Notes on stratospheric ozone depletion follow. The saga of ozone depletion is a classic example of unanticipated consequences from human emissions of what were thought to be inert, ecologically harmless gases. Notes are organized according to the outline given below. You can click on a topic in the outline to jump to that area. When you finish with a section, you can either scroll on down through the following notes, or can return to the outline to jump to a new topic. The "Contents" box at the bottom of the text will return you to the master table of contents for this BI 301 home page. Navigate will give you reminders on how to move about within and among these documents. Click on "study guide" to use the study guide covering stratospheric ozone depletion, and for additional references on this topic.
To review the 2006 quadrennial assessment
of the state of the ozone layer, an international report prepared
by about 250 scientists, click United
Nations Environmental Programme, Ozone Secretariat . The British Antarctic
Survey maintains a site that gives up-to-date information
not only on what is happening over Antarctica, but also over the
Arctic and globally. In addition, this NOAA
website contains clearly writren answers to 20 questions about
stratospheric ozone and its depletion. You can see animations of the ozone "hole" over Antarctica here.
I. Background on ozone in the stratosphere
II. History of realization that stratospheric ozone may be in trouble
III. The Antarctic "hole"
A. Natural explanations?
B. Why Antarctica, why spring, and why such rapid depletion?
IV. Global losses
V. Policy steps
VI. Repair of stratospheric ozone?
VII. Approaches to control losses
VIII. Biological implications of ozone loss
We have talked about the hazards of excess ozone in the troposphere. The troposphere is the layer of the atmosphere that is closest to Earth, running from 0-12 km above the earth's surface.
What we will talk about now is hazards of too little ozone in the stratosphere the region of the atmosphere that extends from about 12 - 50 km above Earth.
Ozone forms naturally in the stratosphere
via dissociation of molecular oxygen (O2) by ultraviolet radiation,
and subsequent combination of atomic oxygen (O) with molecular
oxygen. Ultraviolet radiation is short wave, high intensity radiation,
indicated on the reaction below as hv. (I can't seem to
make this hv show as being above the line, indicating that
the reaction takes place in the presence of hv, but that's where
it should be.)
O2---(hv)-------> O+ O
O + O2 -------->O3
The ultraviolet radiation that dissociates molecular oxygen is referred to as uv-c radiation. It is very short wavelength (<242 nm [recall a nanometer [nm] is one billioneth of a meter]) and very high intensity radiation. This is not the same as uv-b radiation, which is biologically damaging wavelengths, and which we'll say more about in a moment.
IT IS IMPORTANT TO HAVE O3 IN THE STRATOSPHERE --
It is earth's primary shield against damaging uv-b radiation. This radiation is also short wave (280 - 320 nm) and high intensity. Ozone absorbs radiation in the 220 - 320 nm band and so keeps most of it from penetrating to earth. Why do we want to be shielded from uv-b radiation?
Well, uv-b radiation is biologically active radiation. It is responsible for causing damage such as:
damage to immune systems
injury to plants
injury to marine organisms
Ozone concentrations peak in lower stratosphere (25 - 35 km above Earth), where it serves this protective function. Peak concentrations are generally <10 ppmv. Above this height, the density of gases is so low that O rarely finds O2 to collide with, so there is not much O3 formation, and below this height, too little high intensity solar radiation (uv-c) penetrates to dissociate much molecular oxygen.
Ozone is also destroyed naturally in the stratosphere, being dissociated by ultraviolet radiation:
O3 ----(hv)----> O + O2
This is the reaction that is responsible for O3's absorbtion of uv radiation that would otherwise reach earth.
The O atom then typically joins with an O2 molecule to reform O3. The O3 goes on to dissociate and reform many times, usually until it collides with a free O atom, forming two relatively stable O2 molecules.
O3 + O --------> 202 (conversion to relatively stable O2)
That is, O3 formation and destruction are normally in a steady state in the stratosphere, such that the rate of its formation is equal to the rate of its removal. (Ozone is also destroyed by odd nitrogen and hydrogen radicals in catalytic cycles which won't go into, but which have generally been in balance with rates of formation.)
So, what is the concern all about? It is actually a fascinating story.
Concern about human influences on stratospheric ozone were first raised in 1971, with concern that SST (supersonic transport) aircraft emissions of nitrous oxides (N2O) and water vapor in the stratosphere could adversely affect ozone levels. N2O is generally nonreactive, but in the stratosphere it produces NO+ NO2 which convert O3 to O2 in a three step reaction and aren't consumed in the reaction that is, it acts catalytically.) These concerns vanished, however, when the SST fleet didn't materialize.
The current concern began as a pencil exercise, aimed at solving what seemed like a minor academic mystery. The time was 1974, and two chemists at the University of CA, Irvine (F. Sherwood Rowland and Mario J. Molina) were trying to answer a question about a class of industrial chemicals known as CFC's.
CFC's (chlorofluorocarbons) were inert gases that had been around since the late 1920's and had then (1974) been in wide use for more than a decade . These compounds were increasingly important in refrigeration (freon), as aerosol propellants (they were nonflammable and wouldn't react with the active ingredients), in making plastic foams (they make nice, uniform bubbles), as cleaners for electronic parts (e.g., microcomputer chips), and as coolants in automobile air conditioners.
They were very stable -- didn't react with anything, essentially -- and were not toxic, hence they were regarded as near-miracle chemicals. (You might remember people thought the same thing about DDT at first?)
As their name suggests, they are molecules consisting of chlorine, fluorine, and carbon. For example, CFC -11 (freon) is CCl3F and CFC-12 is CCL2F2.
Puzzling was the fact that traces of the chemicals had been turning up far from the world's industrial centers. The potential hazard associated with CFC's was discovered by a chance event. In the late 1960's, a British chemist (Lovelock) was interested in tracing the motions of air masses. He was using CFC's to do so, as they were ideal for tracing air motions, being chemically stable and not naturally-occurring (they are only man-made) so their presence in an air mass could not be confused with CFC's coming from natural sources. CFC's culd be released from a point, and the motion of the air mass could be tracked by sampling air at distant points for their presence.. What was surprising was that traces of CFC-11 were found over the ocean deep in the Southern Hemisphere, incredibly far from any sources. It seemed that they just persisted and spread out in the atmosphere.
Rowland heard of Lovelock's finding of CFC's at remote sites while attending a scientific meeting in Florida, and Rowland teamed up with Molina to try to calculate where the molecules would end up. If they were stable enough to travel thousands of miles, where would they stop? They weren't worried about it at this time; simply interested as chemists.
