Carbon Dioxide: The No. 1 Greenhouse Gas

Coal plant smokestacks
Martyn Goddard / Getty Images

Carbon is an essential building block for all life on earth. It is also the main atom making up fossil fuels’ chemical composition. It can also be found in the form of carbon dioxide (CO2), a gas that plays a central role in global climate change.

What Is CO2?

Carbon dioxide is a molecule made of three parts, a central carbon atom tied to two oxygen atoms. It is a gas making up only about 0.04% of our atmosphere, but it is an important component of the carbon cycle. Carbon molecules are real shapeshifters, often in solid form, but frequently changing phase from CO2 gas to liquid (as carbonic acid or carbonates), and back to a gas. The oceans contain vast amounts of carbon, and so does solid land: rock formations, soils, and all living things contain carbon. Carbon moves around between these different forms in a series of processes referred to as the carbon cycle – or more precisely a number of cycles that plays multiple crucial roles in the global climate change phenomenon.

CO2 Is Part of Biological and Geological Cycles

During a process called cellular respiration, plants and animals burn sugars to obtain energy. The sugar molecules contain a number of carbon atoms which during respiration are released in the form of carbon dioxide. Animals exhale excess carbon dioxide when they breathe, and plants release it mostly during nighttime. When exposed to sunlight, plants and algae pick up CO2 from the air and strip it of its carbon atom to use in building sugar molecules – the oxygen left behind is released in the air as O2.

Carbon dioxide is also part of a much slower process: the geological carbon cycle. It has many components, and an important one is the transfer of carbon atoms from CO2 in the atmosphere to carbonates dissolved in the ocean. Once there, the carbon atoms are picked up by small marine organisms (mostly plankton) which make hard shells with it. After the plankton dies, the carbon shell sinks down to the bottom, joining scores of others and eventually forming limestone rock. Millions of years later that limestone may emerge to the surface, become weathered and release back the carbon atoms.

The Release of Excess CO2 Is the Problem

Coal, oil, and gas are fossil fuels made from the accumulation of aquatic organisms that are then subjected to high pressure and temperature. When we extract these fossil fuels and burn them, the carbon molecules once locked into the plankton and algae get released back in the atmosphere as carbon dioxide. If we look over any reasonable time frame (say, hundreds of thousands of years), the concentration of CO2 in the atmosphere has been relatively stable, the natural releases being compensated by the amounts picked up by plants and algae. However, since we have been burning fossil fuels we have been adding a net amount of carbon in the air every year.

Carbon Dioxide as a Greenhouse Gas

In the atmosphere, carbon dioxide contributes with other molecules to the greenhouse effect. Energy from the sun gets reflected by the surface of the earth, and in the process it is transformed into a wavelength more easily intercepted by greenhouse gases, trapping the heat within the atmosphere instead of letting it reflect out into space. Carbon dioxide’s contribution to the greenhouse effect varies between 10 and 25 % depending on the location, immediately behind water vapor.

An Upward Trend

The concentration of CO2 in the atmosphere has varied over time, with significant ups and downs experienced by the planet over geological times. If we look at the last millennia however we see a steep rise in carbon dioxide clearly starting with the industrial revolution. Since pre-1800 estimates the CO2 concentrations have risen by over 42% to current levels over 400 parts per million (ppm), driven by the burning of fossil fuels and by land clearing.

How Exactly Do We Add CO2?

As we entered an era defined by intense human activity, the Anthropocene, we have been adding carbon dioxide to the atmosphere beyond the naturally occurring emissions. Most of this comes from the combustion of coal, oil, and natural gas. The energy industry, especially through carbon-fired power plants, is responsible for most of the world’s greenhouse gas emission – that share reaches 37% in the U.S., according to the Environmental Protection Agency. Transportation, including fossil fuel powered cars, trucks, trains, and ships, comes in second with 31% of emissions. Another 10% comes from the burning of fossil fuels to heat homes and businesses. Refineries and other industrial activities release a lot of carbon dioxide, led by the production of cement which is responsible for a surprisingly large amount of CO2 adding up to 5% of the total worldwide production.

Land clearing is an important source of carbon dioxide emissions in many parts of the world. Burning slash and leaving soils exposed releases CO2. In countries where forests are making somewhat of a comeback, like in the United States, land use creates a net uptake of carbon as it gets mobilized by the growing trees.

Reducing Our Carbon Footprint

Lowering your carbon dioxide emissions can be done by adjusting your energy demand, making more environmentally sound decisions about your transportation needs, and re-evaluating your food choices. Both the Nature Conservancy and the EPA have useful carbon footprint calculators which can help you identify where in your lifestyle you can make the most difference.

What Is Carbon Sequestration?

Besides reducing emissions, there are actions we can take to reduce atmospheric carbon dioxide concentrations. The term carbon sequestration means capturing CO2 and putting it away in a stable form where it will not contribute to climate change. Such global warming mitigation measures include planting forests and injecting carbon dioxide in old wells or deep into porous geological formations.