Humans as a Geological Force Study Guide
Unlike other species, humans deploy vast arrays of chemical processes outside their biological bodies, via factories, residences, and forms of transportation. Chemical inputs and outputs from these have created a new kind of force within the systems of Earth, altering many aspects of the biosphere's reservoirs. What has changed? Can we reach a balance with nature?
Human Impacts on the Atmosphere
Historically, acid deposition was one of the first recognized human impacts on the atmosphere that occurred across a large geographical scale. Although natural rain is slightly acidic, humans have made the rain in many areas of the world even more acidic. In addition, the acids that humans add to the atmosphere come down to Earth's surface in other forms: acid snow and acid attached to minute particles in the air (so-called dry deposition). Acid rain, acid snow, and dry deposition are collectively known as acid deposition. To keep things simple, we will sometimes refer simply to acid rain in the following paragraphs.
Acid deposition begins with the combustion of fossil fuels, with coal being the biggest culprit. Deposits of coal are the remains of ancient plants from hundred of millions of years ago, preserved in a rock-like form that we can burn. Burning coal is possible because the coal contains carbon. Coal also contains sulfur, which is one of the biologically essential elements for all life. When coal is burned in power plants to obtain energy, most of which comes from converting carbon to carbon dioxide (CO2), the sulfur is also combined with oxygen to create sulfur dioxide (SO2). The SO2 enters the atmosphere as a gas. There, it further combines with water vapor to become sulfuric acid (H2SO4) in cloud droplets. The rain that falls from these clouds is acidic, thus acid rain.
Nitrogen also contributes to acid deposition. In high-temperature combustion of fossil fuels in power plants and automobiles, some of the nitrogen gas (N2) in the air is inadvertently combined with oxygen (O2) in the air to create the pollutants known as nitrogen oxides. In the atmosphere, these also react further to create nitric acid (HNO3), which combines with rain drops and contributes to acid rain. On average, the contribution of sulfuric acid to the acidity of rain is about twice the contribution of nitric acid.
Acid rain falls mostly in the regions downwind of power plants and large urban areas. In particular, large power plants that use coal with a relatively high sulfur content are the biggest makers of acid rain.
Acid deposition causes numerous kinds of damage. The most important is damage to lakes and forests. Many organisms are sensitive to the level of acidity in their environment. Fish have died in thousands of lakes in Scandinavia, over a thousand lakes in Canada, and in hundreds of lakes in the United States. Certain forests in New England and in Europe have also suffered severe damage. Finally, acid deposition erodes limestone and marble and has caused damage to buildings and priceless outdoor sculptures around the world.
In some geographical regions, the ecology of the lakes is fine, despite the acid deposition. The effects all depend on the local chemistry. Some areas have a natural buffering capacity in their soils and sediments, and so the acids are consumed by these natural chemical buffers. This process ultimately prevents damage by the acid rain.
Laws governing the release of acids from power plants are in place. Between 1980 and 2000, for example, the total emissions of sulfur dioxide from the worst-offending power plants were cut in half. Some of the improvement has come from adding sulfur scrubbers to power plants to "clean" the exhaust gases from those power plants; these scrubbers remove the sulfur. Switching to low-sulfur coal has been another strategy. With regard to the nitrogen emissions from automobiles, better catalytic converters have lowered the nitrogen oxides created by the combustion process.
Another human impact on the atmosphere has come from the release of substances that destroy the earth's ozone layer. Recall that ozone (O3) is a gas that occurs naturally in the stratosphere. Ozone absorbs the ultraviolet portion of the sun's energy that strikes the earth and it protects organisms from the potential damage from ultraviolet radiation. Because of its protective properties, the stratospheric ozone has been called a protective shield. Without this protective ozone layer, life would be in big trouble because biologically damaging ultraviolet rays would reach the surface. Ultraviolet radiation is a main cause, for example, of skin cancer.
Ozone is made when ultraviolet radiation strikes oxygen molecules in the stratosphere, initiating a chemical reaction that creates ozone. When ozone absorbs ultraviolet radiation, the ozone is destroyed. A balance is reached between creation and destruction, resulting in a natural amount of ozone that is constantly present and acts as the shield.
