1. Something In the Air

On the morning of December 5, 1952, the residents of London woke to find their city heavily blanketed by a toxic and stagnant mix of impenetrable fog and sooty black coal smoke. The source of what came to be known as "The Killer Fog" was the thick sulfurous output of local factories and over a million coal-burning stoves in the London basin. Stan Cribb, a funeral worker who lived through what has come to be recognized as one of deadliest anthropogenic environmental crises in recent history, recalled during a media interview that he attempted to drive to a morning wake in a line of mourners' cars but was "stunned by the blackness of the gathering fog." (Nielson 2002). As a result, London's roads were soon littered with abandoned cars. The event lasted for four days, until winds finally cleared the air.

It is estimated that sulfur dioxide levels in London during the month of December of 1952 reached 700 ppb, several times greater than normal. Particulate matter concentrations are estimated to have ranged between 3,000-14,000 μg/m^³, with the highest levels being 50 times greater than normal (De Angelo and Black 2008). While many health and political authorities did not fully appreciate the magnitude of the cataclysm, "undertakers and florists knew there was a problem. They ran out of caskets and flowers" (Davis et al. 2002: 374).

An investigation by the British Ministry of Health (1954) concluded that during the first three weeks of December the deadly fog caused an extra 3,000 mortalities in London beyond what would have been expected at that time of year, or about three times the norm. In addition, there were significant jumps in hospital admissions for respiratory conditions and applications for emergency services. A re-analysis of hospital, insurance and other records by Bell and Davis (2001:393) concluded that, "The true scope and scale of the health effects linked with London's lethal smog extended over a longer period than originally estimated… [with] unusually high mortality rates [of approximately 12,000] for a period of 2.5 months," affirming the deadly nature of the event.

Since the Killer Fog, changes in policies and practices facilitated a successful abatement of the specific pollutants that produced the disaster. In recent years, however, anthropogenic air pollution has re-emerged as a significant public health research issue, as a result of photochemical processes leading to high levels of ground level ozone (O₃) and nitrogen dioxide (NO₂). Exemplary of this work is Deborah Dreschslet-Parks (1995) experiment with 80 healthy adults between the ages of 56-85 years of age who she placed in a sealed environmental chamber and exposed them for two hours during exercise and resting periods to ozone and nitrogen dioxide, alone and in combination. This experiment was prompted by the knowledge that the world's ever growing number of motor vehicles and industrial sources of pollutants pump millions of tons of these toxic gases directly or indirectly into the atmosphere each year. Cardiac output was measured before and after chemical exposure. Dreschslet-Parks found that cardiac output increased by 1% upon exposure to O₃, was reduced 5% during exposure to NO₂, and was reduced 14% when the two gases were combined. Moreover, heart stroke volume, which is the amount of blood that is pumped from a heart ventricle with each beat, was reduced 2% during exposure to O₃, by 7% during exposure to NO₂, and by 12% when the two gases were combined. She concluded that the interaction of these two gaseous pollutants causes vasodilatation which adversely impacts the normal functioning of the human heart beyond the mere addition of the effects of each individual gas.

Dreschslet-Parks' findings are consistent with evidence of an association between long-term exposure to multiple air pollutants and deaths associated with cardiovascular and respiratory diseases. Research on diesel fuels, for example, shows rising rates of respiratory mortality resulting from exposure to: 1) sulfur dioxide which constrictions lung airways (Schwela 2000); 2) particulate matter small enough to pass deep in the lung and carry with it anthropogenic carcinogenic compounds from the atmosphere to this vulnerable body site (Pope 2000); and 3) oxides of nitrogen which aggravate common respiratory conditions and the aging of lung tissue (Janssen et al. 2003). In turn, combustion of diesel fuel is an important source of global warming, producing 25 to 400 times greater mass of particulate black carbon and organic matter than gasoline (Jacobson 2002). Global warming, in turn, contributes to higher rates of ozone and the toxicity of other air pollutions. As Noyes et al. (2009:971) conclude, "climate change coupled with air pollutant exposures may have potentially serious adverse consequences for human health in urban and polluted regions."

