v1.1.0 / chapter 8 of 15 / 01 sep 10 / greg goebel / public domain
* In the mid-20th century, researchers became increasingly aware of the effects of human activities on the environment. This has led to ever-increasing efforts to understand and control this impact, with one of the most prominent aspects being "environmental chemistry" -- the study of the normal chemical interactions of the Earth's environment and how human activities affect them.
* All heavenly bodies have an environment of sorts, but they're not
necessarily very complex: our airless Moon is characterized by vacuum, rock,
and dirt. The Earth, in contrast, has a very complicated environment that we
still don't understand in full detail. The Earth is a rocky planet with a
diameter of about 12,750 kilometers, with a mass of about 6 x 10^21 tonnes.
Its elemental composition is as follows:
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iron: 34.6%
oxygen: 29.5%
silicon: 15.2%
magnesium: 12.7%
nickel 2.4%
sulfur 1.9%
titanium 0.05%
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The Earth has a layered arrangement, with a thin external crust and a number
of deeper layers down to a molten iron core. The crust is divided into about
a dozen "tectonic plates", which are rigid in themselves but can move
relative to each other, driven from one side by upwellings of rocky materials
from the "mid-ocean ridges" of undersea volcanoes, and pushed slowly back
down into the Earth at the other side into "oceanic trenches". This
phenomenon is known as "continental drift".
About 70.8% of the surface is covered with water, mostly in the Earth's salt-water oceans, Only about 3% of the Earth's water is fresh, with about two-thirds of that locked up in polar icecaps, and the remaining third in fresh-water lakes and rivers.
Current evidence from radioactive dating points to the Earth being about 4.5 billion years old. In the beginning, the Earth's atmosphere was nothing like it is today, being heavily loaded with carbon dioxide and nitrogen. There was no free oxygen to speak of: oxygen of course is reactive and tends to form carbonate minerals, such as limestone (calcium carbonate / CaCO3) or magnesium carbonate (MgCO3), which eliminate it from the air. It wasn't until organisms arose that could produce oxygen, 2.2 billion or more years ago, that oxygen began to appear in the Earth's atmosphere. It wasn't until about 700 million years ago that oxygen became a significant component of the atmosphere, resulting in more or less the atmosphere as we know it today.
In the modern era, the primary components of the Earth's atmosphere are:
These figures assume no water vapor, but the water content of the atmosphere can vary over time and place, ranging from effectively zero to up to 4% in terms of numbers of molecules. The remaining fraction of the atmosphere is a mix of trace gases, measured in parts per million:
The upper layers of the atmosphere have high concentrations of ozone, O3, produced by radiation from space breaking up O2 molecules. The "ozone layer" provides a safety barrier for the Earth's organisms since ozone is opaque to much of the high-energy radiation that sleets down from space.
The Earth's land, sea, and atmosphere exist in a complicated equilibrium, with weather patterns transporting water vapor, particularly from the tropical seas, to become rainfall elsewhere. Erosion and volcanic processes slowly adjust the shape of the land over long aeons, changing the configuration of the seas and influencing climate patterns. The relatively thin layer of organisms that covers the Earth -- the "biosphere" -- both is affected by and affects this shifting equilibrium.
While natural processes do change the Earth's environment over time, as human population climbed up into the billions, human activities have begun to have a major and worrying impact on the operation of the system of the world. The rest of this chapter addresses this issue in more detail.
* The fact that human activities could have a damaging effect on the environment was long more or less ignored. Deforestation and short-sighted agricultural practices often resulted in the "desertification" of lands once green and fertile. With the arrival of the Industrial Revolution, pollution from factory smokestacks and the production of chemicals became widespread. It wasn't until after World War II that awareness of environmental issues became widespread. In December 1952, unusual weather conditions carpeted London with a smoky fog -- what would be called "smog" -- for the better part of a week, killing thousands of people.
Another one of the roots of modern environmental consciousness was the development of the pesticide DDT during World War II. It was a strategic tool during the conflict, allowing control of mosquitoes that carried malaria and other diseases, and was put to widespread use to protect civilian populations after the war. However, research indicated that DDT tended to become concentrated by biological activity up the food chain, which became a public issue after American biologist Rachel Carson (1907:1964) published her famous book SILENT SPRING in 1962. Although some believe Carson overstated the case against pesticides, the book helped create the modern environmental movement, and from that time, like it or not, the environmental impact of chemicals was an issue that couldn't be ignored.
