Some nutrients cycle at a global level. We will use a simple example of the carbon cycle to illustrate an example of a global cycle (see figure at top of this post).
At the global level the major locations for carbon are living organisms, the atmosphere, water, and rocks. The two most important biological causes of movement between one reservoir and the next are photosynthesis and cellular respiration.
1. Photosyntheis- moves carbon from the atmosphere (for terrestrial organisms) or water (for aquatic organisms) to living organisms. Carbon dioxide ===> glucose
2. Cellular Respiration- moves carbon from living organisms to the atmosphere or water.
glucose ===> carbon dioxide.
Carbon in the Atmosphere
The major source of carbon in the atmosphere is carbon dioxide. Historically, the rate at which carbon dioxide was added to the atmosphere was about equal to the rate at which carbon dioxide was removed from the atmosphere were about equal. Thus, over time the atmospheric contentration of carbon dioxide didn't change very much.
Human Alteration of the Global Carbon Cycle
Humans have altered the global carbon cycle in three main ways.
1) By burning fossil fuels (e.g., gasoline in our cars and coal in our power plants) we have increase the rate at which carbon is moved from rocks to the atmosphere.
2) Deforestation- By cutting down trees for agriculture or development we have reduced the number of plants conducting photosynthesis. Thus, we have decreased the rate at which carbon is moved from the atmosphere to living organisms.
3) By burning trees after deforestation we have increased the rate at which carbon moves from lving organisms to the atmosphere.
Thus, humans have increased the rate at which carbon is being added to the atmosphere while decreasing the rate at which carbon is removed from the atmosphere.
Prediction- the amount of carbon in the atmosphere should be increasing.
Test of this prediction- scientists in Mauna Loa, Hawaii have tested this prediction by measuring the amount of carbon dioxide in the atmosphere over the past 50 years.
MaunLoa Curve- http://www.eoearth.org/article/Mauna_Loa_curve
This curve shows two things. First, that the concentration of carbon dioxide in the atmosphere has indeed increased over time. Second, that there is seasonal variation in the concentration of carbon dioxide in the environment. Carbon dioxide is most abundant in the atmosphere in the North American winter and lowest in the North American winter. This pattern is caused by seasonal variation in the amount of photosynthesis. In the summer, when photosynthetic rates are the highest, carbon dioxide is removed from the atmosphere at a high rate which reduces the amount of carbon in the atmosphere. Because there is more land mass in the Northern Hemispere and most photosynthesis happens on land, the global pattern is determined by seasons in the Northern Hemispere (this is truly a global cycle, carbon dioxide move so quickly though the environment that the conentration is virtually the same all over the world).
Expected Learning Outcomes
At the end of this course a fully engaged student should be able to
- diagram the global carbon cycle and be able to explain how human activity has altered this cycle (TEKS 112.43. 12A).
What is Happening to the Rate Carbon Dioxide is Being Added to the Atmosphere?
Here is an article from the Associated Press entitled- Climate Warming Gasses Rising Faster than Expected that was published yesterday. Chris Field, one of the scientists quoted in this article, was one of my professors at University of Utah.
CHICAGO (AP) -- Despite widespread concern over global warming, humans are adding carbon to the atmosphere even faster than in the 1990s, researchers warned Saturday. Carbon dioxide and other gases added to the air by industrial and other activities have been blamed for rising temperatures, increasing worries about possible major changes in weather and climate.
Carbon emissions have been growing at 3.5 percent per year since 2000, up sharply from the 0.9 percent per year in the 1990s, Christopher Field of the Carnegie Institution for Science told the annual meeting of the American Association for the Advancement of Science.
''It is now outside the entire envelope of possibilities'' considered in the 2007 report of the International Panel on Climate Change, he said. The IPCC and former vice president Al Gore received the Nobel Prize for drawing attention to the dangers of climate change.
The largest factor in this increase is the widespread adoption of coal as an energy source, Field said, ''and without aggressive attention societies will continue to focus on the energy sources that are cheapest, and that means coal.''
Past projections for declines in the emissions of greenhouse gases were too optimistic, he added. No part of the world had a decline in emissions from 2000 to 2008.
Anny Cazenave of France's National Center for Space Studies told the meeting that improved satellite measurements show that sea levels are rising faster than had been expected. Rising oceans can pose a threat to low level areas such as South Florida, New York and other coastal areas as the ocean warms and expands and as water is added from melting ice sheets.
And the rise is uneven, with the fastest rising areas at about 1 centimeter -- 0.39 inch -- per year in parts of the North Atlantic, western Pacific and the Southern Ocean surrounding Antarctica, she said.
Also, highly promoted efforts to curb carbon emissions through the use of biofuels may even backfire, other researchers said. Demand for biologically based fuels has led to the growing of more corn in the United States, but that means fields were switched from soybeans to corn, explained Michael Coe of the Woods Hole Research Center. But there was no decline in the demand for soy, he said, meaning other countries, such as Brazil, increased their soy crops to make up for the deficit.
In turn, Brazil created more soy fields by destroying tropical forests, which tend to soak up carbon dioxide. Instead the forests were burned, releasing the gasses into the air. The increased emissions from Brazil swamp any declines recorded by the United States, he said.
Holly Gibbs of Stanford University said that if crops like sugar and oil palm are planted after tropical forests are burned, the extra carbon released may be balanced by lower emissions from biofuel in 40 to 120 years, but for crops such as corn and cassava it can take hundreds of years to break equal.
''If we run our cars on biofuels produced in the tropics, chances will be good that we are effectively burning rainforests in our gas tanks,'' she said.
However, there could be benefits from planting crops for biofuels on degraded land, such as fields that are not offering low productivity due to salinity, soil erosion or nutrient leaching.
''In a sense that would be restoring land to a higher potential,'' she said. But there would be costs in fertilizer and improved farming practices.
In some cases simply allowing the degraded land to return to forest might be the best answer, she said.
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