Anthropogenic carbon emissions

Figure 1. Carbon is trapped underground in reservoirs, extracted, and used by humans through the process of combustion. This causes a release of carbon dioxide (and other carbon compounds) which would otherwise not reach the atmosphere, ocean, or soil.

Anthropogenic carbon emissions are any emissions of carbon that are associated with human activities. The two most concerning forms are carbon dioxide and methane. Examples of human activities which emit carbon include the burning of fossil fuels, deforestation, changes in land use, livestock, fertilization, and anything else that results in a net increase in emissions.

Increasing Carbon Levels

A major concern is the emission of carbon dioxide (CO₂) - a type of greenhouse gas - which contributes to global warming and ocean acidification. CO₂ and other compounds of carbon are exchanged throughout carbon pools in the global carbon cycle. The natural carbon cycle is kept in a near perfect balance. However, human emissions are constantly added into the cycle which causes a net increase in concentrations of carbon in the atmosphere, soil, and oceans, as seen in Figure 1.

Methane (CH₄), carbon monoxide, and black carbon are all present in the atmosphere, and each have varying effects on the planet. The amount of each of these in the atmosphere has been increasing rapidly: the amount of carbon dioxide in the atmosphere has increased by about 40% since the industrial era, and methane by about 150%. These levels had been relatively stable for around 10 thousand years prior to this.

See carbon cycle for more information on increasing levels of carbon in Earth's crucial systems.

Measuring Emissions

Scientists have had to work hard to show that climate change is caused by the greenhouse gasses that humans emit. Part of the reason that this was tricky is that nature does emit nearly 20 times more CO₂ into the atmosphere than humans do, but it's the new carbon that humans introduce that changes the amount of carbon in the system. This section highlights techniques and observational evidence to support the impact that humans have had on the environment.

There are many scientific methods that allow for the measurements of human emissions, two of which will be talked about here:

1. The first method is the investigation of carbon-14 (¹⁴C), a radioactive isotope of carbon. ¹⁴C is found naturally in atmospheric CO₂, however it is not present in fossil fuels that humans extract from underground. This is because fossil fuels were formed millions of years ago, far longer than the half life of ¹⁴C (t_1/2 = 5730 years), meaning that all of the ¹⁴C has decayed away. This means that carbon emitted by these fuels does not contain any ¹⁴C. By measuring levels of ¹⁴C and CO₂ over time, it is found that the vast majority of excess CO₂ does not contain ¹⁴C and therefore can be attributed to fossil fuels.
2. The second method relates to changes in atmospheric oxygen levels. It is employed in order to determine whether this excess CO₂ is indeed from human fossil fuels, or if it is instead from the likes of volcanic eruptions. This method works off of basic hydrocarbon combustion knowledge - when a fossil fuel is burned, its hydrocarbons consume oxygen in order to give off energy, along with water vapour and carbon dioxide. Therefore oxygen levels are expected to decrease when humans burn fossils fuels, and when measuring levels over time it is found that they do, the amounts of which are in accordance with measured carbon dioxide levels. If the emissions were instead dominantly from volcanoes, there would be no measured decrease in atmospheric oxygen levels.

Where do these Emissions Go?

The net destination of emissions from human activities is seen in Figure 2 below. This figure shows the flow of anthropogenic carbon from its emission (dominantly from fossil fuel combustion), to the atmosphere. Once these greenhouse gasses have been emitted, natural processes move these around and they settle in different carbon pools: the atmosphere, the ocean and the land. Over the past few decades the amount that's ending up in the different carbon pools is shifting more heavily towards the atmosphere.

Figure 2. The net emissions from human activity go into the atmosphere, and natural processes distribute those GHGs into three broad carbon pools. With roughly half staying in the atmosphere and the other half being somewhat evenly split between the land and the ocean.

Where do these Emissions Come from?

The Sankey diagram below shows where the world's GHG emissions come from. As seen in figure 3, the majority of the emissions come from natural gas, oil and coal that are used for the world's primary energy. While methane and other greenhouse gasses are a problem, carbon dioxide is the biggest contributor to changing the world's climate.

Figure 3. Roughly 3/4 of the GHG emissions from human come from activities related to human energy use. Note also that most of the GHGs, when put on an even footing of CO2 equivalent is still carbon dioxide.

The World Resource Institute has a nice article with an interactive Sankey diagram here.

For Further Reading

• High energy society
• Climate forcing
• Indicators of a warming world
• Climate change
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