Rowland and Molina began to run a series of calculations -- based on known chemical reactions, the molecular stability of CFC's, atmospheric composition and so forth. These calculations suggested that the "miracle compounds" were "unleashing a demon" (in Rowland's words) high above Earth. They concluded that CFC's would never break down on Earth's surface or in the troposphere. Instead, they would rise slowly, over several years, into the stratosphere. There, intense uv radiation would sever their bonds, releasing highly reactive chlorine atoms, which would react quickly and repeatedly with ozone. (They calculated that there was enough of this high intensity radiation to break the CFC's bonds in the stratosphere but not in the troposphere.)
The released chlorine atoms would then react quickly and repeatedly with O3:
(1) CCl3F ----hv ------> CCl2F
(The other chlorine's would eventually be released similarly -- all carbon-bound Cl would eventually be released.)
This chlorine is highly reactive:
(2) Cl + O3 ---------> ClO + O2 (ClO is chlorine monoxide)
Basically, the chlorine steals ozone's third O atom, hastening the conversion of O3 into the relatively stable O2 molecule. (Incidentally, F is not effective in this process; it is released, but is not effective at scavenging O3.)
The chlorine, importantly, acts catalytically -- that is it is unchanged in the process:
The ClO (chlorine monoxide) radical produced in reaction #2, above, has an odd number of electrons and is quite reactive. When it meets a free O atom, the O in the ClO is highly attracted to the O atom, and breaks away to form a new O2 molecule, freeing the Cl.to attack another ozone molecule:
(3) ClO + O --------> Cl + O2
The net effect of the reactions above is:
O + O3 ---> O2 +O2 (That is, one ozone
is consumed directly, and the reactions take up an O atom, which
might otherwise have formed another O3 molecule.)
Each chlorine atom can destroy as many as 100,000 O3 molecules before it is inactivated or eventually returned to the troposphere, where precipitation and other processes remove it from the atmosphere, usually as HCl.
(Incidentally, the only long-lived natural source of chlorine in Earth's atmosphere is methyl chloride, which comes from the oceans and is present in the atmosphere only in low concentrations. HCl is not long-lived, nor is NaCl; both tend to be washed out of the atmosphere before they reach the stratosphere, for the most part.)
There is a whole family of chlorofluorocarbons, and a wide array of reactions that accomplish basically this thing:
Rowland and Molina had realized that CFC's had the potential to destroy O3 and its ability to shield Earth from damaging uv-b radiation. They were scared and unsure of their results they calculated and recalculated but ended up being quite sure that they were onto something very important. Rowland wrote, "The feeling was as if the bottom had dropped out. It was like looking into the abyss." (Rowland is a good one for excellent quotes. More recently, he was quoted as saying "My work is going well; unfortunately it means that the world is coming to an end." They realized that the continued release of CFC's would allow them to accumulate in the stratosphere to a level capable of seriously impairing the protective O3 layer.
Furthermore, they recognized that the destruction would continue well into the next century even if release of all CFC's was to cease immediately because the chemicals remain in the atmosphere for decades. For example, the atmospheric residence time of CFC 11 is about 75 years, and that of CFC 12 is about 100 years. (It is the CFC's themselves that persist; once Cl is freed from them, it is usually washed out of the stratosphere in a year or so, usually in a form such as HCl.)
Bromine (Br) is also effective at depleting stratospheric ozone -- in fact more effective atom for atom than is chlorine. Bromine is found in halons, which are used in fire extinguishers (CFBr3 = example of halon). Another long-lived source of bromine is methyl bromide (CH3Br), which is used as a soil sterilant in agriculture, and is emitted from marine phytoplankton and during biomass burning (as from forest or grass fires). Actually, much methyl bromide is destroyed in the troposphere, but a small percentage reaches the stratosphere (it lasts only about 2 years in the atmosphere, whereas halons have atmospheric lifetimes on the order of 20 - 65 years.
THIS IS A TRULY GLOBAL PROBLEM:
While CFC's are (or were) mainly produced and used in North America and Europe, they mix throughout the lower atmosphere. Thus, as many CFC molecules end up in the stratosphere over remote Southern Hemisphere sites as over North America, for example.
THE "AEROSOL BAN"
Rowland and Molina published their findings in the prestigious scientific journal, "Nature" in 1974, but it received scant notice by the scientific community. That fall (1974), however, the press picked it up, and the issue remained in the public eye until 1977 when the US announced its intention to ban CFC's as aerosol propellants beginning in 1978, and did so (at least, 90% of their aerosol use was banned). Canada, Norway, and Sweden issued similar bans, and a suite of other nations joined in the bans as well.
Before leaving Rowland and Molina, you should know that they shared the Nobel Prize in chemistry in 1995 for this discovery, also sharing it with Dr. Crutzen who had worked on N2O and its potential to deplete stratospheric ozone.
The US (and other nations) initially didn't ban all uses of CFC's, banning only most aerosol uses. (See figure "A" on the handout from class, illustrating global CFC production by category over time.) Why was the ban so restricted to this one use of CFC's?
(1) The stratospheric chemistry is very complex, and so scientists weren't sure how effective CFC's would be at destroying O3
(2) At the time, there was no actual evidence that losses of stratospheric ozone were occurring. Thus, all was based on theory and it wasn't dramatic enough to really attract and maintain public attention. Unlike concerns about some other kinds of environmental issues, the public couldn't be shown pictures of clear problems such as poisoned lakes, dead trees, and so forth.
(3) It was an intensely political issue the CFC industry was quite powerful and few members of Congress willing to tackle it on the basis of some scientists' theory.
(4) Furthermore, President Reagan was elected in 1980, and he and his EPA Administrator (Anne Burford) thought it was all a big scare tactic. The industry sensed that they weren't going to be regulated by them, and hence stopped conducting research into alternatives (which they had begun doing). DuPont and other CFC manufacturers claimed that the CFC market was "mature" and that production wouldn't increase further (which turned out to be far from the truth).
Under these circumstances, I actually find it impressive that even a use-specific ban was put into place! The tragedy is that the ban wasn't more complete at the time, as we'll see shortly.
