmosphere (currently at about 380 parts per million, ppm), but when levels rise even minutely, it has a profound and global effect on the dynamics of the atmosphere. Its atmospheric concentration, in turn, affects the concentrations of other gases such as water vapour and that kind of cascading effect can force the atmosphere's equilibrium to shift. We have seen that atmospheres that don't have functioning equilibrium mechanisms do not have the chance to evolve. They slide toward extremes instead, as the examples of Venus and Mars taught us.

EARTH'S ATMOSPHERE MAINTAINS EQUILIBRIUM

Now that we know some atmospheric chemistry and evolution, we might wonder how likely Earth is to slip out of its atmospheric equilibrium state. This, in fact, is the central question I will attempt to answer. We know that Earth has maintained equilibrium through many extremes over the eons, not by being static and unchanging but by virtue of various chemical reactions that, by their very nature, tend toward equilibrium states. Those equilibrium states can and do migrate over time, and that is how atmospheres evolve. When we pollute our atmosphere, however, we may impose such significant and rapid changes on it that we run the risk of pushing various equilibrium mechanisms to extremes as they attempt to compensate for the change, triggering unforeseen ecological damage. These compensations may occur too rapidly and violently for many plant and animal organisms to adapt.

I am stressing equilibrium mechanisms and what can go wrong with them because this tends to be overlooked when we think of pollution. These reactions also give us an idea of just how complex the problem of pollution can be.

GLOBAL WARMING

Global warming is an excellent example of how complicated interrelationships between various equilibrium mechanisms affect each other, sometimes in unexpected ways. They may exaggerate a particular outcome, for example. Our climate is currently warming far more rapidly than climate experts anticipated even a few decades ago, especially in the arctic. Organisms at the tops of food chains, in general, are especially vulnerable to rapid climate change, a vulnerability starkly exemplified by the plight of Canada's endangered polar bears:

Earth naturally cycles through extended periods of colder and warmer climates. Ice core data, extending as far back as 800,000 years shows this cycling beautifully. This data comes from the Vostok team in Antarctica, shown here:

Atmospheric temperature, carbon dioxide (CO₂) level and atmospheric dust level (believed to increase when the climate is cold and dry) can be measured from ice core data. The graphed data below goes back 400,000 years:

(Vostok-ice-core-petit.png: NOAA)

All three parameters show significant directly corresponding cyclic changes. These cycles, called Milankovitch cycles, are obviously natural and not man-made. They are believed to coincide with long-term variations in Earth's orbit, which affect the amount of solar radiation it receives. Notice that CO₂ varies within a range from approximately 180 ppmv (parts per million volume) to 300 ppmv. At times when it dropped below this level or exceeded it, a variety of chemical equilibrium mechanisms compensated to bring it back within this narrow range. This compensation process is called Chatelier's principle. Earth's atmospheric CO₂ level is currently (and significantly) out of this range. This is data from 1960 to the present:

(Image:Mauna Loa Carbon Dioxide.png: Sémhur)

(Inset Note: The grey zigzags indicate seasonal changes in atmospheric CO₂ concentration. In the northern hemisphere, carbon dioxide is reduced throughout the summer as plant life consumes it during the process of photosynthesis)

Climatologists are concerned that Earth's various equilibrium mechanisms are no longer limiting the range of CO₂ levels in the atmosphere. Earth is likely in a natural warming climatic phase, but these levels are far higher than what could be explained by that.

Changes in Earth's orbit, variations in solar luminosity, and volcanic activity all influence Earth's climate. They are all examples of natural external forcing processes. Carbon dioxide pollution is also an external forcing process, one with the potential to be more extreme than any natural example. Let's take a look at the major equilibrium mechanisms involved.

THE MECHANISMS

Three Carbon Dioxide Negative Feedback Loops

Earth's atmosphere maintains equilibrium through a variety of negative feedback mechanisms. The most important mechanism occurs in Earth's oceans.

(1) Oceans

We learned that Earth's vast liquid surface water sequestered much of its early CO₂, taking it out of the atmosphere. CO₂ easily dissolves in water. Otherwise, Earth today would have a much higher level of atmospheric CO₂.