From the mid-1960s, environmental regulations controlling the emission or dumping of toxic substances were established in industrialized nations. Companies that dumped toxic chemicals were fined, and improved techniques for disposal of public and industrial wastes were developed to prevent contamination of the soil and water. Remediation of wastes focused on incineration or chemical neutralization, or when that wasn't possible, disposal in secure toxic waste dumps.
The issue is not a simple one and work on the matter continues, with "green chemistry" efforts now a prominent component of chemical research, focusing on a number of avenues of investigation:
* The most visible component of work to reduce environmental pollution has
been in air pollution control. Motor vehicles are a significant component of
air pollution, the problem being traditionally a matter of incomplete
combustion. Ideally, a combustion process should produce nontoxic emissions,
such as diatomic nitrogen (N2) and CO2, but incomplete combustion produces
nasty NO and NO2 (generally called "NOx" and definitely not the same sort of
beast as N2O or "laughing gas"), carbon monoxide (CO), and partly burned
hydrocarbons. Worse, the effect of sunlight on NOx produces reactive and
toxic ozone (O3), a particularly unpleasant component of air pollution:
NO2 --UV--> NO + O
O + O2 --> O3
Most modern vehicles have emission control subsystems. The first line of
defense has been to develop engines that feature more efficient combustion,
using higher operating temperatures, improved combustion chambers that
encourage better mixing of air and fuel, and smarter electronic ignition
systems. This has the side benefit of improving fuel efficiency, though at a
cost in expense. The second line of defense is to introduce a "catalytic
converter" system in the engine exhaust line that converts reactive
components of incomplete combustion, such as NOx and CO, to benign emissions
such as N2 and CO2.
* Power plants and factories also now feature sophisticated emission control
systems. A modern coal-fired powerplant burns reasonably cleanly, but coal
is not a particularly clean fuel. It contains minerals that won't burn,
ending up as particulate ash, and sulfur, which becomes sulfur dioxide on
combustion and will combine with water in the air to form sulfuric acid,
producing "acid rain":
2SO2 + O2 --> 2SO3
SO3 + H2O --> H2SO4
The "fly ash" that flies up the flue has to be removed, this being done
either by a "baghouse" or an "electrostatic precipitator". A baghouse is
conceptually simple, just a set of heavy cloth filters in the form of long
tubes open at one end, with the cloth allowing the gases to pass through
while capturing the fly ash. Since the fly ash will eventually clog the
bags, the airflow is reversed occasionally to force out the ash, which falls
down into a hopper for removal. Some baghouses have a shaker system to help
dislodge the ash.
An electrostatic precipitator consists of rows of vertical plates, with arrays of fine wires energized to high voltage arranged between the plates. The gas passes up through the gaps between the plates, with the ash particles acquiring an electric charge and sticking to the plates. A "rapper" system knocks the ash loose into a hopper for collection.
Not surprisingly, neither of these schemes works for sulfur dioxide, with a "scrubber" system used instead, installed "downwind" from the baghouse or electrostatic precipitator system. Scrubbers are tall cylinders into which a slurry of limestome -- calcium carbonate (CaCO3) -- is sprayed down from the top. The calcium carbonate dissociates into lime (CaO) and carbon dioxide, with the lime combining with the sulfur dioxide to form calcium sulfate, or gypsum (CaSO4*2H2O).
The gypsum is collected in a hopper and hauled off to a dump. The process produces a substantial amount of gypsum and disposing of it is troublesome. Not all coal-fired powerplants have scrubbers: some coal has low sulfur content and doesn't need a scrubber. Incidentally, sometimes the stack of a coal-fired power plant will emit a visible white plume. Although news clips often show the plume when discussing powerplant emissions, it's not smoke -- it's steam from the scrubber, the actual emissions are mostly invisible. In cold weather, the plume may appear whether there's a scrubber or not. A number of powerplants also have "selective catalytic reduction" system to help get rid of toxic NOx, with the NOx catalytically reacting with ammonia (NH3) to form nitrogen and water.
* Environmental problems have a nasty tendency to appear out of nowhere. Early household refrigerators used noxious gases such as sulfur dioxide and ammonia as coolant fluids, with documented cases of families being killed by coolant leaks. In the 1930s, CFCs were introduced as a replacement coolant, and they seemed all but perfect for the job: they were effective, cheap, nonflammable, noncorrosive, and in particular nontoxic. A person can breathe CFCs and suffer no harm, except through oxygen deprivation.