In 1985, something happened that fanned the fire of concern about CFC's and their impacts on stratospheric O3. The British Antarctic Survey announced the discovery of a massive "hole" in the ozone over Antarctica. The British had been monitoring stratospheric O3 over Antarctica for decades, used ground-based detectors. They had first noticed a puzzling seasonal drop in O3 levels each spring since 1977. The team reported that each October (Antarctic spring), O3 levels dropped by as much as 40% below 1960's baseline levels before recovering by November.
However, the British hadn't reported this phenomenon, because they knew that the US had a sophisticated observational satellite monitoring global O3 levels, and the US hadn't reported it. Apparently, the British figured that if the drop were real, the sophisticated satellite equipment would have picked it up.
As it turns out, the NASA satellite had picked up the O3 drops, but NASA hadn't reported it. I've heard 3 conflicting reasons for their failure to report these events: (1). The data were read by a computer that was programmed to reject such large fluctuations as "improbable." (2) By another report, the NASA scientists did look at the raw data, but disregarded them because the British hadn't reported it. (3) The most probable explanation: scientists had fallen behind in processing the voluminous satellite data, and simply hadn't noticed the seasonal plunges in O3 levels.
The sad part, in retrospect, is that we could have bought almost 10 years on the destruction of stratospheric O3 if someone had announced the declines when they were first noticed.
This announcement shocked the scientific community and the public. Satellite data showed that the hole had been enlarging in terms of the percentage of O3 depletion each year since 1977. Not only was the hole "deepening," but the area of the hole was widening too. (It is now bigger than the continent.) For example, in 1987 and 1989, there was 50% depletion below levels in 1979, which were already about 20% below the 1960's baseline level! In the lower stratosphere (15-20 km), some years had 95% depletion below 1960's baseline levels.
Basically except for 1988, there has been extreme depletion every year since 1987. The area of the hole widened to the point where it is larger than the Antarctic continent (and larger than the North American continent, for scale) (and it is constrained to that size by something that we'll talk about later.) Minimum O3 levels fell to as low as 92 Dobson units (compared to minima of 220, and norms of about 280 - 300 Dobsons before CFC's were big), with losses generally worst in the lower stratosphere. In some years, losses of ozone were actually total i.e., no O3 at some elevations in the stratosphere. The hole tended to worsen progressively in terms of how long it lasts into the Antarctic spring. The spatial extent, degree of ozone depletion, and duration of the hole vary year-to-year, depending in part on how cold the winter and spring are; to give you an idea about the variability, in Sept 2011, the hole was one of the deepest in terms of depletion and area, while in Sept 2012, it was the 2nd smallest in terms of depletion and area. The hole in 2013 was a bit smaller than the average for recent decades.
Many questions were raised by the discovery of this "hole:"
1. Was there a natural explanation?
2. What could account for the hole being so big?
3. Why over Antarctica?
4. Why in the spring?
5. What, if anything, did it imply about what was happening to O3 elsewhere over the globe?
Several possible natural explanations were raised, including the possibility that volcanic Cl was the culprit. These explanations are discussed in some articles on the study guide for this section of the course -- if you are interested, read those for more information on why natural causes are now discounted, as I treat them only briefly here.
One of the major claims was that O3 destruction was being caused by Cl released by volcanic activity. The problems with that claim, in a nutshell, are:
(1) There was significant O3 loss in the 1980's, but no major volcanic activity then.
(2) There has been major volcanic activity since O3 monitoring began in the 1950's, but it was not necessarily associated with declines in O3. That is, O3 losses and volcanic activity appear to be uncoupled in time (lack temporal consistency)
(3) Measures of hydrogen chloride in the stratosphere after the relatively recent eruptions of Mt. Pinatubo and El Chicon showed less than a 10% increase in stratospheric HCl following those eruptions, while stratospheric HCl has increased steadily across recent years. Furthermore, it is estimated that < 1% of the Cl released by the eruption of Mt. Pinatubo Cl made it to the stratosphere, judged by the increase in HCl in the stratosphere following the eruption and the estimated release of Cl by the volcano.
(4) Stratospheric hydrogen fluoride has also increased steadily in parallel with HCl, as would be consistent with CFC sources.
(5) Much of the HCl produced by volcanoes (or Cl from sea salt) is injected into the troposphere and very little of that makes it to the stratosphere, as it is washed out first. Volcanic emissions include abundant water vapor, and HCl and NaCl are quite soluble in water, while CFC's are not.
(6) Most of the HCl that does make it to
the stratosphere is rapidly washed out -- that is the major removal
mechanism for Cl from the stratosphere.
(7) After volcanic eruptions, scientists find enriched sulfate in ice caps, suggesting that the eruptions inject sulfate into the stratosphere, where it gets widely distributed before being washed out. However, ice caps are not enriched with Cl following volcanic eruptions, suggesting that most Cl doesn't make it to the stratosphere where it could get dispersed as sulfate does.
Basically, natural explanations for Cl in the stratosphere and losses of O3 there have been discounted. The remaining questions then were why is it so big, why in Antarctica, why in the spring, and what did it imply about losses of stratospheric O3 elsewhere in the globe?
Basically, no one had expected that depletion would occur as rapidly as it was apparently occurring: levels of depletion were much greater than any of the models of stratospheric chemistry had predicted. The resolution of this quandary provides answers to the questions posed above.
Models had predicted less depletion than
was occurring in part because there are normally interference
reactions in the stratosphere that should have impeded the ability
of Cl to attack O3. Without going into too much detail, these
reactions involve methane and nitrogen dioxide, which should temporarily
tie up the Cl released from CFC's (or the ClO produced along the
way; see reactions 2 and 3, above). These
reactions should provide relatively stable reservoirs for the
Cl, temporarily "tying it up" (like the stratosphere's
immune system). These reactions should put the Cl into molecules
which are either relatively less responsive to sunlight, or into
molecules, such as HCl, that are more readily washed out of the
stratosphere. Examples of these reservoir reactions include:
Cl + CH4 --->HCl + CH3 (reservoir reaction involving methane [CH4])
ClO + NO2 --->ClONO2 (chlorine nitrate)
Both HCl and ClONO2 are less reactive forms of chlorine, which are less responsive to sunlight and less likely to yield free Cl atoms.
Largely because of these known interference reactions, models of the stratosphere had suggested that CFC's should, up to that time, have had only minimal effects on O3 something was clearly wrong with the models!
Thus, taking the circumstances under which marked O3 depletion was occurring into account, investigators deduced that something about the conditions of the Anatarctic spring must minimize the operation of these interference cycles.