This sequestering process cannot keep up with the recent rapid increase in atmospheric CO₂, however. Only about one third of current emissions are sequestered in Earth's oceans. Some experts estimate that it would take 300 years to sequester 75% of today's emitted CO₂, leaving a permanent 25% atmospheric increase over the historical natural range. That accounts for only the current emission total, not future releases of CO₂ from as yet untapped fossil fuel deposits. It also assumes that the rate of CO₂ take-up in the oceans is not affected by increasing levels of dissolved CO₂. As you will see, this assumption is already being questioned.

Increasing levels of CO₂ dissolved in the oceans is presenting a serious problem for ocean ecosystems. CO₂ doesn't just dissolve in the ocean, like oxygen does. It reacts with the water itself:

CO_2(aq) + H₂O ↔ H₂CO₃ ↔ HCO^3- + H⁺ ↔ CO₃^2- + 2 H⁺

It creates carbonic acid, bicarbonate ions, carbonate ions and hydrogen ions, all in equilibrium with each other. Eventually the rate of net CO₂ uptake will slow down to zero when the water's saturation point is reached. Some experts estimate the world's oceans will reach saturation by 2100. Two factors are speeding up the saturation process. First, as the oceans warm (through the increased greenhouse effect caused by CO₂ which I will explain shortly), the water's solubility to CO₂ will decrease. Second, warming increases ocean stratification, isolating the surface water from deeper water and reducing the opportunity for CO₂ to dissolve to saturation in the entire volume of water.

Ocean CO₂ saturation may come faster than experts previously thought. There is now evidence that the Southern Ocean, shown below in blue, is already saturated with CO₂.

(Author:Connormah (Wikipedia))

This unexpected finding is linked to wind. There is increasing windiness in the southern hemisphere because the Antarctic ozone hole has led to a strong cooling of the stratosphere in that region, strengthening the pressure gradient force. Global warming is also linked to increasing winds from storms. Wind increases the surface mixing of water in the ocean. You, and most experts, would expect this to enhance CO₂ uptake by making more water available for absorption, but instead an increased release of carbon dioxide back into the atmosphere occurs, reducing the net absorption of CO₂ into the ocean. The Southern Ocean, the fourth largest ocean in the world, was thought to absorb 15% of all CO₂ emissions but now scientists are discovering that it has been absorbing less and less each decade since 1981. There is also evidence that other oceans, such as the North Atlantic are being affected the same way. It means that the climate models the IPCC now uses are overestimating the capacity of the oceans to absorb CO₂ out of the atmosphere.

Carbonic acid, one of the ions of CO₂ dissociation, acidifies ocean water. It increases the concentration of hydrogen ions, thereby decreasing its pH. As the pH changes, the ratios of various dissolved ions changes too, as shown below:

This graph shows that as pH decreases, CO₂ dissociation products are no longer favoured and eventually the water is saturated with CO₂ (both acidification and heat reduce CO₂ solubility in water). The current rate of acidification is estimated to be 100 times faster than any change in ocean acidity over the last 20 million years, and there is great concern that marine life sensitive to pH won't be able to adapt fast enough. Species that make shells or plates out of calcium carbonate are especially sensitive. Tiny animals with very thin shells that live in warm ocean water, such as planktonic foraminifera, are the first ones affected. These microscopic unicellular organisms, shown below, secrete a carbonate shell that is very sensitive to pH level. Though tiny, huge populations of them provide food for a wide variety of ocean life.

This photo courtesy Colomban de Vargas, EPPO/SBRoscoff is from the National Geographic website gallery. Each pink bubble is a separate organism.

In order to make calciferous shells, ocean pH needs to be at a level where calcium carbonate exists at an equilibrium just barely favouring dissolution into Ca²⁺ and CO₃^2-, so that calcium can be both taken up and deposited. As pH decreases, corals, certain algae and shellfish will experience significantly reduced calcification and enhanced dissolution.

Several coral reefs are already stressed by water that is too warm. Recent El Niño years have been especially hard on them, causing bleaching and death. Coral reefs, like the one below featuring a tube sponge, provide habitats for a wide variety of marine animals.

(Photo courtesy Nick Hobgood (Wikipedia))

In addition to stress from warming, scientists are now looking for evidence of damage to sea life linked to acidification. Ocean surface pH has decreased from 8.25 to 8.14 since the start of the industrial revolution. A pH decrease of 0.1 may seem small but it corresponds to a significant 30% increase in H⁺ ions in seawater, affecting the availability of other biologically important ions.

