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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Carbon dioxide equivalents

Posted on 1 September 2010 by Chris McGrath

Guest contribution by Dr Chris McGrath

There is considerable confusion surrounding climate stabilization targets based only on atmospheric carbon dioxide (CO2) levels and targets that group together all greenhouse gases and other factors using the term carbon dioxide equivalents (CO2-e or CO2-eq).

Because of this confusion some authors avoid using carbon dioxide equivalents altogether when discussing targets for stabilizing anthropogenic climate change.

The major advantages of carbon dioxide only targets are that they are simple to understand and can be measured directly with precision as illustrated by the famous Keeling Curve. The level of carbon dioxide is also the most relevant measure when considering ocean acidification, the “evil twin” of climate change.

However, because carbon dioxide is not the only greenhouse gas, policy targets based on stabilizing the combined effect of all greenhouse gases and other factors affecting the climate are important.

“Carbon dioxide equivalents” is a unit of measurement that allows the effect of different greenhouse gases and other factors to be compared using carbon dioxide as a standard unit for reference. The term is defined and used in slightly different ways in the context of emissions and atmospheric concentrations of greenhouse gases.

In the context of emissions of greenhouse gases, “carbon dioxide equivalents” refers to the amount of carbon dioxide that would give the same warming effect as the effect of the greenhouse gas or greenhouse gases being emitted. It is normally used when attributing aggregate emissions from a particular source over a specified timeframe. It is used in this way at national and international levels to account for greenhouse emissions and reductions over time. For instance, Article 3 of the Kyoto Protocol states targets for emissions reductions in terms of “aggregate anthropogenic carbon dioxide equivalent emissions of the greenhouse gases”. For example, Canada’s net greenhouse gas emissions across all sectors in 1990 totaled 593,998,462 tonnes of carbon dioxide equivalents.

In the context of atmospheric concentrations of greenhouse gases, the term “carbon dioxide equivalents” is used in two main ways:

  • Some authors use “carbon dioxide equivalents” to refer to the concentration of carbon dioxide that would give the same warming effect as the collective effect of all of the greenhouse gases in the atmosphere. This approach excludes the cooling effect of aerosols (e.g. Stern 2007).
  • Some authors define carbon dioxide equivalent concentrations as the net forcing of all anthropogenic radiative forcing agents including greenhouse gases, tropospheric ozone, and aerosols but not natural forcings. This approach includes the cooling effects of aerosols (e.g. Meinshausen et al 2006).

Targets for stabilizing temperature rises between 450-550 ppm “carbon dioxide equivalents” generally refer to the term in the latter usage, thereby considering all components of the atmosphere affecting global temperature rises. However, Gavin Schmidt noted the confusion surrounding the term on RealClimate in the context of comments by Tim Flannery that “we are already at 455 ppm carbon dioxide equivalents.”

The differences between the usages of these terms are very significant and care must be taken not to confuse them. The terms are very different quantitatively as is evident from the following points draw from IPCC (2007: 102):

  • Atmospheric carbon dioxide reached 379 ppm in 2005 and was increasing by around 2 ppm per year.
  • Including the effect of all long-lived greenhouse gases such as methane, the total concentration of atmospheric greenhouse gases was around 455 ppm carbon dioxide equivalents in 2005 (range 433-477 ppm).
  • However, the cooling effects of aerosols and landuse changes reduce radiative forcing so that the net forcing of human activities was about 375 ppm carbon dioxide equivalents for 2005 (range 311-435 ppm).

While net forcing of all greenhouse gases, aerosols and landuse changes is roughly equal to the effect of carbon dioxide alone at the present time, this may not remain the case in the future particularly if atmospheric pollution levels decrease (thereby reducing the cooling effects of aerosols). Stabilizing at 450-550 ppm carbon dioxide alone could mean significantly higher levels of stabilization when measured in carbon dioxide equivalents, leading to much higher levels of temperature rise than 2-3°C.

