Show While most of the Earth’s carbon can be found in the geosphere, carbon is found in all living things, soils, the ocean, and atmosphere. Carbon is the primary building block of life, including DNA, proteins, sugars and fats. One of the most important carbon compounds in the atmosphere is carbon dioxide (CO2), while in rocks carbon is major component of limestone, coal, oil and gas. Carbon cycles through the atmosphere, biosphere, geosphere, and hydrosphere via processes that include photosynthesis, fire, the burning of fossil fuels, weathering, and volcanism. By understanding how human activities have altered the carbon cycle, we can explain many of the climate and ecosystem changes we are experiencing today, and why this rapid rate of change is largely unprecedented in the Earth’s history. What is the carbon cycle?Carbon is transferred between the ocean, atmosphere, soil, and living things over time scales of hours to centuries. For example, photosynthesizing plants on land remove carbon dioxide directly from the atmosphere, and those carbon atoms become part of the structure of the plants. As plants are eaten by herbivores and herbivores are eaten by carnivores, carbon moves up the food web. Meanwhile, the respiration of plants, animals, and microbes returns carbon to the atmosphere as carbon dioxide (CO2). When organisms die and decay carbon also returns to the atmosphere, or is integrated into soil along with some of their waste. The combustion of biomass during wildfires also release large amounts of carbon stored in plants back into the atmosphere. On longer timescales, significant amounts of carbon are transferred between rocks and the ocean and atmosphere, typically over thousands to millions of years. For example, the weathering of rocks removes carbon dioxide from the atmosphere. The resulting sediments, along with organic material, can be transported (eroded) from the land to enter the ocean where they sink to the bottom. This carbon from land, as well as carbon atoms in CO2 absorbed by the ocean from the atmosphere, can become incorporated into calcium carbonate (CaCO3) shells made by algae, plants, and animals. These shells become buried. As the successive layers of sediment are compressed and cemented they are turned into limestone rock. Over millions of years these carbon-bearing rocks can be exposed to sufficient heat and pressure to melt, causing them to release their carbon back into the atmosphere as carbon dioxide via volcanism. Some of these rocks will also be exposed at the surface of the Earth through mountain building and weathering, and the cycling begins again. Carbon from the mantle (see plate tectonics) is also released into the atmosphere as carbon dioxide through volcanic activity. Carbon is also transferred to rocks from the biosphere, via the formation of fossil fuels, which form over millions of years. Fossil fuels are derived from the burial of photosynthetic organisms, including plants on land (which primarily forms coal) and plankton in the oceans (which primarily forms oil and natural gas). While buried, this carbon is removed from the carbon cycle for millions of years to hundreds of millions of years. Human activity, especially the burning of fossil fuels, has dramatically increased the exchange of carbon from the ground back into the atmosphere and oceans. This return of carbon back into atmosphere as carbon dioxide is occurring at a rate that is hundreds to thousands of times faster than it took to bury it, and much faster than it can be removed by the carbon cycle (for example, by weathering). Thus, the carbon dioxide released from the burning of fossil fuels is accumulating in the atmosphere, increasing average temperatures through the greenhouse effect, as well as dissolving in the ocean, causing ocean acidification. A simplified diagram showing some of the ways carbon dioxide moves through the Earth system, and the overall increase in atmospheric carbon dioxide from 2004-2013. The rate of exchange and the distribution of carbon in the Earth system is affected by various human activities and environmental phenomena, including:
Earth system model about the carbon cycleThe Earth system model below includes some of the processes and phenomena related to the carbon cycle. These processes operate at various rates and on different spatial and temporal scales. For example, carbon is transferred among plants and animals over relatively short time periods (hours-weeks), but the human extraction and burning of fossil fuels has altered the carbon cycle over decades. Additionally, processes that include weathering and volcanism affect the carbon cycle over millions of years. Can you think of additional cause and effect relationships between the parts of the carbon cycle and other processes in the Earth system? Explore the Earth SystemClick the bolded terms (e.g. burning of fossil fuels, greenhouse effect, and weathering) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you. Links to Learn More
How do humans release carbon into the atmosphere?Human activities that lead to carbon dioxide emissions come primarily from energy production, including burning coal, oil, or natural gas.
Which human activities release the most carbon?The largest source of greenhouse gas emissions from human activities in the United States is from burning fossil fuels for electricity, heat, and transportation.
What human activities release carbon monoxide to the atmosphere?The greatest sources of CO to outdoor air are cars, trucks and other vehicles or machinery that burn fossil fuels. A variety of items in your home such as unvented kerosene and gas space heaters, leaking chimneys and furnaces, and gas stoves also release CO and can affect air quality indoors.
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