In light of current discussion of the possibility that we have entered a sixth period of mass species extinction, this brief paper examines the mounting threat to species survival of multiple, diverse, interacting ecocrises of our own making.

2. Pluralea Interactions

The terminology used to describe the combined effects of two or more pollutants or other hazards in the environment varies across disciplines. In epidemiology the term effect modification is used when the health impacts of two or more agents are found to be interdependent. In toxicology, interaction that multiplies effects beyond what would be expected as a result of the additive impact of two or more pollutants is called synergy. As Dreschslet-Parks' experiment, as well as the work of many other epidemiologists and laboratory scientists indicates (Mauderly and Samet 2009), interaction among pollutants is especially risky. I have suggested the term "pluralea interactions" (derived from the Latin words plur, meaning "many" and alea, meaning hazards) to refer to the growing number of health-related interactions that are occurring not just among anthropogenic contaminants but across the full panoply of environmental degradations in our increasingly human-dominated environment (Singer 2009).

Ecocrises interactions increasingly are putting humans and other species at risk for catastrophic outcomes (Rees 2003). Consequently, rather than framing the various environmental disasters we face as standalone threats, which is the conventional but narrow outlook that facilitates delayed and fragmented mitigation efforts as well as denialism, a focus on how the negative human impacts on the environment interact and enhance their effects raises questions about the extinction of species, populations, and ways of life.

Illustratively, one of the major difficulties facing scientists working to document the planetary effects of global warming involves establishing which of the many changes across physical and biological systems -- including the disappearance of glaciers, melting permafrost, warming of the oceans, drying up of lakes, coastal erosion, species movement to higher latitudes and altitudes, changes in migration patterns of bird migrations, shifts of plankton and fish from cold- to warm-adapted communities, and the spread of vectors of infectious disease -- are the direct consequence of anthropocentric greenhouse gas release and which have other causes (Parry et al. 2007; Rosenzweig et al. 2008) As Rosenzweig (2008), observes, "Separating the influence of human-caused temperature increases from natural climate variations or other confounding factors, such as land-use changes or pollution is a real challenge." As this statement indicates, the focus of such analyses is on single-cause understandings. Yet, as Spratt and Sutton (2008:xi) emphasize, global climate change comprises only the exposed "tip of [a far] broader globalsustainability iceberg" that includes numerous other environmental disruptions that are not only coterminous but are "converging rapidly in a manner not previously experienced."

3. Emptying of the Oceans

In addition to atmospheric changes, the ecocrises interactions that threaten planet species include significant transformations of the oceans, which, in turn, reverberate on the health of human and other populations. Most notable among the pluralea interactions occurring in marine environments is the entwined processes of overfishing and global warming. As summarized by Feldman (2008), because of overfishing and global warming, "we're facing a world 40 years hence of rising seas empty of fish and devoid of ice."

Notably, during the second half of the 20th century, the ocean fishing industry increased its catch 400% by doubling the number of boats and using more efficient fishing technology (McGoodwin 1990). A result has been a dramatic level of overfishing, involving harvesting of one species after another a t a faster pace than populations can reproduce. As a consequence, many fisheries have collapsed. An analysis (Worm et al. 2006) of almost 8,000 species of seafoods found that 29% of species are now 90% below averages of 50 years ago. Most heavily hit are large predatory fish (e.g., cod, tuna), (Myers and Worm 2003). Notes Myers (quoted in Melville 2003), "From giant blue marlin to mighty bluefin tuna, and from tropical groupers to Antarctic cod, industrial fishing has scoured the global ocean.… Since 1950, with the onset of industrialized fisheries, we have rapidly reduced the resource base to less than 10% – not just in some areas, not just for some stocks, but for entire communities of these large fish species from the tropics to the poles." A total collapse of commercial wild seafood is projected for 2050 if current patterns of industrial fish harvesting continue (Myers and Worm 2003).