By the 1970s, CFCs were not only in widespread production and use as coolant
fluids in refrigerators and air conditioners, they were also used as "blowing
agents" to bubble up foam plastic insulation, cleaning agents, and spray
propellants. In 1974, however, researchers discovered that CFCs might well
be depleting the ozone layer. Once released, the CFCs could migrate to high
altitudes and be broken apart by ultraviolet radiation that didn't reach
lower altitudes:
CF2Cl2 --UV--> CF2Cl + Cl
The reactive chlorine atoms would then react with ozone in a two-step process
to produce oxygen:
Cl + O3 --> ClO + O2
ClO + O --> Cl + O2
The particularly unpleasant thing about this reaction was that it was
catalytic: the chlorine was not consumed in the reaction, which meant that a
small amount of chlorine could convert a vastly larger amount of ozone into
diatomic oxygen. The argument was theoretical at the time, but by the early
1980s satellite observations were showing a spreading region of ozone
depletion over the South Pole, with the hole getting bigger and bigger every
winter. The matter became a public controversy.
The fuss over CFCs tended to give the public the impression that CFCs are toxic. The reality is that they are very inert, extremely safe in themselves, vastly safer than the noxious coolants they replaced -- the irony being that the problem with them is their very lack of reactivity. Once they escape into the atmosphere, they persist and migrate upward through the stratosphere into the ozone layer, where intense solar ultraviolet breaks them down, which in turn leads to ozone depletion and a potential thinning of the protective ozone layer.
Not everyone agreed that CFCs were a real threat. There was no serious dispute that CFCs could cause ozone depletion, but the trick was that the depletion was strongly enhanced by cold temperatures. That was why the Antarctic ozone hole only appeared in the winter. On that basis, it was possible to argue that ozone depletion by CFCs would not amount to a threat at higher latitudes. However, on the basis of the notion of "better safe than sorry", in 1987 representatives from 43 nations signed the "Montreal Protocols", which mandated the gradual phaseout of CFCs.
CFCs are being replaced by hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). HFCs contain no ozone-depleting chlorine. HCFCs do contain chlorine, but the addition of one or more hydrogen atoms causes them to break apart faster in the lower atmosphere. Their tendency to damage the ozone layer is judged to be about 2% to 10% that of CFCs, and they have much shorter atmospheric lifetimes, from 2 to 25 years, compared to a century or longer for CFCs. The disadvantages are that HFCs and HCFCs are not as efficient refrigerants, are more expensive, and in some cases are flammable. Refrigerators and air conditioners have to be redesigned to make use of them.
* Although it seemed in the 1970s and 1980s that the air pollution challenge was being met, in the 1990s worries began to spread that human activities had an effect that promised to be much harder to deal with: global warming.
Over the past 3 million years, the world has gone through a sequence of Ice Ages, periods in which glaciers increasingly covered the Earth, to then fade away for a time. In 1896, Svante Arrhenius published a paper in which he suggested that Ice Ages might be linked to atmospheric concentrations of CO2. The Sun pours light down on the Earth, heating it up; the warm Earth then produces infrared radiation, much of which escapes off into space. Atmospheric CO2 tends to "trap" infrared radiation, preventing it from escaping and making the Earth warmer; in modern terms, CO2 is a "greenhouse gas". The trapping effect is proportional to CO2 concentrations, and so low CO2 concentrations might have led to the Ice Ages.
There was concern at the time, and later, that the Earth was headed for another Ice Age, which would undoubtedly have a savage impact on human population, but in a later book Arrhenius suggested: not to worry. Human industrial emissions of CO2 would be strong enough to prevent the Earth from slipping back into another Ice Age, and the warmer Earth that would result from these high CO2 levels would allow humans to grow more crops to feed an expanding population.
Climate scientists generally believed that Arrhenius was right in principle, but in the period after World War II there was actually a cooling trend. Some climate scientists even suggested that a new Ice Age might be imminent, and in fact as late as 1975 the US news weekly NEWSWEEK ran a cover article titled "The Cooling World", which predicted that a disastrous Ice Age was then in the making. However, the midcentury cooling trend, it turned out, was ironically also due to emissions -- of particulate pollutants, which reflected sunlight back into space and help cool the world. Effective pollution control measures dropped the concentration of particulates, and the temperature began to climb again. By the 1990s, climatologists had become increasingly worried about what might happen to the Earth if CO2 concentrations continued their climb, and spoke out about their concerns.