There is a real potential for governments and policy-makers to misunderstand and mis-apply information based on using either carbon dioxide only figures or carbon dioxide equivalents. However, there is no doubt that these terms will continue to be used in the future. In such circumstances, the best solution is to clearly flag the differences in the usage of these terms.

Authors writing about policy targets for climate change and ocean acidification can minimise the potential for confusion by including in their introduction a statement similar to the following:

“Here we use atmospheric carbon dioxide concentrations to discuss policy targets for anthropogenic climate change and ocean acidification. The use of carbon dioxide alone should not be confused with stabilization targets based on ‘carbon dioxide equivalents’, which includes the effects of all long-lived greenhouse gases and, in some contexts, the effects of other radiative forcing agents such as aerosols and landuse changes (IPCC 2007).”

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Comments 1 to 21:

  1. Isn't it possible we are lucky we have induced global warming because otherwise we may have well been on the way into another ice age? Today the Earth warms up and cools down in 100,000- year cycles. Geologic history reveals similar cycles were operative during the Carboniferous Period. Warming episodes caused by the periodic favorable coincidence of solar maximums and the cyclic variations of Earth's orbit around the sun are responsible for our warm but temporary interglacial vacation from the Pleistocene Ice Age, a cold period in Earth's recent past which began about 2 million years ago and ended (at least temporarily) about 10,000 years ago. And just as our current world has warmed, and our atmosphere has increased in moisture and CO2 since the glaciers began retreating 18,000 years ago, so the Carboniferous Ice Age witnessed brief periods of warming and CO2-enrichment. Following the Carboniferous Period, the Permian Period and Triassic Period witnessed predominantly desert-like conditions, accompanied by one or more major periods of species extinctions. CO2 levels began to rise during this time because there was less erosion of the land and therefore reduced opportunity for chemical reaction of CO2 with freshly exposed minerals. Also, there was significantly less plant life growing in the proper swamplands to sequester CO2 through photosynthesis and rapid burial. It wasn't until Pangea began breaking up in the Jurassic Period that climates became moist once again. Carbon dioxide existed then at average concentrations of about 1200 ppm, but has since declined. Today, at 380 ppm our atmosphere is CO2-impoverished
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    Response: Isn't it possible we are lucky we have induced global warming because otherwise we may have well been on the way into another ice age?