(2) Rock

As we have learned, Earth started out with an atmosphere containing 90% carbon dioxide. This came from outgassing from the hot interior through the young planet's molten crust and later through intense volcanic activity. Earth had surface water very soon after its formation - much of its atmospheric CO₂ was soon sequestered in water as well as in surface rock. Most of Earth's carbon is stored in sedimentary rocks such as limestone. The slow process of subduction takes this carbon deep into Earth's interior, while rock erosion and volcanic release of CO₂ release it back into the atmosphere and oceans. Earth's crust is saturated with carbon dioxide. Like the oceans, it acts as both a sink and a flux - geologic processes of erosion, volcanism and plate tectonics cycle carbon and help maintain its atmospheric levels in equilibrium.

(3) Biological Material

Carbon dioxide is consumed by plants, which take it out of the atmosphere and lock it in tissues. It cycles through the rest of the ecosystem as plants die or consumer organisms eat them. As plants and animals die, they decompose, releasing some carbon dioxide back into the atmosphere and contributing some carbon to the soil or the bottom of a body of water. This organic matter accumulates very slowly over time.

Earth was once covered with shallow seas supporting tremendously lush plant and algal growth as well as a rich variety of animals that fed on it, perhaps like this image from the National Geographic Carboniferous Photo Gallery:

(Artwork by Dorling Kindersley/Getty Images: National Geographic website)

Organic material that accumulated from that period (360 to 300 million years ago) was eventually buried through slow geologic processes. Today we drill and mine for those deposits which have chemically transformed over time into energy-rich hydrocarbon fuels.

The Carbon Cycle

Carbon dioxide cycles through Earth's lithosphere (rock), hydrosphere (oceans) and biosphere (life), as shown in the diagram below. Doing so, it maintains an overall equilibrium within a very narrow range.

(Source:PhysicalGeography.net)

Carbon has gradually been taken out of the atmosphere through these same processes because they function as sinks for the gas. Subduction is a particularly important but very gradual CO₂ sink. Carbon dioxide sequestered in rock is slowly pushed deep underground. As Earth's volcanic activity gradually eased over millennia, less and less of that sequestered CO₂ has been added back into the atmosphere. Our use of fossil fuels is releasing carbon dioxide from its biological sink. This activity releases far more atmospheric carbon dioxide than what is gradually lost from the carbon cycle and sequestered through natural processes. It results in a net increase in atmospheric CO₂, the rate of which over the past 200 years is unprecedented, as shown in the graph below:

(Prepared by Robert A. Rohde from a compilation of data sources (Wikipedia))

Stefan's Law - A Weak Negative Feedback

A negative feedback mechanism, called Stefan's law, operates on Earth as a whole. The total energy radiated from a body is proportional to the fourth power of its temperature. What this means is that as Earth's atmosphere warms up, it radiates more energy back out. This negative feedback loop, however, is weaker than Earth's greenhouse mechanisms. Greenhouse gases in the atmosphere (such as water vapour, carbon dioxide and methane) tend to absorb the Sun's radiation in the longer wavelengths, such as infrared (heat), while reflecting more of the shorter wavelength radiation back out, thus trapping heat in the atmosphere. Earth, as a result, has a higher equilibrium temperature (288K or 14°C) than the temperature predicted using Stefan's law (255K) and higher even than the temperature a perfect black body (exhibiting maximum radiation absorption) would have (279K).

Three Positive Feedback Loops

(1) Albedo

Albedo means the fraction of the Sun's radiation reflected from a surface. Earth has experienced long periods of climatic cooling resulting in ice ages followed by warming. The 100,000-year Milankovitch cycles (see the red-green-blue graph data near the beginning of this article) closely match Earth's 100,000-year pattern of ice ages. When a significant amount of ice covers the planet, temperatures tend to remain cool because (white) ice exhibits high albedo; it reflects much of the solar radiation striking Earth back into space. When ice melts, dark seawater absorbs more radiation than it reflects and, as a result, oceans warm. As the oceans warm, more ice melts and the warming effect continues to build. This mechanism operates only over a narrow range of temperatures (ice-melting range) and it works in both directions. As Earth cooled and slipped into past ice ages, areas of ice enhanced climatic cooling. Its overall effect is that of hastening the rate of warming or cooling once it is already underway.