WHAT IS IT ABOUT CONDITIONS OF THE ANARCTIC SPRING (especially the early spring) THAT MINIMIZES OPERATION OF THESE INTERFEENCE REACTIONS?
Well, during the year, O3 formed over the tropics is delivered, along with CFC's and Cl, to the stratosphere over Antarctica and the Arctic. That is, the stratosphere is mixed, globally. However, in winter, the Antarctic stratosphere is isolated from the rest of the stratosphere by winds swirling around Antarctica, creating what is referred to as the "polar vortex." Because of its extreme isolation, temperatures inside the vortex drop to very low levels (minus 90 degrees C, or minus 160 degrees F!). Strange things can happen in that isolation and extreme cold.
Winter temperatures within the vortex are cold enough for the scant water vapor present in that dry air to freeze (must be -80C [-112 F] for freezing to occur). It does so, forming "polar stratospheric clouds" or "PSC's". (These are more common over Antarctica than over the Arctic, as the stratosphere over the Arctic isn't as isolated and hence not as cold during winter.) Perhaps the PSC's have something to do with minimizing the interference reactions?
Indeed they do, and in more than one way!
First, water vapor isn't the only thing that freezes at those low temperatures. Nitrogen compounds, such as NO2, that otherwise could tie up Cl and ClO can condense and freeze in winter. When they are frozen, they are basically nonreactive; they are highly reactive only when they are in gaseous form. Thus, during winter, the Antarctic stratosphere is basically denitrified; essentially no NOx compounds are present in gaseous form. Thus, they are not available to react with and tie up chlorine!
(Technically, the various NOx's get converted to HNO3 and freeze (as HNO3.3H2O -- ie with 3 molecules of water.)
Second, the cloud particles act as surfaces on which reactions that free chlorine from its reservoirs can proceed more rapidly than in a purely gaseous environment. Hence, all during winter, Cl is being freed rapidly from various reservoirs (even though in darkness).
For example, the ClONO2 and HCl react together on these surfaces, in the dark, as follows:
HCl + ClONO2 --->Cl2 + HNO3
The chlorine compounds so produced then
escape into the gas phase, while the HNO3 freezes in the clouds
and can't take it up!
SO, that's why the cold matters the cold sets the stage for what happens in the spring. With the first return of sunlight in spring, the Cl2 (or other freed forms of Cl, such as HOCl) photodissociate (photolyze) into atomic Cl. However, even though some sun has returned, the vortex is still intact and temperatures in the very early spring are still incredibly cold. Hence, the NOx compounds are still frozen and can't act to form reservoirs such as ClONO2, and the Cl is free to react rapidly and repeatedly with O3. (The chemistry gets a bit more complex, involving what happens with ClO; see me if you want more information.)
Thus, winter sets the stage for unimpeded destruction of O3, which takes place for 5 - 6 weeks after the return of sunlight, during which time nitrogen compounds are still condensed or frozen and surfaces, provided by PSC's are still present for the reactions to occur on. This operates either until all the O3 is gone, or until the PSC's evaporate, restoring NO2 (from HNO3), which then can combine with ClO to reform the ClONO2 reservoir. What is critical is the overlap between cold and sunlight. It has been said that these conditions are like "hitting the fast forward button on O3 depletion."
To summarize conditions of the Antarctic winter and spring that are conducive to O3 destruction:
1) NO2 and CH4 are frozen
2) Thus they can't tie up Cl
3) Reactions that free Cl from relatively stable reservoirs (HCl or ClONO2) take place faster on surfaces, as provided by PSC's, than they do in a gaseous environment
4) With the first sun, atomic Cl is freed (Cl2 and HOCl photodissociate)
5) But, NO2 is still frozen
6) Thus, Cl attacks O3 unimpeded as long as sunlight and frozen conditions co-occur.
There may also be some positive feedback going on:
1. The decrease in O3 in the hole means that less solar radiation is absorbed in the stratosphere. This leads to longer perpetuation of the lower temperatures, and of the vortex itself, and
2. Increasing concentrations of CFC's in the atmosphere lead to increases in Earth surface temperatures (CFC's are also "greenhouse gases"), and to decreases in stratospheric temperatures, so they also may exacerbate the situation.
Results from a variety of studies are fully consistent with this understanding of what is happening to O3 in the Antarctic stratosphere, and CFC's (and some Br-containing compounds) are clearly implicated as the villains. For example, results from aircraft sampling of stratospheric air (using satellite O3 readings to direct balloons into the center of the hole) implicate CFC's. In spring, levels of ClO are elevated in the hole compared to elsewhere, while O3 levels are way down. Levels of nitrogen oxides are also depressed there as compared to elsewhere.
Furthermore, concentrations of Cl in the
stratosphere increased until quite recently. In fact, concentrations of CFC's themselves in the stratosphere
were increasing at about 3 - 5% per year through the 1970's, 1980's
and even the first part of the 1990's.
Bromine, of course, takes place in similar reactions. In fact, Br atoms are about 50x > efficient than Cl at attacking O3 in the chlorine-rich stratosphere, such that, even though present at lower concentrations, Br may be responsible for about 20% of Antarctic ozone depletion, with 5-10% of the total depletion due to methyl bromide alone.
We now know that global ozone is being affected, rather than just that over the Antarctic. First we'll talk about losses over the Arctic, and then about losses elsewhere.
An obvious question is whether something similar takes place over the Arctic. Indeed, the Arctic is experiencing depletions, but not as great as those over Antarctica. So far (1999) losses over the Arctic have been more in the 5-10% range, compared with 50 - 66% losses over Antarctica (and total losses there at some elevations). (However, during some very cold winters in the 1990's, losses over the Arctic were as high as 30% for total column ozone.) Why aren't losses as severe over the Arctic as over Antarctica?
(1) The polar vortex over the Arctic during winter is not as tight as that over Antarctic. Antarctica is completely surrounded by water, so land masses don't interfere with the vortex - this is not true for the Arctic. Thus, there are successive small losses over the Arctic, rather than cumulative losses.
(2) The Arctic stratosphere warms faster in the spring than that over Antarctica. This means that there is a briefer time for that critical overlap between cold and sunlight (i.e., ClO levels don't stay high for as long because PSC's disappear sooner, freeing NOx's faster).
(3) The Arctic stratosphere doesn't denitrify as completely as the Antarctica stratosphere does . (Because it doesn't stay cold for as long over the Arctic, there is less sedimenting of PSC's, which include frozen NOx compounds, out of the stratosphere, thus it is less denitrified.)
