Options to Reduce CO2

COâ‚‚ Stabilization Wedges video Energy and Carbon: The Big Picture
Cutting Carbon Dioxide Emissions

Most of the carbon dioxide emissions from human activities (also known as anthropogenic CO2) result from burning fossil fuel for energy, making cement from limestone, and plowing land for farming. As a result of these activities, the amount of anthropogenic CO2 emissions to the atmosphere is increasing, and the level of CO2 in the atmosphere is rising.

CO2 from Fossil Fuels

Significant fossil fuel use began in the 1850s with the growth of steam engines, coal gas manufacturing, steelmaking (and eventually railroads), the internal combustion engine, and electricity generation. Nearly every year since, humans have used more fossil fuels and thus emitted more CO2—totaling more than 1600 billion metric tons of CO2.1 The fossil fuel and cement-making portion of annual human global CO2 emissions since 1751 is shown in the figure below.

The Numbers — Carbon Versus Carbon Dioxide

When talking about carbon in the atmosphere and anthropogenic emissions, the numbers can get confusing quickly. Scientists use the metric system to describe these amounts of carbon; engineers might use U.S. tons. Metric tons (also called tonnes) are about 10% larger than U.S. tons (2204.6 lb per metric ton).

More confusing are the two methods to quantify these concepts:

  1. tonnes of carbon dioxide (CO2)
  2. tonnes of carbon equivalent (C or Ce)

Both notations are commonly used in the scientific literature and public forums. Both are correct, but—unfortunately—not interchangeable. Ce is also used to represent all greenhouse gases, further complicating data comparisons.

CO2 is heavier than carbon (because of those two oxygens attached to the carbon), so as long as the number refers only to carbon dioxide, the numbers can be converted using the ratio of CO2 to C, i.e., 3.67 tonnes CO2 = 1 tonne carbon.

Here is how some key numbers on this page look using tonnes Ce versus tonnes CO2

billion tonnes*
Ce CO2
CO2 Emissions from 1751 to 2018431=1580
CO2 Emissions from 1751 to 2018431=1580
Global CO2 Emissions in 20068.3=30.5
Global CO2 Emissions in 205012=44
Projected Annual CO2 Emissions from OECD Countries3.5=13
Total CO2 from OECD Counties Between 2019 and 2050110=400
Annual CO2 Emissions in 2010 (S&P**)9=33
Estimated Annual CO2 Emissions in 2060 (S&P)18=66
CO2 Emissions Avoided to Stabilize Emissions at 2010 Levels225=825
Annual CO2 Emissions Avoided by Each Wedge after 50 years1=3.67
Cumulative CO2 Emissions Avoided by Each Wedge During Those 50 years25=92
*Billion tonne is the same as a gigatonne, metric gigaton, and Gt.
**S&P stands for Sokolow, R., 2011, Wedges Reaffirmed, Climate Central (see Note 6).

Graph of annual global CO2 emissions from fossil fuels Source: https://ourworldindata.org/grapher/co2-by-source

Every year, humans add CO2 to the atmosphere. Annual global CO2 emissions were 30.5 billion metric tons in 2006—the last year that the United States was the world leader in CO2 emissions. Annual emissions are projected to grow to 44 billion metric tons a year by 2050.2 This is another 1300 billion metric tons of anthropogenic CO2 over the projected period. Emissions are expected to remain relatively stable in the industrialized world (countries that belong to the OECD—the Organization for Economic Cooperation and Development)3, which means these countries will still be adding about 13 billion metric tons of CO2 to the atmosphere every year (about 400 billion metric tons of CO2 over 31 years). Most of the increase in the rate of CO2 emissions is expected to come from non-OECD countries, especially the rapidly developing economies of China and India.

The Numbers — Annual Versus Cumulative

Throughout this page, we reference annual CO2 emissions. Remember that those data are for just 1 year. For example, the second figure (Annual Global Carbon Dioxide Emissions, 2006–2040) depicts CO2 emission projections for every fifth year from 2015 to 2040. The total emissions over the period from 2015 to 2040 is the sum of those data plus the other 20 years not listed (roughly, 1030 billion metric tons CO2).

Graph of global energy-related carbon dioxide emissions in IEO2019 reference case (1990 to 2050) Source: U.S. Energy Information Administration, International Energy Outlook 2019

A significant part of this growth is for power generation. For example, one study estimated that the existing fossil fuel electricity generators would emit more than 300 billion metric tons of CO2 over their remaining life span, which is assumed to be 40 years. Two-thirds of the emissions would come from coal-fired plants (natural gas and oil providing the remainder). About 20% would come from the United States and European Union, with 80% of the emissions from China, India, and the rest of the world.4

Coal and Global Development video Coal and Global Development

Tackling CO2 Emissions — Stabilization

For more than a decade, scientists from Princeton University have advocated tackling the problem of increased CO2 emissions in stages, first by stabilizing emissions at current levels using technologies available now and then focusing on reducing emissions further using technology advances that will happen in the future.5 This strategy, called Stabilization Wedges or The Wedge Concept, lays out the magnitude of the problem, suggests practical steps to eliminate emissions, and realistically estimates the impact. The authors chose a 50-year time line for holding CO2 emissions constant while developing and adopting new technologies. The emissions eliminated with existing technology are illustrated in the graph to the right as the stabilization triangle. The results show that no one action—or even a half dozen—will be enough to eliminate all CO2 emissions above 2010 levels.

