Environmental scientists are interested in how the natural environment works. The natural environment includes all of the living and nonliving things found naturally in an area. Some of these scientists are interested in how the natural environment responds to change. In this research, scientists wondered about change occurring over a span of more than 100 years. To study this change, the scientists used data that had been collected over that many years.

The scientists entered the data into a computer program called a model. The computer model included mathematical equations. The scientists observed changes recorded over a 100-year period. They then developed equations based on the recorded changes. For example, a scientist may observe that the water level in a forest stream drops 1.10 centimeters for every 2 weeks without rain. With this knowledge, the scientist can create an equation that will predict the stream’s water level for any number of weeks without rain.

In this research, the scientists were interested in whether an area of land and forests absorbed more carbon than it released into the atmosphere. With the use of a computer model, the scientists were able to identify a pattern over more than 100 years.

All living and once-living matter contains the element carbon. Think about your own body. A human body contains about 18 percent carbon. The carbon in plants is produced from atmospheric carbon dioxide through photosynthesis. Plants, therefore, are called producers. Humans get their carbon from eating plants and from eating animals that eat plants. Animals, including humans, are called consumers. A plant contains carbon as long as it lives, and until it completely decays or is burned. Soil and water bodies also contain carbon.

Carbon is held by plants, soil, and water bodies, but some carbon is also released back into the atmosphere. Carbon dioxide, therefore, continually cycles between the atmosphere and Earth (fig. 1). This exchange occurs in the form of carbon dioxide. A carbon source is when more carbon is released by the environment than is produced through photosynthesis. A carbon sink is when more carbon is produced through photosynthesis than is released to the atmosphere.

The scientists in this study were interested in environmental changes occurring in the Southern United States (fig. 2).

The scientists were interested in changes occurring over a long time. In particular, the scientists were interested in the following environmental changes:

• Increasing atmospheric carbon dioxide
• Increasing ozone
• Increasing nitrogen on Earth
• Climate change (Climate change is the long-term change in global weather patterns, especially increases in temperature, storm activity, and precipitation)
• Changes in land use (fig. 3)

The scientists were interested in how these five changes will affect the carbon cycle of the Southern United States (See fig. 1).

Carbon dioxide continually cycles between Earth and the atmosphere. In the past, this cycling created an average balance over time between atmospheric carbon dioxide and carbon on Earth. Over the past 100 years, however, human activities have increased the amount of carbon dioxide released to the atmosphere. Carbon dioxide is released when trees are removed for development (fig. 4) and when fossil fuels are burned for energy.

Human activities have caused other changes. Tropospheric ozone, another gas, helps protect Earth from the Sun’s ultraviolet radiation (fig. 5). When fossil fuels are burned, however, too much ozone is produced near Earth’s surface. This ozone contributes to creating a kind of air pollution called smog.

Another long-term change involves nitrogen. Human activities have caused an increase in nitrogen on Earth’s surface. Nitrogen is released when fossil fuels are burned for energy. Much of this nitrogen returns to Earth. Nitrogen is also used as a crop fertilizer. Over time or in large amounts, nitrogen adds too much acid to plants, soil, and water bodies. In the short term or in smaller amounts, nitrogen can cause an increase in plant growth. This growth occurs because the plants are able to use nitrogen as a nutrient.

The scientists were interested in land use changes over the past 100 years. Over time, for example, forests have been cut and regrown. Farms have been cultivated and abandoned. Land developed for buildings and roads is one type of land use change that is not usually reversed (fig. 6).

The scientists were also interested in the changing climate. Over the past 100 years, the average yearly temperature has been rising around the world. This average rising temperature may have an effect, for example, on where and how much rain falls.

The scientists in this study wanted to know how these environmental changes have affected how land in the Southern United States holds carbon or releases it to the atmosphere. The scientists wondered if the Southern United States has been a carbon source or a carbon sink over the past 100 years. A carbon source is when more carbon is released by the environment than is produced through photosynthesis. A carbon sink is when more carbon is produced through photosynthesis than is released to the atmosphere.

The scientists were interested in studying environmental changes that have occurred over the past 100 years. They used data that were collected between 1895 and 2007 across the Southern United States (fig. 7).

The scientists divided the Southern United States into a grid of blocks 8 kilometers by 8 kilometers in size (fig. 8). A value for each variable in figure 7 was calculated and assigned to each block.

The scientists entered all of the data into a computer. The scientists developed a computer program, called a model, to analyze the data. The model used equations to calculate NCE. The scientists then compared all combinations of variables in figure 7 with the value NCE. This process was done for each block in the grid over the entire 112 years. Then all of the data in all blocks were averaged to produce values for the entire Southern United States.

The findings in figure 9 show values for NCE in the Southern United States over three time periods.

The Southern United States was a carbon sink for the period of 1895–2007, over which there was an increase in terrestrial carbon of 0.80 Pg (See fig. 9). This amount accounted for 2.7 percent of the total carbon stored across the entire Southern United States in 1895. Between 1895 and 1950, however, the Southern United States was a carbon source of 1.20 Pg. Between 1951 and 2007, the Southern United States was again a carbon sink, storing an additional 2.0 Pg of carbon. The scientists believe that the Southern United States was a carbon source between 1895 and 1951 because many acres of forest land were cleared for agriculture. From the 1950s until the late 1900s, many of these agricultural lands were abandoned and forests grew back.

Overall, the scientists found that changes in land uses over the past 112 years have had a negative impact on carbon storage in the Southern United States. The scientists discovered that different kinds of land use stored different amounts of carbon (fig. 10). The application of nitrogen fertilizer and an increase in nitrogen released on Earth had a positive impact on carbon storage. This positive impact is from the beneficial effects of nitrogen on plant growth. When plants grow, more carbon is stored.

Climate change, as an individual variable, appeared to have little direct impact on carbon storage in the Southern United States. However, when considering that most of the other variables studied had an impact on climate change, some of the change in carbon storage can be indirectly related to climate change.

Although the Southern United States was a carbon sink over the past 112 years, many factors influenced carbon storage positively and negatively. Some factors related to climate change had a negative effect, for example, but were offset by increased nitrogen and the application of nitrogen fertilizer. Land use change had a negative effect, but was offset by an increase in the amount of forest land in the Southern United States.

The scientists found that an increase in atmospheric carbon dioxide and nitrogen created a carbon sink over the time period. Increases in ground-level ozone, urban development, and a changing climate contributed most to carbon losses.

The scientists noted that the list of variables they considered was incomplete. Forest management, for example, could affect NCE but was not included in the model. Hurricanes, fires, and other natural disturbances were also not included in the model. If these variables were included, NCE estimates might be different.

The scientists said that additional research is needed to better understand how changes on the planet contribute to the carbon balance between Earth and the atmosphere.