It has been suggested, however, that the accumulation of greenhouse gases in the atmosphere may trigger a springtime O3 hole over the Arctic much like that over Antarctica. Why might this happen? Well, the greenhouse gases warm the lower atmosphere, largely by trapping and re-radiating heat coming from the earth. They also, however, simultaneously tend to cool the stratosphere, trapping heat near Earth's surface and changing atmospheric heat transport. It is projected that greenhouse gases may chill stratospheric temperatures at the poles by as much as 8 - 10 degrees C. This cooling of the stratosphere at the poles isn't likely to have much effect on depletion over Antarctica, as losses are already so severe there. However over the Arctic, such cooling might exacerbate losses, for reasons that you can deduce by reading the list, above, of reasons that losses over the Arctic have not yet been as severe as over Antarctica.
LOSSES IN NON-POLAR REGIONS OF THE GLOBE
First, how do we know what is happening to stratospheric O3 globally? Well, there are "long term" O3 records from two sources:
Ground-based instruments that have been monitoring it since 1957-1959 (about 100 stations)
Satellite-based instruments since the 1970's.
These data indicate that:
(1) There is essentially no loss of stratospheric O3 near the equator (latitudes from about 25 degrees S to 25 degress N).
(2) At 35 - 60 degrees N latitude (where we live) total column ozone levels, averaged across the year, between the mid- to late- 1990's and the present are about 3 - 6% lower than the average values from pre-1980. Losses in our region are most severe in winter and spring, extending into April and May at our latitude in Corvallis. (So be extra careful when you are out trying to catch those welcome sun rays in April!) Some years see more losses than others, depending on climate and other factors. At corresponding latitudes in the southern hemisphere, losses tend to be a bit higher than they do here (averaging more consistently close to 6% compared to pre-1980 levels.)
(3) At latitudes above 60 degrees N, losses tend to be in the 6 - 14% range, compared to pre-1980, and are worse in some years -- in the winter of 1995, for example, losses over Siberia averaged 35%.
(4) Global total column ozone levels, averaged over the year, tend to be in the range of 6 - 13% below pre-1980 average levels.
WHAT MECHANISMS ACCOUNT FOR THIS GLOBAL DEPLETION?
First, when the Antarctic and Arctic vortices break up, O3-poor air diffuses out through the stratosphere. Monitoring stations in Australia and New Zealand, record sharp drops in stratospheric O3 at that time of year.
However, this diffusion is not sufficient
to explain global drops of the magnitude that have been observed.
Something else must be going on, and this something turns out, at least in part, to be
a process that early models of stratospheric chemistry didn't
take into account. (Hence their underestimation of the rate at
which global O3 would be lost.)
The non-polar stratosphere is not cold enough (in general) to support the mechanisms of loss that I described above for the Antarctic and Arctic. However, droplets of sulfuric acid and water seem to be able to support some of same reactions that take place on PSC surfaces. That is, these droplets both speed the release of compounds such as Cl2 that then release Cl in sunlight and tie up NOx compounds and CH4 that would normally take part in interference reactions.
Where does the sulfuric acid come from? Both from anthropogenic sources, such as coal burning, and from volcanoes. Global O3 depletion was particularly acute in 1993, a year when the stratosphere contained an abundance of sulfur from the eruption of Mt. Pinatubo. (Here is a good example of mixed roles for pollutants in the atmosphere; while sulfate speeds losses of stratospheric O3, it also tends to slow global warming!)
Fortunately, the international community has moved relatively swiftly, and apparently effectively, to slow the destruction of stratospheric O3. This problem is much more amenable to solution than is the problem of arresting global climate change. The problem of O3 depletion can be addressed largely by regulating a couple of industries (primarily the CFC and halon industries), while regulation of greenhouse gases requires the regulation of a huge array of industries. Both CFC's and halons are less vital to core functions of society than are the processes that result in emissions of many of the greenhouse gases, such as CO2.
However, in regulation of O3 depleters, "he who hesitates is lost." The trick is that to simply hold levels constant -- with whatever resultant damage -- emissions have to be cut very deeply, because CFC's persist for so long in the atmosphere. It takes 6-8 years for CFC's to reach the stratosphere, and, once there, they persist for 75 - 100 years! This means that presently we are seeing effects from CFC's that were emitted in the 1980's and earlier, a time during which emissions were increasing rapidly.
What actions have been taken to reduce this threat? What follows is a roughly chronological account of various steps that have been taken.
The situation regarding stratospheric O3 depletion has been very different than that regarding global climate change. In the case of stratospheric O3 losses, the US has been a leader in encouraging strong international treaties, while it has been a foot-dragger in climate change negotiations. In addition, action has been slow regarding climate change, while policy steps were taken with remarkable speed in the case of stratospheric O3 depletion. (It is encouraging to know that major policy steps can happen fast on occasion!)
THE MONTREAL PROTOCOL
In 1987, 40 industrial nations signed the Montreal Protocol. They agreed to freeze consumption of CFC's at 1986 levels by 1990, and to decrease consumption 50% by 2000. This was before global depletion had been announced, but the Antarctic "hole" had been discovered by this time.
The Protocol included provisions for review if:
(a) it was decided that the problem wasn't as bad as had been thought
(b) OR if it was decided that the problem was worse than had been thought.
This treaty was the first international treaty regulating an air pollutant in the history of Earth!
THE LONDON AGREEMENT
In 1988, evidence suggesting that O3 depletion was occurring globally, rather than just over the poles, was published. That spring, 11 days after scientists reported that O3 was diminishing globally, rather than just over the poles, DuPont -- one of leading producers of CFC's -- announced its intention to voluntarily phase out their production. It also encouraged other manufacturers to do the same, and many did.
Then, in June 1990, the signers of the Montreal Protocol plus representatives from many other nations (for a total of 93 nations) met and signed London agreement The meeting was called because of the announcement of global depletion and also because scientists and policy-makers realized that the steps called for by the Montreal Protocol wouldn't slow the destruction (or allow repair) fast enough.
This time, they agreed to totally ban all major depleters (CFC's and halons) by 2000. (We'll talk below about the ins and outs of what is considered a "major depleter." )
In November of 1992, in light of mounting evidence that depletion was proceeding faster and more seriously than had been anticipated, 87 of the nations met again and signed yet another treaty. By this treaty, they agreed to a total phaseout of major depleters by Jan 1, 1996.