(2) Methane Deposits

Earth has a lot of peat bogs, such as this one in Germany shown below. They cover about one quarter of the planet's surface to a depth of about 25 meters:

(Photograph by Jan van der Crabben (Wikipedia))

Peat, which is generally acidic marsh vegetation matter, decomposes very slowly, generating methane as it does so. Methane is an extremely potent greenhouse gas, 70 times more potent than carbon dioxide. There is only 1.7 ppm of methane in Earth's atmosphere, but raising that level by only 1 ppm would be equivalent to raising CO₂ from 380 ppm to 450 ppm. During the last ice age much of Earth's peat was frozen in permafrost, trapping methane in the ice itself. The arctic climate alone has warmed enough to trigger the melting of these frozen peat bogs, releasing methane into the atmosphere and accelerating the melting of the permafrost, forming a positive feedback loop. It is a natural process associated with past warming periods, but this particular melting cycle is accelerated by human carbon dioxide emissions. Atmospheric methane levels nearly doubled during past interglacial periods, as shown below, but the current rate of increase seems significantly higher (see the blue graph below this one).

Atmospheric methane levels are not easy to measure. They vary widely from region to region depending on vegetation, industry and other factors, complicating global predictions.

Most researchers don't believe that methane release from melting peat bogs has the potential to cause catastrophic run-away climate change, but many are concerned about another source of methane:

The warming of the oceans could trigger a sudden release of methane from frozen methane hydrate compounds buried in the ocean floor. These compounds are frozen because they are under pressure and the water temperature there is below around 15°C. This is a deposit of methane hydrate embedded in sediment taken from the subduction zone about 1200 m deep off the coast of Oregon:

(Image by Wusel007 (Wikipedia))

No one is sure how large these deposits are but some experts expect there could be as much as 3000 billion tonnes of it around the globe. If it were all released into the atmosphere that would translate into staggering 1000 ppm. As the oceans warm up (especially the warmer ones), this methane could be released, warming the atmosphere, and oceans, and releasing yet more methane. A hypothetical process, called the clathrate gun, might be triggered (a clathrate is a crystalline water-based solid, in this case it is synonymous for hydrate). Under the frozen methane hydrate layer, experts expect to find hot compressed methane gas. If released it could lead to an explosive rate of global warming, hence the word gun. Researchers need to know how large the methane hydrate deposits are and what their composition is in order to make a better prediction of their global warming risk. Sudden warming linked to the melting of sub-ocean methane hydrates may have contributed to the Permian-Triassic extinction 250 million years ago, the Paleocene-Eocene Thermal Maximum 55 million years ago and the sudden warm-up of Snowball Earth, 630 million years ago.

(3) Water Vapour

When the atmosphere warms, its saturation vapour pressure increases. That means it can hold more water vapour gas. Water vapour accounts for an average of half of Earth's total greenhouse effect in clear skies to over 70% when including clouds. Human activity can significantly contribute to regional differences in water vapour concentrations, which fluctuate naturally from a minimum of less than 0.01% in very cold regions to up to 20% in warm humid regions.

Water vapour is itself a greenhouse gas. When the atmosphere warms because of the greenhouse mechanisms of increasing carbon dioxide and methane, water vapour increases too and its contribution amplifies those effects, creating a powerful positive feedback loop.

This feedback loop is believed to be the primary reason why Venus's surface is so hot compared to Earth. However, the mechanisms involved in Venus's heating are much different. As the young Sun evolved, its output of energy increased. Venus' surface warmed and the amount of water vapour in its atmosphere increased, setting off a positive feedback loop that eventually boiled away its oceans and unlocked carbon dioxide from surface rock. Of course, Earth was similarly affected by the Sun (it orbits 1.4 times further from the Sun than Venus), but Earth had and still has an effective carbon cycle. There is growing evidence that Venus did not. Most experts now believe that such a runaway effect is not possible on Earth because it has active carbon cycling and, therefore, a way to sequester carbon dioxide before its atmosphere can warm enough to boil the oceans. The fact that Earth, having experienced many climatic extremes, has always settled back into an equilibrium state, offers strength to this idea. Having said this, the IPCC Fourth Assessment Report states that human activity could lead to some effects that are abrupt and irreversible, depending on the overall rate of climate change, which depends on some uncertain factors. These disruptive effects are not expected to be sufficient to lead to runaway global warming though.

