Carbon dioxide - A story

The concentration of carbon dioxide (CO₂) in Earth's atmosphere is of interest because
of its impact on the greenhouse effect. arbon dioxide (CO₂) is the primary greenhouse gas emitted through human activities. In 2011, CO₂ accounted for about 84% of all U.S. greenhouse gas emissions from human activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle—both by adding more CO₂ to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO₂ from the atmosphere. While CO₂ emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution. 

U.S. Carbon Dioxide Emissions, By Source

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011.

The main human activity that emits CO₂ is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO₂. The main sources of CO₂ emissions in the United States are described below.

• Electricity. Electricity is a significant source of energy in the United States and is used to power homes, business, and industry. The combustion of fossil fuels to generate electricity is the largest single source of CO₂ emissions in the nation, accounting for about 38% of total U.S. CO₂ emissions and 32% of total U.S. greenhouse gas emissions in 2011. The type of fossil fuel used to generate electricity will emit different amounts of CO₂. To produce a given amount of electricity, burning coal will produce more CO₂ than oil or natural gas.
• Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods is the second largest source of CO₂ emissions, accounting for about 31% of total U.S. CO₂ emissions and 26% of total U.S. greenhouse gas emissions in 2011. This category includes transportation sources such as highway vehicles, air travel, marine transportation, and rail.
• Industry. Many industrial processes emit CO₂ through fossil fuel combustion. Several processes also produce CO₂ emissions through chemical reactions that do not involve combustion, for example, the production and consumption of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. Fossil fuel combustion from various industrial processes accounted for about 14% of total U.S. CO₂ emissions and 12% of total U.S. greenhouse gas emissions in 2011. Note that many industrial processes also use electricity and therefore indirectly cause the emissions from the electricity production.

Carbon dioxide is constantly being exchanged among the atmosphere, ocean, and land surface as it is both produced and absorbed by many microorganisms, plants, and animals. However, emissions and removal of CO₂ by these natural processes tend to balance. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO₂ and other heat-trapping gases to the atmosphere.
In the United States, since 1990, the management of forests and non-agricultural land has acted as a net sink of CO₂, which means that more CO₂ is removed from the atmosphere, and stored in plants and trees, than is emitted. This sink offset about 14% of total emissions in 2011 and is discussed in more detail in the Land Use, Land-Use Change, and Forestry section.
To find out more about the role of CO₂ warming the atmosphere and its sources, visit the Causes of Climate Change page and the Greenhouse Gas Indicators page in the Science section.
A recent report on global atmospheric CO2 level has created a concern among the scientific community associated with global warming and climate change research. The report released by National Oceanographic and Atmospheric Administration (NOAA) on 9th of this month has stated that the daily average CO2 concentration has surpassed the level of 400 ppm as observed in the Mauna Loa Observatory at Hawaii Islands. As stated by NOAA, such a higher level has been reached for the first time in the last 800,000 years. The same has also been reported by another climate research group ‘Scripps Institute of Oceanography’ who has also added that similar concentration of the green house gas was present during the Pilocene Age, 3-5 million years ago.Daily measurements of CO2 at a US government agency lab on Hawaii have topped 400 parts per million for the first time.
The station, which sits on the Mauna Loa volcano, feeds its numbers into a continuous record of the concentration of the gas stretching back to 1958.The last time CO2 was regularly above 400ppm was three to five million years ago - before modern humans existed. Scientists say the climate back then was also considerably warmer than it is today.
Carbon dioxide is regarded as the most important of the manmade greenhouse gases blamed for raising the temperature on the planet over recent decades.Human sources come principally from the burning of fossil fuels such as coal, oil and gas.The usual trend seen at the volcano is for the CO2 concentration to rise in winter months and then to fall back as the northern hemisphere growing season kicks in. Forests and other vegetation pull some of the gas out of the atmosphere.This means the number can be expected to decline by a few ppm below 400 in the coming weeks. But the long-term trend is upwards.