Carbon Sinks and Sources

When you think of carbon, what is the first thing that comes to mind? Climate change, fossil fuels, and pollution are all hot topics when it comes to carbon. Have you taken time to consider the role humans play in the global carbon cycle? Many of our daily activities, including driving, heating our homes, and using electricity release carbon dioxide (CO2) into the air. So, we release all of this additional heat-trapping CO2 gas into the air, but is there any process responsible for taking it out? Yes!

The answer is carbon sequestration. There are several biological processes responsible for removing carbon from the atmosphere as it moves through the global carbon cycle.

This simple graphic from the UCAR Center for Science Education illustrates the key processes of the carbon cycle. Carbon moves into plants from the atmosphere via photosynthesis, which allows plants to produce food from carbon for growth. When animals eat plants, the carbon is transferred into the food web, and continues to move through as animals eat other animals. As plants and animals die and decay, carbon is brought to the ground. Some of this carbon may become fossil fuels after millions of years; when fossil fuels are burned, carbon is released to the atmosphere. Humans and other animals also release CO2 into the atmosphere each time they exhale. Approximately 5.5 billion tons of carbon enter the atmosphere from human activity annually; 3.3 billion tons will linger in the atmosphere, and a large portion of the remainder dissolves into seawater, ultimately lowering the pH.

While this carbon is moving through the cycle, processes that release carbon are called “sources”, while means of absorption are referred to as “sinks”.

The term “carbon sequestration” refers to the process of carbon removal through biological and technological processes.

Carbon Sources

Natural sources of atmospheric CO2 are volcanoes, forest fires, decomposition, animal respiration, and digestion. Human activities, such as extraction and use of fossil fuels such as oil, natural gas, and coal are also sources of carbon.  The primary issue with our activities as carbon sources involves a “net” prospective. It is not so much that we cause the release of carbon in general, but the fact that we cause more carbon to be released than can be sequestered by carbon sinks, resulting in a net release of carbon into the atmosphere, rather than neutrality between carbon uptake and release. 

Carbon Sequestration and Sinks

Oceans and forests are the two largest natural carbon sinks, which extract carbon from the atmosphere through biological processes. In the ocean, plankton, coral, fish, algae, and photosynthetic bacteria capture CO2 from the water. Unfortunately, when CO2 is absorbed into the water at a higher rate than it can be taken up by organisms, acidification results from excess hydrogen ions. In forests, plants absorb CO2 from the atmosphere, storing some within their tissues, and powering photosynthesis, which returns oxygen (O2) to the atmosphere. Humans have negatively impacted natural forest carbon sinks via logging and clearing land for agricultural use, which shrinks the possible pool size of stored carbon. We have a variety of intentional means, natural and technological, to capture and sequester carbon from the atmosphere. Some of these atmospheric carbon sink processes are more refined and efficient than others. All said, to truly slow climate change we need to capture more carbon than we release.

This graphic from Project Drawdown gives a visual representation of the relative proportions of some major carbon sources and sinks. Electricity production is the largest source, accounting for a quarter of our carbon emissions, followed by agriculture and land use, then the industrial/manfacturing sector. Land, ocean, and coastal sinks sequester 41% of our emissions, leaving 59% to linger in the atmosphere.

The remaining 59% represents the gap we need to close to achieve carbon neutrality. Drastically reducing or even stopping our emissions entirely is crucial to ensure this gap shrinks rather than grows.

There are several ways this goal can be reached. Carbon Footprint’s website lays out this portfolio of carbon offset projects, including tree plantings, renewable energy system installations, energy-efficient housing, and more. These projects are just one avenue for individuals, businesses, and other organizations to make a positive difference in their carbon balance. There are several organizations which sponsor counterbalancing projects, which, of course, are not a get-out-of-jail-free card, but are still a concrete way to make a difference. Closer to home, participating in a community choice aggregation program such as SOPEC’s, which connects customers with 100% Green-E certified electricity, is a concrete avenue to reducing the proportion of carbon emitted through electricity production. SOPEC also supports and creates opportunities for innovative clean energy projects, all keeping energy dollars local.