A TIPPING POINT?

There has been much speculation in the media that greenhouse gas emissions could eventually trigger a climatic tipping point, a point at which positive feedback loops overwhelm the restoring effects of negative feedback loops, causing rapid escalating climate warming. Consider this concept as we examine some challenges to the idea of catastrophic global warming.

Almost all experts agree that Earth's climate is in a warming period. A few experts believe that this period is almost entirely due to natural cycles in climate. Others believe Earth's climate is headed toward a Venus-like oblivion. Some climate data is ambiguous and current computer models predicting future climate effects based on CO₂ emissions must rely on various assumptions. They don't agree on any single well-defined outcome. There is significant wiggle room in which to interpret all the complex factors involved in assessing climate data. I offer an example of an interpretation that challenges my own:

Finding a Solution Requires an Interdisciplinary Approach

If we look at data going back several millennia, we find that atmospheric carbon dioxide equilibrium levels have experienced significant fluctuations. See the chart below. I found an interesting 2009 guest post about carbon dioxide and climate change in which engineer Bob Heiderstadt challenges our current concern over global warming. In particular, he questions whether Earth could experience run-away global warming based on positive feedback mechanisms. There is a common concern among scientists and laymen that human CO₂ emissions will tip the balance of sensitive equilibrium reactions, overwhelm negative feedback mechanisms and drive positive feedback mechanisms. As I mentioned, some worry that this switch could trigger a runaway greenhouse effect that could ultimately push Earth toward a disastrous Venus-like outcome.

Heiderstadt argues that, while it is a fact that CO₂ emissions are driving global warming, this is nothing new for Earth. During the Cambrian period, around 550 million years ago, CO₂ levels ranged between 3000 and 8000 ppm, much higher than current levels. Several sources confirm similar data, as shown in the graph below:

(Prepared by Robert A. Rohde (Wikipedia))

This makes sense. Recall that early Earth likely had an atmosphere dominated by carbon dioxide and, through a variety of processes, it was sequestered into oceans and rock, as well as into biological sinks. One sink, plate subduction, in particular acts very slowly. This process could account for a gradual lowering of atmospheric CO₂ levels over a period of billions of years. Heiderstadt notes that life not only survived but flourished in this ancient humid CO₂-rich and warm (22°C average versus about 14°C today) atmosphere. In fact, as we've seen, much data suggests that Earth's CO₂ levels (and average temperature) experienced several significant fluctuations over the hundreds of millennia since life first appeared on Earth. We are, in effect, through current emissions, simply re-introducing carbon dioxide that was once atmospheric back into the atmosphere. CO₂ emissions ultimately come from fossil fuels, which is ancient biological matter that locked CO₂ out of the atmosphere millions of years ago. I agree with his argument that negative feedback mechanisms in the atmosphere back then existed just as they do today and they prevented Earth from a runaway greenhouse effect. Their equilibrium levels were simply set a bit higher in terms of average atmospheric CO₂ level. I also agree that, thanks to CO₂ emissions, our global climate may indeed shift back to a higher equilibrium level where CO₂ levels of perhaps 3000 ppm are achieved. Our world may then come to resemble that ancient carboniferous epoch - warmer, more humid and perhaps richer in plant life. Oxygen levels, as a result of much higher levels of photosynthesis, could in turn increase to a new equilibrium level as well. This, in and of itself, would not spell catastrophe for humankind - we are remarkably adaptable and it might even spell a more livable climate than what we experience on average now.

The problem with this thesis, however, is with the rate of expected change and its potential ecological impact. These issues were not considered in sufficient detail. Natural carbon sinks cannot keep up with the increase in CO₂ emissions because it is occurring so rapidly, on a scale that could not be replicated naturally except perhaps through a catastrophic series of simultaneous global volcanic events or a supervolcano.

Volcanic Events and Manmade CO₂ Emissions - Similar Effects?

