CHAPTER SUMMARY

• Keeling began a program to monitor carbon dioxide levels in the atmosphere in 1958. Two patterns emerged: (1) carbon dioxide levels oscillate on a yearly basis, and (2) carbon dioxide levels have increased steadily to the present day since measurements began.
• The first pattern, seasonal oscillation, can be explained by the imbalance between photosynthesis and respiration in the summer and the winter months.
• The second pattern, of steady increase, can be extended further back in time by measuring the composition of air trapped in arctic gas bubbles. These measurements indicate that carbon dioxide levels in the atmosphere remained steady for more than a thousand years, but then started to increase in the mid-1800s, the time of the Industrial Revolution.
• The ratio of different isotopes of carbon in the atmosphere indicates that most of the carbon added to the atmosphere in recent decades comes from human activities, particularly the burning of fossil fuels.

• Physical processes, which act at slower rates and over longer time scales, are the major drivers of the long-term carbon cycle.
• In following carbon around its cycle, it is important to consider where carbon is stored (reservoirs) and rates at which it is added or removed from these stores (fluxes).
• Important reservoirs of carbon include organisms, the atmosphere, soils, the oceans, and sedimentary rocks. Sedimentary rocks are by far the largest carbon reservoir.
• Physical processes that remove CO₂ from the atmosphere include the weathering of rocks, and those that add CO₂ to the atmosphere include volcanoes and the oxidation of ancient organic matter in sedimentary rocks.
• The short-and long-term carbon cycles are linked by the slow leakage of organic matter from biological communities to sediments accumulating on the seafloor.
• Over the last 400,000 years, CO₂ levels in the atmosphere have gone through periodic shifts that reflect repeated cycles of glaciation.
• CO₂ levels in the atmosphere 500 million years ago may have been as much as 15 to 20 times higher than present-day levels. Over timespans of hundreds of millions of years, movements of Earth’s plates and the evolution of new life-forms are major players in the carbon cycle.

• Photosynthetic and other autotrophic organisms are primary producers, converting carbon dioxide into organic molecules.
• Heterotrophic organisms that eat primary producers are called primary consumers. Other heterotrophs that prey on these grazers are called secondary consumers.
• Interacting networks of species, linked by predator–prey interactions, are called food webs, and they are an important feature of the carbon cycle.
• Communities contain a much greater biomass of producers than consumers because the movement of energy through communities is inefficient.
• Photosynthetic organisms are well adapted to obtaining sunlight and nutrients in different environments.
• Respiring organisms are adapted to obtain different sources and sizes of food.
• The evolution of life on Earth can be considered a long history during which organisms have evolved different ways of obtaining energy and carbon from their environment.
• The interactions between biological and geologic processes at work in the carbon cycle have not only produced our current biological diversity. They are also responsible for Earth’s oxygen-rich atmosphere and oceans.