365 Days of Climate Awareness 7 - Carbon Sources and Sinks

365 Days of Climate Awareness

7 - Carbon Sources and Sinks

One of the basic analytical tools in climate change is to identify carbon dioxide sources and sinks. These can be used to create a complete carbon dioxide budget for the atmosphere, but even without carrying things that far, it's useful, especially in terms of policy, to have an intelligent idea of where carbon dioxide comes from and where it goes to. Increasingly, it is necessary to include the ocean in these analyses, as it is by no means at all a limitless reservoir of either heat or CO₂ gas. It's currently estimated that world society emits roughly 55 gigatons of carbon dioxide equivalent (GtC) per year, and that total is climbing.

A carbon source is anything which supplies carbon dioxide to the atmosphere. It can be a point source, like a single factory or power plant, or a collective, such as the entire fleet of automobiles in the US. A carbon sink is anything which removes CO₂ from the atmosphere, and likewise can be point sinks (which are rare) such as carbon capture installations (which do exist), or distributed, such as forests.

Energy production, including for transportation, accounts for nearly 3/4 of global CO₂ emissions. Energy use for industry is the dominant component, 24% of overall emissions, with energy use to light, heat and cool buildings next at 17% and transportation a very close third at 16%. Agriculture collectively emits 18% of global CO₂, almost 6% due to livestock. Industrialized farming has turned that entire sector into a net carbon source. Curing of cement is another measurable source, 3% of global CO₂ emissions.

Carbon sources and sinks. (Pg = Petagrams = quintillion (10¹⁵) grams)

Photosynthesis, on land and in the ocean, is the dominant carbon dioxide sink, drawing down 120 GtC per year, but much of that carbon is not removed from the atmosphere for long, due to respiration and decomposition. Weathering of rock also consumes CO₂. There is a correlation in geological records between increased global orogenic (mountain-building) activity and decreased carbon dioxide content in the atmosphere (Earth's current level of mountain-building appears to be fairly moderate). The ocean itself removes about 90 GtC per year but most of that is returned, and only a small portion--2-3 GtC--is retained in the water. The added CO₂ in ocean water has other effects which will be discussed later.

Sinks and sources can be variable. In 2020, due to global COVID pandemic, economic activity was impaired and global emissions shrank. On the other hand, previously known carbon sinks such as the forests of western Canada and the Amazon basin in Brazil have lost much of their potency as sinks due to the persistent wildfires of British Columbia and the deliberate deforestation taking place in Brazil.

Tomorrow: measuring CO₂ concentration.

Be well!