    Possible but unlikely. The current configuration of the Earth's orbit meant even without our CO2 emissions, we were most likely in for a long interglacial - the next ice age was tens of thousands of years away. The question is now moot - CO2 emissions have meant a planetary ice age is indefinitely off the table. The negative impacts of global warming over the next century impacting us, our children and grandchildren are of much more concern than a speculative ice age tens or thousands of years into the future.
  2. miekol, you need to read the following on this site : Climate's changed before It's not bad CO2 was higher in the past Animals and plants can adapt to Global Warming CO2 was higher in the Late Ordovician
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  3. Aerosols are in itself a pollution that is supposed to be avoided. The geoengeneering that counts on them has side effects and is not considered to be a safe solution to the warming. Its inclusion in the total CO2e figures could lead to unwanted incentives.
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  4. The obvious solution/use for higher CO2 levels is man made absorption via planting trees and crops where no trees or crops now grow. It is interesting that China hasn't been targeted as a GHG emitter. They use twice the coal that is used in the US. For the most part it is burned without the pollution controls used in the US. Perhaps this is a good thing as the SOx aerosols help counter the increase in CO2. "...long-lived greenhouse gases such as methane" Methane oxidizes to CO2 in roughly ten years. That is not long lived.
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  5. Re TOP @ #4 above:
    Methane oxidizes to CO2 in roughly ten years. That is not long lived.
    Which then has a residence time on the century timescale. Actually surprised and disappointed you nit on this one. Inre: Aerosols helping counter the GE of CO2 see here as well as here and here for a better discussion and background. The Yooper
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  6. "There is a real potential for governments and policy-makers to misunderstand and mis-apply information..." Good thing most of the world is sold on "DEMOCRACY". What exactly does this statement imply, McGrath?
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  7. RSVP, That statement clearly means that if I say carbon dioxide equivalents (not counting the effect of aerosols) and a policy maker thinks I mean carbon dioxide equivalents (counting the effect of aerosols) the policy maker might misunderstand what I said. Since the costs and benefits are different the decisions made are likely to be different. How does this relate to DEMOCRACY for those of us who are not paranoid?
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  8. TOP, firstly, where do we put all those trees and crops ? Where I live in London (and the South East of England in general) there doesn't appear to be much available land to give over to all those trees and crops - how many/much will we need ? Where do you suggest we put them ? Secondly, who, exactly, hasn't 'targetted' China ? Who HAS been targetted and by whom ? Anyway, although China is now the largest emitter of CO2, that is only a recent occurrence and they are only just ahead of the US, while having four times the population of the US. Per capita, they are well down the list (only just in the top 100), and far behind the US, Australia and Canada, for instance. China are also only just above the US in emissions of ALL GHG. But again, per capita, they are well down the list of offenders. As for coal, China may well use twice (maybe even three times) as much coal as the US, but don't forget that population, and therefore per capita, difference. China are also increasing the difference between themselves and the US in the use of renewable energy.
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  9. Yooper Which then has a residence time on the century timescale. Actually surprised and disappointed you nit on this one. It is an important distinction because CH4 is a much more powerful GHG. CO2 residency can be reduced by planting new trees. CH4 can't. Big point. Your points about the thinning of the haze about corresponds to the drive to reduce pollution. I remember driving in rush hour traffic where you could gag on the fumes. And there was the acidification of lakes in the East. That has all stopped in your time frame and now we have another problem, but one with a simple solution. Plant trees. JMurphy I could see where a Londoner would have trouble with trees in London proper. But then London isn't the only treeless place on the planet. Look at the Sahara. Big tree farm possible there. Anyplace you have water and sunlight you can grow trees. No need for doom and gloom about places to plant trees. I have a motto, "Make hockey sticks from trees, make trees from hockey sticks." If you look at the Western US you will see a lot of logged out areas. There is another prime candidate. States like Wisconsin that once were huge forests and now have a lot of worn out farm land are another target for trees. In addition to China burning a lot of coal (you can see the plume of CO out over the Pacific by satellite) they are contributing to the deforestation of the Amazon in order to grow soybeans and other foodstuffs. Huge farms have been created in what was once rain forest. Since China is a growing economy and things are constantly changing there, it should be the prime target to implement improvements. But while they may have the technology, they aren't using it. Per capita doesn't work with China or anywhere else. A small town in China is 4 million people. If the entire world was emitting GHG per capita like China, CO2 would drastically increase from today's numbers. Bad suggestion. Adding India to the loop would be even worse. China has no choice but to look to renewable energy. At the current rate of coal mining they will be out of coal in 50 years. They take a long view.