Many of the lesser developed nations had been reluctant to sign these treaties, arguing that they were not responsible for the problem in the first place, and that it was unfair that they be prohibited from using these chemicals (many of which had important uses) to foster their development. In recognition of this, the developing nations have been given a longer time frame for their phaseout. Production levels in these nations were to be frozen at the average of 1995 - 1997 levels by 2000, reduced by 50% by 2005, and by 85% by 2007, with a complete phaseout date set for 2010 rather than 1996. Several of the nations, however, voluntarily committed to eliminate use of these compounds earlier than that. To encourage developing nations to sign the treaty, the industrialized nations pledged to assist the developing nations with finding and purchasing alternatives and with buying recycling equipment. This encouragement is believed to be responsible for some of the more recalcitrant and globally very important nations, such as China and India, agreeing to sign the treaty.
These treaties have been effective. Global production of CFC's has diminished rapidly since the signing of the first treaty regulating their production. In fact, by 1995, production had decreased 75% below 1988 levels, and use of CFC's decreased, globally, by 96% between 1986 and 2005!! (Vital Signs 2007-2008).
The repair of the O3 layer will be accelerated by nearly 20 years as a result of moving the phaseout date for CFCs from 2000 to 1996. Even more dramatically, it is estimated that if everyone abides by the Copenhagen treaty, O3 will repair to pre-hole levels nearly 50 years sooner than if the original Montreal timetable and quantities had been adhered to.
(A black market in CFC's has emergeed, however, and has grown steadily since 2000 when the developing nations were to have stabilized their use of CFC's.)
The soil fumigant methyl bromide (MeBr) has been particularly problematic. Originally, it was to be phased out for use in developed nations (including the US) by 2001, with lesser developed nations allowed more time. The time for its elimination from use in the developed countries was then delayed to 2004. However, use didn't stop in 2004 -- the US along with 12 other developed nations requested and received a "critical use exception" for this compound, which will allow these nations to continue its use beyond the end of 2005. Given that 35 other nations have phased out use of this compound, it seems odd that the US and the 12 other nations who received this exception have been unable to do so....The story isn't all bad, though; production of methyl bromide in these 13 countries is capped at 25 - 30 % of 1991 baseline levels, so will be diminished. In some cases, a nation may be allowed to use more methyl bromide than it is allowed to produce, in which case they will use existing stockpiles to supplement the new production.
A decline in Cl concentrations in the troposphere became noticeable in about 1995; by 2005, the total concentration of ozone-depleters in the tropsphere had dropped 8 - 9% below the peak levels measured in 1992 - 1994 (Vital Signs '07-'08). This decline began to be detected in the stratosphere in the late 1990's. (While global production of CFC's, halons, carbon tetrachloride, and methyl chloroform has declined steeply, production of HCFC's has continued to climb.) As of 2007, concentrations of CFC's in the lower stratosphere appeared to have stabilized, which is a good sign.
These declines in concentrations of Cl and Br in the stratosphere suggest that O3 levels will be repaired to those of 1979, near levels when the hole was first noticed, by about 2050, with noticeable repair in place by 2010 - 2015 or so. (This assumes that nations continue to abide by the treaty, and that climate change doesn't influence the rate of destruction of stratospheric ozone, either increasing or decreasing it.) In fact, at latitudes between 50 degrees N and 50 degrees S, the average concentration of O3 in the stratosphere during 2002 - 2005 was about the same as that during 1998 - 2001 (e.g. concentrations were 3 - 4% below pre-1980 concentrations), indicating that depletion is no longer worsening.
Importantly, this stabilization of global ozone levels seems to have stablized surface uv-b exposures in areas outside of the poles too! While exposures are still higher than normal, their stabilization is an excellent sign -- and, in a few places, exposures have even decreased slighly (Vital Signs '07-'08).
The Antarctic hole, however, will repair more slowly, since conditions favoring depletion are so extreme there. Severe losses are predicted to continue to be severe there for at least another decade or two, and pre-1980 levels probably won't be observed until 2060 - 2075, or even later (Vital Signs '07-'08). The Arctic hole is expected to repair faster, at a rate more similar to that of global ozone.
Remaining CFC's (as in old refrigerators and automobile air conditioners) are to be captured when the refrigerator or automobile is disposed of (by law), and then the CFC's are either incinerated or used as intermediates in the manufacture of other products. (Incidentally, one of the single largest sources of CFC to the atmosphere in the US is believed to be old leaking automobile air conditioners!) As of 1995, it became illegal to discard an old refrigerator in the US without first recovering the CFC's from it, and as of 1993, shops that service automobile air conditioners were required to be equipped with "vampire machines" that suck out the CFC's and clean them for recycling. (Unfortunately, a car air conditioner that was manufactured for use with CFC's as coolant can't use the newer, more "ozone-friendly" compounds, but must be recharged with recycled CFC's.)
Despite crying that it would be impossible to find alternatives to CFC's and halons as fast as the treaties called for, industry has, with its usual inventiveness, moved more swiftly in the effort to find alternatives than they thought possible. In fact, as early as 1992, there were effective substitutes for most of the major O3 depleters. Some of the new compounds or processes cost more to manufacture or use, but in a surprising number of cases, the new processes are better and cheaper than old (e.g., the use of good old soap and water for cleaning in the microelectronics industry!). I am told that the local Hewlett Packard facility found that it saved money and got improved performance with substitutes, including soap and water and a compound made from terpenes from citrus rinds ("bioact ec-7"). Similarly, halons in fire extinguishers have been satisfactorily replaced.
By 1989, the food packaging industry had largely quit using the worst depleters, such as CFC 11 and 12, and had moved to lesser depleters such as HCFC-22. Hydrochlorofluorocarbons (HCFC's) basically substitute a hydrogen for a halogen (such as Cl or F), which makes the compounds less stable and more likely to break down in the troposphere, rather than reaching the stratosphere. Notice that I say "more likely." This is meant to remind you that these compounds are not totally "ozone friendly;" some do reach the stratosphere, but a much smaller proportion than in the case of CFC's. The complete phaseout date for HCFC's in developed nations is currently set at 2020, (except that their use in servicing of existing equipment will be allowed until that equipment is retired), and in developing nations they are to be phased out by 2030.
