Thursday, January 19, 2012

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:

CO2(aq) + H2O ↔ H2CO↔ HCO3- + H↔ CO32- + 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 CO32-, 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.


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.

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