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  10. TOP, have you put in a quote for planting trees in the Sahara, or do you have some figures we can all see which would show how viable a proposition it is ? I also notice that Wisconsin is presently 46% forested and is also important agriculturally, especially in the dairy industry. How much of that do you think they should give up so that they could plant more trees ? And if you reckon that "CO2 would drastically increase from today's numbers" if the "world emitted GHG per capita like China", you obviously agree that it would be far, far worse if the world emitted at US rates, I suppose ? Actually, it's a shame that the world DOESN'T emit at China's rate.
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  11. JMurphy said... "Anyway, although China is now the largest emitter of CO2..." I am not sure China shares any more guilt here than those who consume their products...(which could be almost anyone).
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  12. michael sweet asks... "How does this relate to DEMOCRACY for those of us who are not paranoid?" Not paranoid as much as considering how technically capable your average politician may be.
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  13. And yes it is hopeless, except that God only left so much fossil fuel on this Earth (probably for this very reason).
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  14. Re: TOP
    It is an important distinction because CH4 is a much more powerful GHG. CO2 residency can be reduced by planting new trees. CH4 can't. Big point.
    I understand your first point. Please keep in mind that:
    1. While on a molecule-for-molecule basis methane is about eighty times stronger greenhouse gas than carbon dioxide due to it's much smaller concentration in the atmosphere, the greenhouse effect attributed to methane is 4-9% of the whole, whereas that attributed to CO2 is 9-26%. Thus, the Radiative Forcing (W/m2) of methane is 0.48 while that of CO2 is 1.46. And keep in mind that the concentrations of methane (1745 ppb) are a tiny fraction of that of of CO2 (390 ppm). Yes, the methane will provide a modest increase in RF during its 12-year residence time before it degrades into CO2. But the warming coming from that CO2 concentration will (like the Energizer Bunny) keep on going, and going, and going... And thus rendering the need to sequester the methane moot (because it presto/chango converts to CO2). 2. AFA planting trees. Sure. Yup, in the tropics, it makes sense. But in temperate and more northerly climes, reforestation actually causes the new forests to become part of the warming problem. Because the new tree canopies are darker, they effectively change the albedo of the newly forested area, causing more heat to be retained near the ground. See here and here for a nice discussion.
    And atmospheric aerosol pollution hasn't stopped per se so much as it has leveled off. CO2 concentrations have been doing their Icarus thing. Hence the emergence of the CO2 warming signal from the cooling effects of aerosols. Aerosols are still reducing the warming effect of CO2, not stopping it. Hence, we are buying the current generation time, at the expense of all future living generations. Unrestrained BAU will make this place a living H-E-Double-Hockey-Stick for our grandchildren's generation. Which will continue for centuries.
    States like Wisconsin that once were huge forests and now have a lot of worn out farm land are another target for trees.
    Really? Have you BEEN to Wisconsin lately? Dude, I drive all across Wisconsin and Northern Michigan, daily. Every viable plot of land with soil decent enough for growing is under cultivation. The rest is either houses, swamps or stands of crapwood, like popple or jackpine. All the good soils were "expatriated" to Illinois, Indiana & Ohio by some white lumpy rainish-stuff millenia ago... The remnants of the once mighty forests that stood here (the stumps of which measure up to 8 feet across) are still visible in places. Those giants took many centuries to reach maturity in these poor soils. Reforestation going on now focuses on the fastest-growing pines, which are soft and unsuitable for building purposes (they use them to make paper, toilet paper and particle boards of various sizes & thicknesses). The CO2 drawdown potential of softwoods vs hardwoods is pretty low. The 6 foot-thick oak in my backyard somehow escaped (probably because all of the old hardwoods here are natural bonsai's and don't grow that high compared to their more southern relatives) the loggers of the 1800's (some of them my ancestors) and is estimated to be about 450-500 years old. Lotta board feet, but not useful for Carbon Capture & Sequestration purposes. ---------------------------------------------------------------------------------------------------------------------- Re: RSVP
    I am not sure China shares any more guilt here than those who consume their products...(which could be almost anyone).
    Excellent point. In an intertwined, global, consumeristic society, all are guilty. You guys got me all fired up. I hates that. The Yooper
