Humans as a Geological Force Study Guide (page 2)
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).
Toward a Sustainable Future
Humans are altering the biosphere in ways that will change it for very long periods of time. That's simply what we do. Here we will look at some of the global changes in land use. Then we will focus on several fluxes made by humans that exceed those of nature. Finally, we will note the concept of sustainability, which deals with the question of how we will continue to provide life support for what soon will be 7 billion people without further stressing the systems that nature provides.
As of 2005, the human population was about 6.4 billion people. It is growing at approximately 80 million people per year. Population is rising, but, as they say, land is one thing that doesn't increase. The global land area is 140 million square kilometers. The common land unit in the metric system is the hectare, which is a square 100 meters on a side. How many square meters in a hectare? You will be asked this in a practice question.
Using the conversion for the number of square meters in a hectare and the fact that one million square meters are in a square kilometer, we can calculate the global land area to be 14 billion hectares. That means there are about 2.2 hectares per person. If you are more familiar with the unit of acres, this figure is about 5.4 acres per person (1 hectare = 2.47 acres).
How much of this do humans currently use? How much could they use? First of all, 31% of the world's land (4.4 billion hectares) is unusable, because it is rock, ice, tundra, or desert. That leaves 9.6 billion hectares for potential use.
The major human land use is for agricultural production, which currently uses 4.7 billion hectares. So agriculture (pastureland and cropland) takes 34% of the world's land (4.7/10.4). Of that, 70% is permanent pasture and 30% is cropland.
Globally, only about 1% to 2% of land (approximately 140-280 million hectares) is considered urbanized. In some local areas, obviously, the urbanized land approaches 100% of land use.
Therefore, summing the unusable land, the agricultural land, and the urbanized land, and then subtracting that from the total land area, we arrive at the figure of 4.8 billion hectares that potentially remain. This is about 33% to 34% of the total land, or about as much as we currently use for all agriculture already. Here is the calculation: 100% - 31% - 34% - (1% or 2%) = 33% to 34%. However, much of the prime land for agriculture has already been used, so what remains is not as high in quality. This is the big picture of global land use.
Obviously, humans have made a big impact on the world's land. There is still more land that could be converted to exclusively human use, and that process is going on yearly. The conversion that is of most concern is the deforestation of tropical rainforests. However, in certain areas of the developed nations, forests are actually regrowing. An example in the United States is in the New England states. These were once nearly all converted into farms. But as the farms became less economical, when farms in the Midwest and California could produce food more cheaply, the farms in New England lost much of their value, and forests were allowed to regrow.
The appropriation of land by humans implies that humans are creating fluxes of matter for their own use. For example, when land is used for agriculture, humans take the organic carbon products of photo synthesis for food. But to produce this food, humans apply fertilizers such as phosphorus and nitrogen to boost the productivity of the agricultural soils. Plowing the soil also allows erosion to proceed at a rate that is faster than what would be the normal rate for nature.
Nature has balanced cycles. Erosion of the continents and the deposition of continental minerals into the ocean are, over the long term, balanced by the uplifting of the continents into mountains from the forces of plate tectonics. The nutrients of phosphorus and nitrogen—required by all life—cycle around and around. These nutrients are incorporated into plants on land and algae in the ocean by the process of photosynthesis. The plants and algae can be eaten, or they die. Eventually, many of the nutrients are recycled back to the soil (on land) or into the deep ocean water, primarily by bacteria. Some of the nutrients leak out and are buried in sediments. These nutrients are replaced over the long term by new nutrients put into the biosphere from the weathering of rocks and the eruption of volcanoes.
How do the fluxes caused by humans compare to the equivalent fluxes in nature? Looking at some numbers instructs us about the power of humans as a new kind of geological force on the planet.
One component or process in the cycle of nitrogen is the conversion of nitrogen gas in the air into forms such as ammonium and nitrate ions. The ammonium and nitrate ions—unlike the air's nitrogen gas—are what plants and algae need for their bodies to grow. The conversion process is called nitrogen fixation, and it is accomplished by special kinds of bacteria that are called nitrogen fixers.
The natural biological flux of nitrogen fixation, performed by the world's nitrogen-fixing bacteria in the soils and in the ocean, is about 130 million tons per year. (All tons are given in the international unit of metric tons, or 1,000 kilograms, approximately 2,200 pounds or 1.1 U.S. ton.)
Humans also create a flux of nitrogen fixation. Some happens inadvertently, as noted previously, in the creation of nitrogen oxides by fossil fuel combustion. Humans also create conditions for extra nitrogen fixation during rice cultivation. And, importantly, humans take atmospheric nitrogen and use high temperature and high-pressure industrial processes to manufacture fertilizer for the large-scale agriculture that humans think needs a boost in productivity. All these forms of human nitrogen fixation add up to 140 million tons per year, more than the global flux of nitrogen fixation by the world's nitrogen-fixing bacteria.
Phosphorus is also required by all organisms. And many agricultural soils are deficient in phosphorus. Therefore, applying phosphorus as fertilizer increases the productivity of many soils.
The natural release of phosphorus into the cycles of the biosphere from the chemical weathering of rocks and minerals is about 3 million tons per year. In contrast, humans mine phosphorus and thereby bring this new phosphorus into the cycles of the biosphere. The flux from mining activities, primarily aimed at fertilizer production, is about 12 million tons per year, or 4 times the flux of phosphorus in nature.
One major environmental concern coming from the increases in the fluxes of nitrogen and phosphorus caused by humans is the fact that these nutrients can runoff from farmlands after rains and go directly into lakes and coastal regions. Though this might seem like a good thing—to fertilize the lakes and coasts for "free" from farm runoff—it is actually bad. The extra nutrients create a condition called eutrophication. During eutrophication, algae go wild with growth. But then the algae die and fall to the bottom, where bacteria consume the bodies of the algae along with oxygen in the water. The water becomes depleted of oxygen, and fish and other creatures such as mussels and oysters die. For instance, there is currently a massive low oxygen zone in the Gulf of Mexico, around the entry of the Mississippi River into the Gulf. The low-oxygen zone has been created by the runoff of fertilizer from the Midwestern farms.
Finally, we look at sediment loads (suspended solids) in the world's rivers. The global sediment load carried by the world's rivers prior to human impact is about 10 billion tons per year (just under two tons per person). With the influence of humans, that number has tripled. In other words, human activity causes about 20 billion tons per year of sediment in the world's rivers that goes into the ocean. Humans are clearly a major geological force!
There is a new science on the horizon that can help us. It is called the science of sustainability. The science of sustainability deals with ensuring the sustained connection between humans and Earth. As we have seen, human needs are often fulfilled by compromising the environment. We drain aquifers, convert forests and prairies into farms, use water and land as waste dumps, fish without adequate limits, even change the radiation balance of the atmosphere. How can we harmonize the arrow of human progress with the cyclic, restorative processes of nature? This problem looms before us significantly. Its solution has been taken to constitute the new branch of knowledge called sustainability science.
Practice problems of this concept can be found at: Humans as a Geological Force Practice Questions
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