v1.0.1 / chapter 8 of 14 / 01 oct 08 / 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 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.
By the mid-1960s, environmental regulations controlling the emission or dumping of toxic substances were being strongly implemented 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 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 engine 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 -- 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. This 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 the realization began to spread that human activities had an effect that promised to be much harder to deal with: global warming.
During the 19th century, when the Industrial Revolution was going full-bore, average global temperatures hardly changed. They began rising slightly after 1900, dropped a bit after 1950, and then started rising again after 1970. Overall, the average temperature from 1900 to 2000 rose about 0.6 degrees Celsius. That didn't seem to be much to worry about, since the Earth's climate is always shifting in one direction or another. However, from the 1990s it became increasingly apparent the temperature increase was part of a process that might lead to an uncontrolled "thermal runaway", with temperatures rising to ever more disastrous levels.
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.
Nobody took Arrhenius's idea too seriously. The general assumption was that a new ice age might be in store 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. That was shortly after the end of a two-decade cooling trend, so it wasn't a completely wild idea, but atmospheric CO2 concentrations were then continuing a steady increase. In fact, as shown by analysis of ice cores from the Greenland icecap, from the beginning of the industrial revolution to the 21st century atmospheric CO2 concentration has increased from 280 PPM to 380 PPM -- a level not seen for 500,000 years. At the current rate of growth, the CO2 concentration will be 800 PPM in 2100.
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 so the temperature began to climb again.
Climatologists became very worried about what might happen to the Earth if CO2 concentrations reached 800 PPM, and spoke out about their concerns. The end result has been a loud quarrel, with advocates of global-warming theories insist that the welfare of the planet is at stake, while the critics insist it's all a big con, a fraud being put over by hysterical "Greens" whose alarmist scenarios have no real basis in fact. The skeptics often point to the fears in the 1970s of a new ice age to suggest that global warming is just another scientific fad.
The skeptics have a point, since the weather is famously unpredictable -- the classic example of an unstable system, where a slight influence in one place can have enormous effects in another by pathways that very hard to nail down. The cycles of ice ages are obvious evidence that climate fluctuates wildly. Climatologists are perfectly aware of this and have traditionally tended to hedge their statements about climate change.
In addition, computer models designed to predict climate change are extremely tricky to write. There are actually about 30 different greenhouse gases. CO2 is the worst offender because it has the highest concentration, but methane is over 20 times more effective in its warming effect than CO2; there are certain HFCs that are over ten thousand times more effective a greenhouse gas and have been banned along with CFCs. The different greenhouse gases not only have different effects, they are introduced to and deleted from the atmosphere in different ways, complicating modeling. There are also concerns about how to factor other effects into the models. As the Earth warms, ice will melt, reducing the reflection of radiation back into space; the oceans will not be able to absorb as much CO2; and enhanced microbial activity in the soil will produce more CO2. Exactly how much will these factors affect the climate? A warmer world will mean more evaporation of water and more cloud cover; on the balance, will that cloud cover's tendency to reflect light back into space outweigh its tendency to trap heat?
Climatologists have been increasingly coming forward about global warming because, nitpicking over the details aside, the big picture seems somewhat frightening. Humanity is performing a massive uncontrolled experiment on the Earth's environment, and that idea would give any sensible person cause to stop and think for a moment. However, governments always have a long list of problems to deal with, and for good reasons actual problems in the present tend to trump theoretical problems in the future. Governments were concerned enough to set up an "Intergovernmental Panel on Climate Change (IPCC)" to provide recommendations. The IPCC deliberated on the matter and released estimates of global warming; the latest, issued in 2001, suggests that global temperatures might increase from 1.4 to 5.8 degrees Celsius by the year 2100.
The wide range of the estimates didn't inspire confidence, though it certainly suggested the trend, and the panel was challenged by the critics. Some claimed the evidence didn't support the idea of global warming; others admitted that it did, but believed that trying to head off global warming would be more trouble than it was worth.
One problem was that at the time of the 2001 IPCC report, the data was contradictory. Earth-based data seemed to show the temperature was rising, while satellite-based data didn't. However, over the past few years the satellite data has been shown to be in error, and now the temperature trendline is clearly up and to the right. Arctic sea ice and glaciers are clearly melting rapidly, and an increase in violent hurricanes and other storms increasingly seems linked to the rise in temperature. Towns built on permafrost are sinking into quagmires, the UK suffered a heat wave in the summer of 2006, and Muscovites found themselves walking around in shirtsleeves in December of that year.
The patterns are admittedly inconsistent, but a general consensus is emerging that global warming is for real. Efforts by contrarians to discredit various aspects of global warming -- for example, that the Earth's recent heating is due to increased solar activity, not human actions -- have been dismissed by careful examination of the evidence, and the increasing desperation of the arguments of the contrarians actually tends to enhance the credibility of global warming.
The issue complicated by the fact that global warming may benefit some countries -- a hotter world will make things better for Russia, where a large part of the territory is snowbound and more or less useless a good part of the year. However, few doubt that the tropics, particularly Africa and South Asia, will suffer badly. Governments are now considering what policies to take to reduce greenhouse gas emissions. Such political considerations are beyond the scope of a chemistry document.
* Some researchers have suggested that if global warming becomes 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; 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.
On the other side of the coin, 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 is speculative but unsettling.