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  15. Daniel Bailey #14 In reference to trees not helping global warming... you point to links in statement... "See here and here for a nice discussion." I went there and read these. I think I understood the reasons explained as to why trees in northern latitudes may not help global warming, but this argument does not appear to make a distinction between conifers and deciduous trees. At least, I did not see this. I know near to nothing on this subject, but can only imagine that the mass of all those leaves that pile up in autumn come from CO2 taken directly out of the air. And as far as how deciduous trees affect albedo, after loosing their "canopy", they would therefore tend to shade snow less in the winter than conifers that do not shed as much. So it would seem that the planting of deciduous trees is good for the environment. The leaves are good for soil mulch, and many of these types of trees even provide food. All of which seems like a win win to me.
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  16. TOP said "States like Wisconsin that once were huge forests and now have a lot of worn out farm land " I was driving through farm land yesterday (not Wisconsin) seeing all that is grown... is the use of land for farming also not a form of carbon secuestration?
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  17. It would only be sequestration if nothing ever consumed the farm products eg wheat was stored in a mine and never used. Once we consume it we convert it back to CO2.
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  18. RSVP at 16:06 PM, well managed improved pasture systems are a more efficient form of CO2 sequestration than forestry and will build up the amount of carbon stored in the soil, and keep it there as long as it remains well managed. Governments are looking at providing incentives to allow primary producers to profit from such capturing and storing.
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  19. Daniel Bailey at 08:09 AM, re "Those giants took many centuries to reach maturity in these poor soils." The soils would not always have been poor, especially if it could grow giant trees. Trees drag a huge amount of essential nutrients out of the soil. In a permanent forestry situation, the foliage, branches and trees that die and fall to the forest floor allow those nutrients to be recycled. However if logging occurs all those nutrients that have been taken from the soil over the life of the tree are permanently removed from the system depleting the natural reservoir of nutrients and thus degrading the soil. Irrespective of what is grown from the soil, unless those nutrients that disappear down the road on a back of a truck are replaced, the system is not sustainable. In farming, the nutrients are generally regularly replaced, and sometimes in plantations, but rarely when natural forests are logged, hence formerly good soil can suddenly become poor soil.
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  20. A plea to our moderator and John Cook for forbearance, in that we are straying perilously off-topic on this thread. But a claim was made that I feel needs to be justified with a source: Re: johnd (19)
    "The soils would not always have been poor, especially if it could grow giant trees... good soil can suddenly become poor soil."
    What source do you reference that shows the soils in Northern Michigan and Wisconsin were ever good? Soils in this area not under cultivation already tend to have 1-12" of acidic loess overlaying either sand dunes or bedrock. Shorter growing seasons relative to warmer, more southerly climes have meant narrow, more densely packed tree growth rings than is typically found. As a result, the forests here take a long time to achieve maximal height and spread of canopy. Compare this to soils farther south in Ohio, Indiana and Illinois, where loessal drift is often measured in thousands of feet. Coupled with a warmer, longer growing season, forests there achieve greater heights and canopy spreads yielding much greater boardfeet yields when harvested. Even hardwoods. Unless you have more direct experience in the soil types and forests of this area than I, please cite your sources which offer testimony different from what my own direct, personal lifelong experience shows? Michigan background info the same for Wisconsin Not trying to be argumentative here. But this is really not a subject I'm willing to debate, nor is this a proper venue for anything other than science-based discussions and sharing of learnings. So please, share the learnings. The Yooper
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    Moderator Response: It's better to post a comment on a relevant thread and then on this thread post a link to that comment.
  21. Daniel Bailey at 06:27 AM, I also appeal to the moderators to allow reply to posts that have previously introduced a line of discussion and have been allowed to remain. I made a general response regarding how logging permanently removes large amounts of essential nutrients thereby depleting the nutrient reservoir within the soil. You made mention of the "remnants of the once mighty forests that stood here (the stumps of which measure up to 8 feet across)" which you indicated as being having been logged out. Mighty forests cannot grow without the nutrients firstly being available in the soil, there is only so much that can be absorbed out of thin air. If you could estimate the quality and quantity of timber that was removed from the forests, it would be easy to arrive at a reasonable estimate of what amount of nutrients the soil had available originally, but not once the timber was removed, thereby allowing some comparison of soil nutritional levels before and after.
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    Moderator Response: Please instead post your comment on a relevant thread, and on this thread post a comment saying you have replied over there, with a link to your specific comment. You can get the link to your comment by right clicking on its date-time stamp and choosing the resulting menu item Copy Link Location.

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