SEE-U 2000: Biosphere 2 - Module 15

## Module 15: Pollution, Global Warming, and Ozone Hole

### Pollution

Defining what is a pollutant is a contentious issue between scientists. For the purposes of our class, we will define a pollutant as any stimulus that is increased due to human activity. Under this definition, there are many types of pollution (clearinghouse of websites discussing pollution).

The first type of pollution most people think of is air pollution. This is usually due to gas emissions from engines burning fossil fuels and the burning of forests, industrial farming, and ranching. Human activity thereby disrupts many of the normal chemical cycles through which the naturally occurring gasses released by these activities would normally cycle. As an example, cattle ranching greatly increases the number of cattle globally. Cattle feed on grass and their digestion of which releases large quantities of methane. Some of the additional methane produced by cattle rises to the atmosphere and could help to contribute to the Greenhouse Effect.

However, there are several other types of pollution including mechanical ground relocations like mine tailings and strip mining, and their concomitant dust disturbances that physically disrupt the natural processes. Sound pollution, such as that emitted by automobiles along highways (Module 13), often keeps animals from coming near roads. Light pollution, like home lights along beachfronts, attract marine hatchlings and lead to significant mortality, reducing population sizes (Module 4) and threatening extinction. Thermal pollution, given off by human activities like water coolant outflows and urban heat retention, can lead to dramatic changes in the local climate and thus the local biotic community. A more recently recognized type of pollution is organismal pollution, in the form of introduced exotic species (Module 14). Great effort is often put forth to extricate these species from the local community in the hope of “cleaning” it and returning it to its pre-introduction state.

### Biotic Responses

The local biome always responds to pollution. A famous example of how an individual species has responded to a specific kind of pollution is what is referred to as industrial melanism. Coal combustion fueled the early years of the industrial revolution in England during the late 1800s. The soot from coal burning covered the light colored bark of the trees in areas surrounding the factories. The Peppered Moth (Biston betularia) has two morphs in England and Germany. A white morph had small black flecks on its wings and was most common in areas with minimal pollution. A dark morph of the Peppered Moth predominated in other areas, where the black soot from the factories covered the light colored bark.

Entomologists (biologists who study insects) later demonstrated that the mechanism for this shift was natural selection (Module 3). Birds preferentially fed on those forms that stood out on the trees on which they rest. If white bark was most common, then the black morph was heavily preyed upon and vice versa (read more in depth about some lingering controversy surrounding this study).

In this way, pollution can change the genetic composition of the species. When this intraspecific change is combined with all the other effects of pollution that lead to habitat degradation (e.g., oil spills, destruction of land, displacement of species, etc.), pollution can affect not only intraspecific variation, but also the species composition of communities and biomes. The different types of pollution influence different levels of the ecological hierarchy: some impact only on a local level (sound, light, and thermal pollution) while others, like gas pollution, have global consequences.

Gas emissions have been suggested to be the cause of two serious global phenomena — global warming and the ozone hole. As you are probably already aware, both of these phenomena have the potential to restructure and redistribute biomes the world over and thereby lead to massive extinction events.

### Global Warming

A global warming trend has been indisputably observed over the last century, with the ten warmest years on record occurring within the last 15 years. In this time, global temperatures have increased by about 1°C over the average found at the end of the 19th century. As a point of comparison, during the last ice age, global temperatures were 5 to 6°C lower than they are at present. If these trends continue, scientists predict that the average global temperature could increase from 1.6 to 6.2°C by the year 2100. A temperature increase of this magnitude could melt the polar ice caps. The huge increase in sea levels would then flood massive stretches of coastal areas, change global weather patterns, lead to the extinction of thousands of species and displace millions of people from their homes.

While everyone agrees that the earth is warming up, scientists disagree over the cause. Some scientists see this change as part of the natural cyclical changes that are a normal part of the Earth’s functioning. But others attribute this rise in temperature to human actions.

The consensus among most of the scientific community is that Global Warming is due to the Greenhouse Effect. This phenomenon is caused by the release of 20 greenhouse gasses including carbon dioxide, nitrogen oxide, chlorofluorocarbons (CFCs), and methane, which combined historically made up less than 1% of the total volume of the atmosphere. At present, however, the levels of some of these gasses have increased more than four-fold since 1950, mostly due to fossil fuel combustion. Burning of these fuels accounts for 75% of the increased global levels of these gasses, while destruction of natural vegetation, which would reabsorb the gasses, accounts for the other 25%.

The greenhouse gasses float up to the tropospheric layer of the atmosphere, allow heat to enter the atmosphere, but prevent it from leaving. In the process, they act like the panes of glass in a greenhouse and ensure a gradual increase in global temperature. Studies looking at the relationship between carbon dioxide and methane levels in ice cores from Antarctica and historical temperature records found an intricate linkage between the two variables.

### Ozone Hole

Caused by similar mechanisms to global warming is the recently observed global decrease of the amount of protective ozone (O3) in the stratosphere. Ozone absorbs 98% of the harmful UV rays from the sun, preventing them from striking Earth. Extensive exposure to UV rays can lead to cancer in mammals and are lethal to all life in extreme amounts. In the late 1970’s a marked decrease in the ozone layer was observed over Antarctica. This Ozone Hole has been increasing in size and intensity ever since.

The depletion in the ozone layer is due to the action of CFCs that have been used in air-conditioners and refrigerators and are also used during the production of polystyrene foam (a.k.a. Styrofoam) and polyurethane. CFCs degrade O3 to O2 and water and a single CFC molecule can degrade more than 100,000 O3molecules. As a consequence, ozone is being depleted much faster than it can be produced by lightning and UV light. During October 1999, the ozone hole, which annually shrinks in the Southern Hemisphere’s fall and expands in their spring, expanded so that parts of South America were also exposed. The threats to plant and animal life of the ozone hole are many and indisputably serious.

### The Future

As a consequence of increasing human population size, global emissions of carbon dioxide from fossil fuel combustion has and will probably continue to steadily escalate, as will likely be the case for most other pollutants. If this happens, the dangers posed by Global Warming and the Ozone Hole will only become more serious.

Although bleak, the picture for global air pollution does have some bright spots. CFC production has been continually declining worldwide and the European community and the United States have signed a treaty to phase out all production by 2000. The Kyoto Protocol, which was adopted in December 1997 by 160 countries, calls for at least a 5% reduction in greenhouse gas emissions below 1990 levels beginning in 2008. Unfortunately, the United States Senate has shown a dramatic unwillingness to ratify even as minor a treaty as the Kyoto Protocol in the fear that it would stifle industry.