There is some evidence that sudden spikes in volcanic activity (pumping out a great deal of CO₂ once sequestered in deep rock and rapidly increasing atmospheric CO₂ levels) may have triggered several global extinction events, but the effects are not as straightforward as they might seem. It is possible that sudden extreme volcanic activity could plunge Earth into a significant cooling period, rather than a warming one. The reason for this is the loading of very fine ash and sulphur dioxide into the stratosphere, reducing the amount of the Sun's radiation reaching the surface, and offsetting the warming effect of additional CO₂. For example, the Toba supereruption approximately 70,000 years ago may have reduced the human population to a mere 10,000, creating a severe population bottleneck. Estimates of the resulting global cooling vary widely, between a drop of 15°C to just 1°C.  Lake Toba in Indonesia, shown here, is a crater lake created by that eruption:

The Triassic-Jurassic extinction event, 200 million years ago, in which at least half the world's species became extinct, may have been triggered either by warming or cooling by increased CO₂ (former) or increased fine volcanic dust and sulphur dioxide (latter). This uncertainty underlies our current knowledge about the atmospheric effects of volcanic activity. Scientists recently linked the Permian extinction about 250 million years ago, which wiped out almost all land and water life, with abrupt global warming caused by a cluster of volcanic eruptions in ancient coal beds in now Siberia. There is geological evidence that these volcanoes spewed out an incredible amount of toxic coal ash and greenhouse gases.

It is difficult establish a direct relationship between volcanic activity and global warming events because there is regional variation in the composition of volcanic gas emissions and because volcanoes also release gas and ash into the atmosphere that have cooling effects, in addition to the greenhouse gas, CO₂, complicating any comparison between the two events.

Ecological Impact of Rapid Global Warming

Even a few °C change in Earth's average global climate, if it occurs too quickly, translates into significant stresses on ecosystems enough so that animals adapted to certain food sources lose them as the plants at the base of food chains die off from climate change related heat, drought or water stress. There is evidence that rapid climate change also brings larger and more violent weather systems, further stressing ecosystems. We can see this firsthand for ourselves in the arctic. While gradual shifting in equilibrium levels, which has occurred naturally over the millennia, offers organisms enough time to adapt to new niches and evolve, sudden shifts run the risk of stressing organisms so greatly that they either die or are so weakened they no longer reproduce sufficiently to keep the population up. Holes in food webs would then start to appear, stressing yet more organisms that rely on them. Eventually a cascade effect could happen in which entire ecosystems disappear altogether, all triggered by a relatively small, but rapid, shift in atmospheric CO₂ equilibrium.

The term "tipping point" mentioned earlier now tends to be used more subtly than its association with runaway global warming. There are several tipping points of current concern: irreversible melting of the Greenland ice sheet, dieback of the Amazon rain forest, the current shift in the African monsoon belt, shifts in global ocean circulation patterns, and increasing weather extremes. Many of these things are interconnected, affecting the impacts of one another, sometimes in unexpectedly complex ways. Together they will have a significant impact on life on Earth. Humans are a very adaptable species - we can move, change our life styles, adapt to new food sources etc., but I worry that the abrupt significant climate change, as outlined in the most recent IPCC report, will put such widespread stress on global plant and animal life that our huge and increasing population will have difficulty securing sufficient and reliable food and water resources. The latest climate change data leads me to believe that continued global reliance on fossil fuels, while not likely to trigger runaway global warming, poses the serious potential of a mass extinction event. There is some evidence that one is already underway.

Based on the above research, even if all the carbon dioxide sequestered in Earth's fossil fuel reserves is released into the atmosphere through emissions, the planet's powerful negative feedback mechanisms will likely re-sequester much or all of it, returning Earth to either a similar or slightly different state of atmospheric equilibrium. New ecosystems will evolve as the climate once again settles into that equilibrium. The concern for us focuses on the impact of climate change on current ecosystems of which we are part. Some of those impacts are already documented. Others are less certain in terms of severity or even cause. As the climate continues to warm, other, as yet unknown, ecological impacts are likely to become apparent as well.

Researching this issue made it clear to me how important an interdisciplinary approach is to understanding global warming and its connection to CO₂ emissions. Experts in ecology, chemistry, geology, climatology, physics and biology are required to further our understanding, and an ongoing collaboration between these and other fields seems essential to success. This is challenging issue that I hope those of us in positions to make environmental policy decisions and those who are heading up the various hydrocarbon fuel industries will make a priority.