Commercial fishing fleets from wealthy countries have benefited from government subsidies in the amount of about $30 billion annually in recent years. Such policies boost the number of working boats, their technology to find, catch and process fish, and, while fish stocks last, an increase in the global catch. Consequently, a vast flotilla of industrial trawlers from the European Union, China, Russia and elsewhere, search the world for dwindling stocks. One place they found them in recent years is in northwest Africa, off the coast of countries like Senegal. It is estimated that 82% of all the commercial fish species in this area are now either depleted, overfished, or at their maximum output (Food and Agricultural Organization 2000). While indigenously caught ocean fish traditionally were the main source of protein for many people in northwest African, some species are now so scarce that local fishermen cannot find them, threatening local protein intake.

As of 2000, the Food and Agricultural Organization (2000) estimated that about one billion people world-wide rely on fish as their primary source of animal protein, but this varies by region. Although fish provide only an estimated 6.5% of the animal protein consumed in North America, in Africa the figure is an estimated 21%. Populations in regions in which fish supply a significant percent of dietary protein are gravely threatened by the continuous decline in quality and quantity of fish being caught and consumed locally.

Analysis by Halpern et al. (2002:948) indicates that no area of the oceans "is unaffected by human influence and that a large fraction (41%) is strongly affected by multiple drivers." Beyond overfishing, global warming has become an potent force exacerbating the decline of fish stocks and marine ecosystems generally. Especially hard hit are cold and temperate water fish. In research on a reef fish community off the coast of North Caroline, for example, Parker and Dixon (1998) found that over a 15 year period characterized by mean monthly bottom warming of 1–6°C during winter months, there was a decline in the size of temperate species and a decrease in abundance by a factor of 22. Further north, the Atlantic cod has been subject to extensive overfishing, b ut, in addition, climate warming has limited the biogeographic range of this species, suggesting that even with strong controls on fishing the potential recovery of cod populations may be compromised. As Mieszkowska et al. (2009) observe, it is uncertain if rebuilding cod populations is possible in a warmer North Atlantic.

Similar patterns abide for other marine species, but direct effects of ocean heating on range is not the only factor involved. As the ocean heats, the solubility of gases deceases and there is the possibility, suggested by climate models with biogeochemical components, that one result of global warming will be a decline in the amount of oxygen dissolved in ocean waters, especially in what is known as the oxygen minimum zone (OMZ) between 200 to 1,000 meters below the surface. Already, there are indications in tropical areas that OMZs are expanding horizontally and vertically, while low oxygen areas at 200 meters, where some large micro-organism cannot reside, have been expanding as well (Stramma et al. 2008). In some Atlantic OMZs, oxygen levels have fallen to a point that could cause anoxic stress. One consequence is that species of tuna, already hard hit by overfishing, may be forced to surface layers of the ocean where they are made even more vulnerable to fishing fleet detection. There is also some evidence that increased ocean hypoxia could amplify the production of nitrous oxide, a potent greenhouse gas, creating a feedback loop that leads to further marine life depletion. Moreover, in summarizing the multiple interacting threats to the oceans, the United Nations Environment Programme (Nellemann et al. 2008:12) notes, "Climate change, with its potential effects on ocean thermohaline circulation and a potential future decline in natural ‗flushing and cleaning' mechanisms, shifts in the distributions of marine life, coral bleaching, acidification and stressed ecosystems will compound the impacts of other stressors like overharvest, bottom trawling, coastal pollution and introduced species" creating, in effect, a perfect pluralea storm.

4. Conclusion

The occurrence of multiple pluralea interactions in the atmosphere and in the oceans indicates the contemporary value of models and analytic approaches that focus on the assessment of connections and relationships rather than unitary causes, and, further, that seek to determine the pathways and mechanism through such pluralea interactions occur, and with what added effect on the life systems of the planet, including their potential role in extinctions.