Hardest to find (among the chlorine-containing
compounds) have been good substitutes for coolants that are safe
(not explosive) and that can be used in existing equipment. Refrigerators
are now available that are cooled with HCFC's, and some helium
cooled refrigerators are also on the market (these refrigerators
also use less than 50% of the energy of a conventional refrigerator).
Many refrigerators are now cooled with HFC's (hydrofluorocarbons)
such as HFC3, which is O3-safe, but more flammable than CFC's.
(Note that HFC's are greenhouse gases less potent at trapping
heat than CFC's, but greenhouse
gases nonetheless.) Work is on-going to develop efficient and
safe refrigerators that are cooled with ammonia, or with hydrocarbons
such as propane and butane.
Automobile manufacturers phased in the use of an HFC (HFC - 134a , which is C2F4H2 with a carbon-carbon bond in the middle) as a coolant for automobile air conditioners beginning in 1993. This compound can't be used in old air conditioners, however, but requires a different system. In addition, work is on-going to eliminate the need for air conditioners in automobiles through means such as redesigned vents, window glazings and solar ventilation systems. (By the way, home air conditioners have historically not been cooled with CFC's, unlike automobile air conditioners.)
Of course, some folks always like to try to think of technological fixes that would allow life to go on as usual. For example, some wonder why we don't just ship some O3 from polluted places, such as Los Angeles, up into the stratosphere. There are many problems with this idea, not the least of which is that stratospheric O3 levels are higher by 1000 times or more than levels even in polluted parts of the troposphere, so that this would actually represent a dilution!
Of course, the reason for all of this concern is the increased exposure to uv-b radiation that we will experience (are experiencing) on Earth as a result of depletion of stratospheric ozone.
Have increases in uv-b been documented? It is difficult to measure uv-b well because it is influenced by many factors, including clouds, haze, aerosols, etc, and because calibration of the instruments is difficult and difficult to maintain. However, in November of 1993, the first good documentation that increases are occurring was published in the journal Science. Reported were measurements of uv-b from Toronto, Canada, where uv-b has been measured since 1989. These spectral measurements indicate that the intensity of light at wavelengths near 300 nm has increased there during winter and summer. The selectivity of the increase (only in wavelengths that stratospheric O3 absorbs) implicates O3 depletion as causal. In addition, in the late summer of 1999, scientists from New Zealand reported that uv-b penetration there now increases markedly during spring, the time when the O3 hole breaks up, diffusing O3-poor air more widely. Data from New Zealand in 1999 suggest that levels of uv-b radiation in summer were about 12% higher than they were during comparable periods in the early 1990's (by which ozone depletion was well-along). Importantly, levels of uv-a radiation in the same area (a wavelength that is not absorbed by ozone in th stratosphere) have not increased, strengthening the concept that the increases in uv-b are in fact related to decreases in stratospheric ozone. The Scientific Assessment of Ozone Depletion, 2012 document, compiled by the World Meteorological Organization and others, reported that the average values for uv-B radiation at the South Pole between 1991 - 2006 were 55 - 85% higher than were estimated values for 1963 - 1980.
The 2002 synthesis report on the status of stratospheric ozone (web link given at the very top of these notes on the topic) summarizes what we knew then about uv-b changes by saying that there has been an increase of about 6% in this radiation since the early 1980's at a range of mid- to high latitude sites over both the northern and southern hemispheres.
That is, uv-b exposure at Earth's surface is (or was until the early 2000's!!) increasing. Not wholly clear is whether the measured increases are enough to be biologically significant. While they sound like large percentages of increase the increases are not particularly large in the absolute sense, because the values for uv-b are small to begin with. (Keep in mind, however, that without vigorous policy actions to stop the O3 destruction, it would have continued to increase with consequent and undoubtedly significant - increases in uv-b exposure! Let's count our blessings, for once, eh?)
HUMAN HEALTH EFFECTS
Increases in uv-b exposure are linked to effects on humans. We know that uv-b can damage DNA, and it is implicated as a likely cause of melanoma skin cancers. (That is, of course, why we are encouraged to wear sunscreens and sunglasses that block uv-b!) To quote an advertisement from Seventeen magazine, "The environment is in trouble and the more it suffers, the tougher it is on your skin...."
In the summer of 1991, the US Environmental Protection Agency estimated that global O3 depletion would cause about 200,000- 300,000 additional skin cancer deaths in the US during the following 50 years. This was based on rates of depletion that were occurring at that time, and hence is now considered a worst case scenario, given that depletion is predicted to slow and reverse over the coming decades. Perhaps more appropriate is a UN estimate from 1992 that there would likely be a 26% increase in melanoma skin cancer incidence worldwide if 03 decreased by 10% overall, which now seems like it won't occur (that is, global depletion is unlikely to reach that level -- yeah!!)
People living on Tasmania (an island South of Victoria, Australia) experienced an 88% increase in the incidence of malignant melanomas and an almost doubling of melanoma mortality between 1978 and 1987. While caution must be used in interpreting the cause of this huge increase, the coincidence with O3 depletion at those southerly latitudes is striking. Skin cancer rates reportedly increased by 66% between 1994 and 2002 in Punta Arenas, Chile's southernmost city (Vital Signs 2002). There are also anecdotal reports of increases in melanomas from Argentina, another region of the globe affected by O3 drops when the Antarctic hole breaks up in the spring.
Cities in Australia and Chile issue O3 warnings on days and seasons when uv-b exposure is likely to be intense, and in Chile's southernmost city, Punta Arenas, many parents keep their children indoors between 10:00 a.m. and 3:00 p.m. and have moved soccer practices to later in the day to minimize uv-b exposure. Even our Corvallis paper publishes the uv-index each day on the page where other weather information is presented!
The lens of the eye is a strong absorber of uv-b radiation, and links between uv-b exposure and the development of cataracts have been established (through epidemiological studies and experiments with animals). Hence, increased frequency of cataracts is anticipated as a result of increased exposure to uv-b radiation. Problems with blind livestock have been reported from areas in Chile and other far-south latitude areas, perhaps owing to their increased exposure to uv-b radiation.
Finally, excess exposure to uv-b radiation is believed to cause diminished functioning of immune systems.
EFFECTS ON NATURAL AND AGRICULTURAL ECOSYSTEMS
As you might imagine, there is uncertainty about what to expect in terms of effects on natural and agricultural ecosystems. We need to consider not only how much ozone is lost but where on the globe losses will be greatest and at what times of year. For example, we know that losses are likely to be greater at higher latitudes than near the equator, and also greatest in late winter and early spring. Recall that, at our latitude, we are seeing yearly average ozone levels that are 3 - 6% below pre-1980 levels. Taking the more conservative number (3%), this indicates an increase of about 6% in uv-b exposrue (averaged over the year), which is analagous to levels experienced 10 degrees to our South in latitude (e.g., like those in S CA).