* The end result has been a loud quarrel, with advocates of "anthropocentric global warming (AGW)" theories insisting that the welfare of the planet is at stake, while the critics insist it's all hysteria, an alarmist scenario with no real basis in fact. The critics claim that the climate-change advocates are being misled by sloppy analysis of climate data, bogus interpretations of climate theory, and dubious computer models. In response, advocates stress the way the elements of their case reinforce each other, pointing out that theory, modeling, and data point overall in the same direction -- the pieces of the puzzle show a generally consistent picture. If there's errors, why are they all in the same direction?
The foundation of global climate theory is the simple fact, established by thermodynamics, that for a planet to maintain a constant temperature, the amount of energy absorbed from sunlight must be matched by the amount of energy the planet loses to space in the form of infrared thermal radiation, with the intensity of this radiation increasing with temperature. The Earth receives an average of 239 watts of sunshine per square meter; a simple body reradiating the energy back into space would have an average temperature of -18 degrees Celsius -- about zero Fahrenheit.
Clearly, on the average the Earth is warmer than that, and the reason that is so is because the greenhouse gases, like CO2, in the Earth's atmosphere block the escape of infrared thermal radiation back into space by absorbing it and reemitting it -- incidentally, in the tenuous upper atmosphere where greenhouse gases are too diffuse to have much of an effect, the average planetary temperature really is about 18 degrees Celsius. Increasing the concentration of greenhouse gases makes it harder for the heat to leak out, with the surface of the Earth and the lower atmosphere heating up. The rise in temperature alters the way the atmosphere transports energy from the warm equator to the cold poles, changing weather patterns.
There are four principal greenhouse gases:
Anyone reading this list might have reason to feel puzzled, because it clear shows that water vapor is the most important greenhouse gas. So why the fuss over CO2? However, although skeptics have proclaimed CO2 "insignificant" as a factor in comparison to water vapor, an effect ranging in possible value from about a tenth to a quarter of the whole is to be reckoned with -- and there are good reasons to believe CO2 is the "lever" in the system.
One of the characteristics of CO2 is that the "sinks" that draw it out of the atmosphere, most significantly through the photosynthetic operation of plants, operate slowly. That means that a rise in CO2 concentrations can take a long time to fall back down. Water, in contrast, simply falls out of the sky as precipitation, and water vapor concentrations can change very rapidly -- everybody knows that the weather can change from humid to dry overnight. CO2 concentrations don't, can't fluctuate anywhere near that rapidly.
Obviously, water vapor is produced by evaporation, mostly from the seas, and also obviously an increase in global temperatures means a higher rate of evaporation. That suggests a temperature increase due to a rise in CO2 concentrations could well be amplified by positive feedback from an increase in water vapor concentrations. As noted, the concentrations of CO2 in the atmosphere are small, a fraction of a percent. That means its effectiveness as a greenhouse gas is disproportionately greater than of water vapor on the basis of the total mass of each gas, and so increments in the concentrations of CO2 could have a disproportionate effect.
All this said, water vapor occupies a very ambiguous position in the global warming debate, with both sides claiming it aids their case. Although it is certainly a greenhouse gas, as water vapor concentrations rise it also produces more cloud cover -- and, in winter, more snowfields -- reflecting more sunlight back into space. However, to confuse matters further, clouds also reflect radiation from below, helping trap more heat; in addition, condensation of water droplets is an exothermic process, it releases energy, and so formation of clouds tends to produce local warming.
* So what does real-world data show? Measurements made from the 1950s show the level of CO2 rose from 316 parts per million (PPM) in 1959 to 387 PPM in 2009. Indirect measurements suggest the rise began about 1750, starting from the 280 PPM that appears to have been the long-term average for the 10,000 years before that -- though as everyone acknowledges, natural CO2 concentrations did tend to vary around that average.
The timing of the rise in CO2 concentrations from 1750 tracks the rise in human population and industrialization. Skeptics point out that the relative proportion of human emissions of CO2 to natural emissions is small, and in fact that's true -- but while natural processes have been producing CO2 for a lot longer than humans have been around, they also provide sinks that soak up the CO2, keeping the levels roughly constant. Human activity has provided a persistent increment of CO2 emissions that natural processes can't quite keep up with, a trickle that is gradually causing an overflow. Estimates suggest that humans produce CO2 in the range of 25 to 30 gigatonnes a year; the rate of growth needed to account for the parts-per-million changes observed is about 15 gigatonnes per year, which is roughly only half the human contribution.
