Case Study: Piecing Together Carbon's Pathways

Carbon Cycles

Greenhouse gases in Earth's atmosphere keep our planet warm enough to sustain life. One such gas—carbon dioxide—attracts a lot of attention from scientists because humans have increased its concentration in the atmosphere by about 30 percent over the past century. This recent increase is a direct result of humans' use of fossil fuels (oil and coal). Samples taken from air bubbles trapped in ice on Antarctica over the last 500,000 years show that levels of carbon dioxide (CO₂) have also risen and fallen naturally. Climate evidence shows that global temperatures are related to the amount of CO₂ in the atmosphere, so scientists are very interested in increasing their understanding of carbon dioxide's role in global climate change.

Searching for the Missing Carbon

Though many scientists focus their studies on carbon, there is a mystery surrounding it: when scientists add up all the carbon they can detect in various places in the environment over a year, between one and two billion metric tons of carbon are "missing." In other words, scientists cannot account for 15 to 30 percent of the carbon that humans release into the atmosphere each year. Because scientists don't know where all of this carbon goes, they cannot construct precise computer models that simulate Earth's actual carbon cycle. Without precise models, they cannot accurately predict future levels of carbon dioxide in the atmosphere nor can they gauge the impacts of rising carbon dioxide levels on the quality of life on our planet.

Plants: Carbon Sources or Sinks?

Currently, there is no way to directly observe the carbon cycle on a global scale, so scientists focus on carbon's "sources" and "sinks"—meaning where it comes from (sources) and where it is stored (sinks). There are a variety of parameters in the Earth system that indicate the movements of carbon. For instance, from its vantage point in space, NASA's Terra satellite can observe the "greenness" of Earth's landscapes using a unitless measure called Normalized Difference Vegetation Index or NDVI. For any location on Earth, the greener the land surface, the higher its vegetation index value, and the greater the amount of carbon that is being used at that location. That's because through photosynthesis, plants are removing carbon dioxide gas from the atmosphere and "fixing" the carbon into their structures, such as leaves. So, plants are a carbon sink. Scientists refer to the measure of carbon absorption by plants over time as "net primary productivity." This measurement is called net production because it indicates the total amount of carbon dioxide taken in by vegetation during photosynthesis minus the amount that is given off during respiration, the process by which they use food to produce energy.

Plants can also be a source of carbon: when they burn, either due to naturally occurring wildfires or human deforestation activities, much of the carbon stored in their structures is released back into the atmosphere in the form of gases and smoke particles. From space, Terra can map the frequency and intensity of fires and help scientists more accurately estimate how much carbon is released into the atmosphere on a global scale.

In this chapter, you'll view and build animations of satellite data that illustrate carbon sources and sinks to help you visualize Earth's carbon cycle.