Several kinds of human substances that find their way into the air and travel up into the stratosphere increase the destruction rate of ozone. The most worrisome substances are known as CFCs, or chlorofluorocarbons (containing chlorine, fluorine, and carbon). These industrial chemicals can be used in refrigerators, air conditioners, and some aerosol cans, in the production of industrial plastics, and in the electronics industry for cleaning. When the CFCs are released into the atmosphere, the chlorine is liberated in the stratosphere, where it acts as a catalyst to destroy the ozone at a rate much faster than its natural rate of destruction.
By the late 1970s, it was clear that humans were changing the balance between ozone creation and destruction in the stratosphere. Most dramatically, in 1985, a large hole in the ozone was discovered over Antarctica. Earth scientists were shaken with alarm at this surprise from nature. We now know that special weather conditions over Antarctica intensify the destructive power of the CFCs, during the Southern Hemisphere's spring. The discovery of this hole led to increased efforts to stop the production of CFCs. Additionally, global levels of ozone had been dropping by several percent per decade during the final decades of the twentieth century.
The world did respond. The countries of the world grew concerned at the situation, and in 1987, most signed the Montreal Protocol, a global agreement to phase out the production and use of CFCs. An amendment in 1990 stepped up the schedule. By 1996, most of the industrial nations no longer manufactured CFCs. Substitute gases were invented to replace the technological uses of CFCs. The transition was well on the way, and the ozone decline was halted around the year 2000. The ozone level in the stratosphere, how ever, is still not back to where it once was as a shield. That recovery is expected to take until about 2050, at which time the ozone layer should have returned to its natural level.
An entirely different problem involving human impacts on a crucial gas in the atmosphere is now of concern—the issue of the rising levels of carbon dioxide and the expected warming of the future world.
Recall that carbon dioxide (CO2) is a greenhouse gas. A greenhouse gas has the following property: It lets visible radiation (light and short wave radiation) from the sun pass into the atmosphere and directly through to the ground (we can't see the CO2). But, significantly, a greenhouse gas absorbs infrared radiation. Infrared radiation (or longwave radiation) radiating out to space is the means by which the earth cools itself, the means to balance the energy received from the sun. Greenhouse gases are like one way insulation, letting light in but blocking the escape of infrared. The earth's surface will simply warm up to compensate for any extra insulation in the atmosphere.
CO2 is typically measured in units of parts per million (ppm), because only small amounts of it exist in the atmosphere. "Million" here refers to a million randomly selected molecules of air (such as N2, O2, and so forth). Today, CO2 is present in somewhat more that 370 ppm (which is equal to 0.037%). Though CO2 is present in such a small amount of the atmosphere, it is of critical importance because it is a greenhouse gas. In contrast, the most abundant gases, nitrogen and oxygen, are not greenhouse gases.
Without CO2 the earth would be very cold, below the freezing point of water, in fact. So present conditions require CO2. But one can also have too much of a good thing. CO2 is emitted as a waste gas from the combustion of fossil fuels (coal, oil, and natural gas, which is mostly methane) and its presence in the atmosphere is rising. Data from bubbles trapped in ice at Antarctica show that for 10,000 years prior to the industrial revolution, CO2 was fairly constant at about 280 ppm. It is now above 370 ppm and rising, from human activities, at the rate of 1.5 to 2 ppm per year.
Basically, fossil fuels contain carbon, originally incorporated by photosynthesis into the bodies of plants on land and algae in the ocean. Some of these bodies escaped the recycling loops of the biosphere. Quite simply, coal deposits are the remains of ancient land plants, oil comes from ancient algae, and natural gas is derived from either coal or oil, as these substances are "cooked" by the tectonic forces of geology over hundreds of millions of years. When we burn the fossil fuels, we combine their carbon with the oxygen in the atmosphere to create energy and the inevitable waste by-product—carbon dioxide (see Figure 19.1).
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