But what is the actual effect of that overflow? It's not entirely clear from the data just how temperature rises with CO2 concentration, or in other words what the "sensitivity" of climate to CO2 concentration really is. Climate is a noisy phenomenon, making it hard to spot and track changes, and the oceans can absorb a good deal of heat, inserting considerable inertia into the system. Climate records now available do support warming -- though critics claim those who organized the records were subtly influenced by various confounding factors, for example the spread of urban areas that tend towards the warm. Advocates reply that they are not ignorant of such effects and have tried to factor them into the data.
* As far as sensitivity goes, theory backed up by lab studies on the absorption of infrared by CO2 suggests that, on its own, a doubling of CO2 concentration would add 1 degree Celsius to the global temperature. Current projections show that CO2 concentrations will double from the pre-industrial level of 280 PPM to 560 PPM by 2070. Notice the changes involve doubling: it would require raising concentrations to 1,160 PPM to raise temperatures another degree Celsius.
On the face of it, a 1 degree Celsius change hardly seems worth making a fuss about. However, climate is complicated, and increasing CO2 has complicated effects -- particularly in terms of increasing the average concentration of water vapor, with its potential positive and negative feedback effects. The current consensus is that, overall, water vapor provides positive feedback, and that theory says a doubling of CO2 concentration will produce a rise in temperature of 1.7 degrees Celsius.
Everyone also admits that the climate scenarios are complicated and the data is difficult to handle. Researchers are stuck with wading through the bog, and have turned to computer modeling to see if they can make things clear. Such models slice the atmosphere up into stacks of blocks or "cells", with each cell being associated with a set of parameters. In execution, the program running the model determines the inputs and outputs of each cells, covering the entire set of cells once per cycle. The models involve vast numbers of cells and require considerable computer power; in fact, given current computer hardware, it's out of the question to build a global model with cells approaching, say, a kilometer on a side, since the computing requirements would go through the roof and climb towards the Moon.
The models are good enough to accurately simulate features of the real-world climate system, such as monsoons, trade winds, and seasons. They also have the interesting feature that all of them predict global warming, and in fact they predict more warming than could be accounted for by simple theoretical considerations of the effects of CO2 and water vapor, the most extreme prediction being a drastic increase of 4.4 degrees Celsius. In these models, clouds end up enhancing warming -- though even some AGW advocates have their doubts over the actual effects of clouds. Observations haven't so far haven't been able to help resolve the issue.
Along with the behavior of clouds, the sensitivity of the models is dependent on concentration of aerosol particles; the eruption of Mount Pinatubo in the Philippines in 1991 dumped a layer of sunlight-diffusing sulfur particles into the stratosphere, leading to a temporary cooling that fits with the models as well. Again, however, there is an ambiguity involved -- some aerosol particles are reflective and provide a cooling effect, others such as soot absorb sunlight and provide a heating effect. The models do seem to track the response of climate to aerosol concentrations over the 20th century fairly well.
Data available on what seems to have happened in the prehistoric past also suggest the sensitivity of the models is realistic. During the Ice Ages, the expanded polar icecaps did increase the reflectivity of the Earth, contributing to cooling, but studies show they couldn't have kept the Earth cool on their own. The Earth also had relatively low CO2 concentrations, and factoring in high sensitivity gives a reasonable fit to the known global temperatures. Before the Ice Ages, the Earth had slightly higher CO2 concentrations than today but was a good deal warmer, again suggesting sensitivity.
The skeptics claim the data analyses ignore the "medieval warm period (MWP)", a time of relatively high temperatures in the North Atlantic region, with vineyards in England and Viking settlements in Greenland. The whole business is, like all the other elements of the climate quarrel, bitterly argued, with critics saying the MWP demonstrates that natural variation dominates the climate system, rendering human CO2 emissions irrelevant -- while advocates claim that a global analysis shows it was a localized episode at most, with no evidence of a warmer planet overall at the time. Besides, why make a fuss about vineyards in England? In 1977, before the warming trend became apparent, over 120 vineyards were in operation in the UK. There were only a handful in the 19th century, but it seems that was more due to the fact that the British of the era simply didn't like wine than due to climate.