The Stabilization Triangle

In 2011, scientists projected an unchecked CO2 emissions rate that would increase our annual output of CO2 emissions by 33 billion tons in 50 years.6 That equates to 825 billion additional tons of CO2 into the atmosphere in 50 years.

Graph of projected annual global CO2 emissions

Divide and Conquer: The Wedge Concept

The CO2 wedge concept suggests ways in which we can apply today's technology to reduce CO2 emissions in large amounts called wedges. Each CO2 reduction wedge would be designed to prevent 92 billion tons of CO2 from entering the atmosphere over 50 years. When the concept was first proposed in 2004, seven wedges were needed to stabilize global CO2 emissions through 2054.7 In 2011, scientists revised the number to nine wedges to account for the higher global emission rate. If we start applying these actions now to create nine wedges, we could prevent 825 billion tons of CO2 from entering the atmosphere in 50 years.6

What Actions Might These Wedges Include?

Stabilization wedges chart Examples of CO2 wedges: actions that could eliminate 92 billion metric tons of CO2 over 50 years.8
Image modified from https://cmi.princeton.edu/wp-content/uploads/2020/01/carbon_plan.pdf (page 5)

The authors of the wedge concept provided 15 examples of actions. Each action would result in one full wedge of carbon dioxide not released to the atmosphere over the subsequent 50 years. We could also achieve the equivalent of one full wedge by combining partial actions from two or more wedges. The wheel does not intend to show all possible actions that could be used to stabilize CO2 emissions over several decades. It does, however, show the magnitude of actions needed to make significant eliminations in CO2 emissions. These and many other actions will be needed to stabilize emissions over the next several decades.

The wheel shows that both terrestrial sequestration (as part of Agriculture and Forestry) and geologic sequestration (as part of carbon capture and storage) play key roles in the CO2 wedge concept. The U.S. Department of Energy is investigating potential locations to implement CCS through its CarbonSAFE Initiative. Two potential locations are in North Dakota and Wyoming.

Coal, Electricity, and CO2 Coal, Electricity, and CO2

Notes and References
  1. Emissions from fossil fuels and cement production from the Carbon Dioxide Information Analysis Center at AppState, https://energy.appstate.edu/research/work-areas/cdiac-appstate (accessed January 2020).
  2. Projected CO2 emissions estimates from the U.S. Energy Information Agency's International Energy Outlook 2019. https://www.eia.gov/todayinenergy/detail.php?id=41493 (accessed January 2020).
  3. The Organization for Economic Cooperation and Development (OECD) consists of industrialized democracies, 36 countries including most of North America and Europe, Japan, Australia, New Zealand, South Korea, and Chile. The bulk of the projected economic growth and growth in anthropogenic CO2 emissions is in the non-OECD countries of China and India. https://www.oecd.org/about/members-and-partners/ (accessed January 2020).
  4. Data from Steven J. Davis and Robert H. Socolow, 2014, Environ. Res. Lett. v.9, no.8, 084018. The paper lays out the methodology, limitations, and uncertainties. https://iopscience.iop.org/article/10.1088/1748-9326/9/8/084018/pdf (accessed January 2020).
  5. Stabilization articles published between 2004 and 2011 are available at https://cmi.princeton.edu/resources/stabilization-wedges/articles-and-videos (accessed January 2020).
  6. Socolow, R., 2011, Wedges Reaffirmed: Bulletin of the Atomic Scientists. https://cmi.princeton.edu/wp-content/uploads/2020/01/Wedges_reaffirmed_-_Bulletin_of_the_Atomic_Scientists.pdf (accessed January 2020).
  7. Pacala, S., and Socolow, R., 2004, Stabilization wedges: solving the climate problem for the next 50 years with current technologies: Science, v. 305, no. 5686 p. 968–972. https://cmi.princeton.edu/wp-content/uploads/2020/01/Stabilization_Wedges_-Solving_the_Climate_Problem_for_the_Next_50_Years_with_Current_Technologies_Science.pdf (accessed January 2020).
  8. Wheel of wedges graphic modified from the article "A Plan to Keep Carbon in Check" by R. Socolow and S. Pacala in Scientific American, 2006, using the updated information at https://cmi.princeton.edu/resources/stabilization-wedges/ (accessed January 2020).