One might think that increased "sunlight" would be good for plants. However, this isn't the case. Plants do not use uv-b wavelengths in photosynthesis, and the majority of plants that have been tested suffer ill effects when subjected to elevated uv-b exposure (exposures that we can anticipate for our area). Effects include interference with photosynthesis and damage to nucleic acids. Plant species vary in sensitivity, with legumes apparently particularly sensitive. Of the species tested, conifers appear to be least sensitive, followed by hardwood trees, and many herbaceous plants are quite sensitive. Australian officials believe that crops of wheat, sorghum, and peas -- early season crops that are grown when O3 levels are lowest -- have already shown effects in terms of diminished yields. While it may be possible for plant breeders to develop uv-b tolerant varieties and some plants in nature may eventually be able to adapt to increased exposure, this may not be possible if the changes happen too fast.
Keep in mind also that plants and other organisms are likely to faced not only with increased uv-b exposure, but also with increased CO2 and associated climate changes!
Effects are not limited to terrestrial systems. Organisms that spend much of their life near the surface of water bodies may be adversely affected. Uv-b doesn't penetrate as far into water as does visible light; at 30 m, uv-b is only 1% of surface levels, compared with 60% for visible radiation. Even so, recent information suggests injurious amounts of uv-b may penetrate more than 10 m in clear open ocean areas. In general however, strongest effects are expected on creatures who live at least part of their life near the surface of the water. (Many marine creatures that ultimately live at depths spend their early life nearer the surface, hence may be exposed at critical times in their lives.)
For example, excess uv-b inhibits photosynthesis in most phytoplankton that have been examined and thus could have an impact on the community productivity and composition of the phytoplankton. This, in turn, could have implications up the food chain, for organisms that depend on phytoplankton -- such as zooplanton, krill, squid, fish, whales etc. Data suggest that productivity of phytoplankton has decreased 6-12% since 1987 in monitored areas of the Antarctic Ocean. (While some species produce protective pigments in response to uv-b exposure, these provide only limited protection.)
You have probably heard about various amphibian declines occurring in many regions of the world? In areas ranging from Brazil to the Cascades of Oregon to Australia and Europe, populations of many amphibians are declining at alarming rates. Why?? Many hypotheses have been advanced and many experiments have been undertaken to address these declines. (See several papers on the supplementary reading list, part of the study guide, for more information on these.) Hypotheses center on the potential role of uv-b exposure, various kinds of pollutants including herbicides and other pesticides, climate changes, natural cycles in populations, and various diseases. As with so many things, it isn't likely that there is one answer that will apply to all cases. Unfortunately (as is also true with so many things!) few long-term data on amphibian populations exist, against which present trends could be evaluated. The International Union for the Conservation of Nature (IUCN) has set up a task force on declining amphibians, which has crated a world wide data base and network for information exchange (based in Corvallis!).
Dr. Andy Blaustein in the Zoology Department here at OSU is one of the leaders in investigations into amphibian declines. One of his study areas is Lost Lake in central Oregon, and one of his study organisms is the western toad, which lives there. His monitoring data showed that between 50 - 100% of toad eggs in the field died from unknown causes over each of several recent years, following about a decade of monitoring in which egg mortality was never higher than about 5% of eggs. However, during years of high egg mortality, adults were normal and eggs brought to the laboratory hatched and developed normally. This suggested that something in the field environment was affecting the eggs. Further studies and experiments, with collaborators, indicate that the Cascades amphibian species that are declining most have the lowest abilities to repair damaged DNA, and that these species also tend to lay their eggs in positions that are exposed to radiation, rather than under cover. Both of these relationships implicate uv-b exposure as at least a contributor to the declines.
Additional experiments by Blaustein and his colleagues have been conducted on long-toed salamanders in Cascade lakes of Central Oregon. They shielded some eggs from uv-b, while others were left unprotected. (Note that they didn't add extra uv-b; simply shielded or did not shield eggs from ambient levels.) They reported, in 1997, that 90% of the embryos that were not shielded either died or were deformed, but that almost none of the shielded embryos developed any problems. This demonstrates a correlation with uv-b exposure, which, of course doesn't necessarily mean that uv-b is necessarily causal. Further work on mechanisms of injury is underway. (For example, fungal pathogens are implicated in some of the die offs, and a trematode (a parasite borne by snails) may be responsible for some of the deformities. In addition, very recent work suggests that nitrogen levels in water at levels below those of concern for human health may be important in contributing to some of the declines -- this elevated nitrogen resulting, largely, from applications as fertilizer.)
Lower levels of ozone in the stratosphere do cause it to cool -- so the stratosphere over Antarctica is particularly affected by this cooling. By mechanisms that I don't claim to understand, that stratospheric cooling accelerated westerly winds that encircle Antarctica and shifts them poleward. This acceleration and shift in westerlies brings more warm air poleward from mid-latitudes in the Austral summer, potentially acclerating warming there. These stronger winds also "tug" the Antarctic circumpolar current, which is the strongest ocean current on Earth, towards the sourth, which pushes ocean heat from warmer areas towards Antarctica. This warmer water changes patterns of ocean oveturning, bringing up more deep warm carbon-rich water. This increases carbonate concentrations in the surface waters and causes more outgassing of CO2 and less uptake of that gas by those waters. [Science 1 Feb 2013 and 16 Nov 2012]. The Southern ocean stores vast quantities of carbon at depth, and is believed to account for ~ 40% of oceanic uptake of atmospheric CO2. So, I think you can see that these effects are not beneficial from the perspective of global climate change induced by increased concentrations of gases such as CO2!
To sum it up, there is potential for the increases in uv-b exposure, caused by depletion of stratospheric O3, to cause wide-ranging damage to a variety of creatures and ecosystems. The very bright side is that we should be dealing with the worst of it over the next 10 - 15 years or so -- or perhaps the worst is even past!-- after which stratospheric O3 should begin to recover - thanks to aggressive policy measures! Without those, depletion would have continued to increase with consequences that we probably don't even want to think about.
This page is maintained by Patricia Muir at Oregon State University. Page last updated VERY partially Feb 27, 2014.