* Anyone interested in simply knowing the facts about the climate debate has good reason to feel confused. On one side, the advocates insist that global warming is guaranteed to happen, on basis of data with clear uncertainties and models even they admit are inexact -- and which give a wide range of results, not all of them particularly alarming. The data and the models do seem to tilt towards the direction of warming, but proclaiming disastrous climate change an absolute certainty seems hard to justify on the basis of the argument offered.
To be sure, if we are confronted with a possibility of disaster, it does make sense to take precautions -- but we are always confronted with an open-ended list of potential threats, leaving us with the issue of determining which ones are real and deserve to be allocated scarce resources, instead of squandering those resources on ones that aren't real. Taking precautions implies an "opportunity cost" that has to be considered.
On the other side, the skeptics simply snipe at bits and pieces of the case for AGW and then proclaim there's nothing to worry about. Unfortunately, not all of their sniping seems particularly well-informed -- for example, a claim that the warming trend over the past few decades was due to increased solar heating fell over when analysis of solar observations showed the Sun's been slightly cooler than normal in that timeframe. Other models proposed by the critics claiming to demonstrate a natural fluctuation in global temperature have done no better so far.
The biggest problem with the skeptics is that they give the impression that they are simply looking for weaknesses in the AGW case to exploit. However, the efforts of the skeptics to play up the uncertainties make their own certainties hard to justify -- and they skeptics also have to deal with the awkward fact that the professional climate community has overwhelmingly accepted AGW scenarios to a greater or lesser extent.
* Although AGW remains politically controversial, the science community has generally accepted the idea. Some researchers have suggested that if global warming does become too immediate a threat, we might need to perform "geo-engineering" to cool the planet. The most exotic scheme proposed so far is the idea of placing a constellation of "sunshade" spacecraft at the Earth-Sun Lagrange point -- the location in space where the gravitational force of the Earth and Sun balance, where spacecraft can be kept on station with relatively little effort. Each spacecraft would be about a meter across, using solar-powered thrusters for positioning. The spacecraft would be shot into space using a magnetic launcher; the total mass of the constellation would be about 20 million tonnes.
A second approach takes a hint from nature. As mentioned above, volcanic eruptions can throw particulates into the upper atmosphere that cause a cooling effect; a massive program could be started to inject harmless aerosols into the upper atmosphere to achieve the same effect. Others have suggested the scheme might be used locally, for example to help preserve the polar icecaps.
A third idea involves spraying droplets of seawater into the air to generate low-lying, highly reflective oceanic clouds. This scheme could be implemented by a fleet of unmanned vessels that could generate the sprays using wind power, with each vessel handling 10 kilograms of seawater a second. About 100 vessels would be needed to cool off the Earth, though only 50 would be needed once the climate was stabilized. The fleet could be dispatched to the North Atlantic in the summer to protect the Greenland ice sheets, and transfer to Antarctica six months later. Cooling clouds could be used to lower sea temperatures in tropical areas and help prevent hurricanes from forming.
Other ideas have included seeding the ocean with nutrients, for example iron, to encourage the growth of photosynthetic plankton that would soak up carbon dioxide; encourage planting of fast-growing trees; setting up networks of CO2 scrubber stations; or to cover deserts with reflective sheets. Critics have been highly skeptical of geo-engineering schemes, questioning their practicality, and there are sensible worries that such "fixes" might well have unintended consequences. There are Greens who oppose the whole notion of geo-engineering on the principle that it encourages people to believe that there is a quick technological fix to the problem of climate change, discouraging efforts to take real action.
However, advocates can point out that if matters go from bad to worse too rapidly, the technological fix might be the only thing available to save the planet from disaster and so we should know what options are available. Some have suggested that global warming is an even greater threat than generally believed. When waters are depleted of oxygen, they tend to support "anaerobic" bacteria that don't require oxygen to live, with the anaerobic bacterial ecology heavily based on production and consumption of hydrogen sulfide (H2S), the "rotten egg" gas. This scenario matches that of the depths of the Black Sea.
The issue is that warmer waters absorb less oxygen, allowing the boundary of the anaerobic ecology to migrate upward. If it reaches the surface, hydrogen sulfide begins to be released into the atmosphere. H2S does not just smell bad: it's toxic. The theory is that some of the mass extinctions in the past history of the Earth were due to volcanic eruptions that led to global warming, depletion of oceanic oxygen, and then the mass infusion of H2S into the atmosphere. The idea was originally seen as speculative, but geological evidence is starting to flow in that suggests an Earth gradually poisoned over millions of years resulted in the grand "Permian extinction" of about 250 million years ago.
