How much warming in the pipeline? Part 1 – CO2-e

You may have heard that the planet is committed to further warming and sea level rise, irrespective of what choices we now make to reduce carbon emissions. The global warming century trend that was observed from 1906 to 2005 was 0.74°C (with a 90% uncertainty range of 0.56°C to 0.92°C), with more warming occurring in the Northern over Southern Hemispheres, and more over land compared to oceans. Yet, based on our understanding of the climate impact of greenhouse gases (GHG) such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and other trace gases, we should have observed even more warming than this. Actually, when you put all the pieces together, the expectation is for much more warming.

But before I tackle the critical issue of just how much more warming is still in the pipeline (in another post), it is important to explain the concept of carbon dioxide equivalents (CO₂-e). This term initially confuses a lot of people, but it’s not really that difficult to grasp once it’s been explained.

To start, you need to understand that from a global warming perspective, we are interested in the changes in GHGs – which causes an energy imbalance. The pre-industrial and current concentrations of well-mixed long-lived GHG are 278 parts per million (ppm) for CO₂ (now 383), 700 ppb (pp billion) for CH₄ (now 1,775), and 270 ppb for N₂O (now 320). Most of the other trace greenhouse gases (there are plenty), such as chloroflourocarbons (CFCs) and sulphur hexafluoride (SF₆), are almost exclusively a result of industrial activity. Now to quantify their relative contribution to global warming, we need to find a way of putting all of these individual gases (and other climate drivers) on an equal – or equivalent – footing. That’s where CO₂-e comes in.

The IPCC has two ways of expressing CO₂-e. The first is known as concentration equivalence, which has units of ppm CO₂-e. This definition asks: for a given change of a climate forcing agent (such as a greenhouse gas or aerosol), what change in the concentration of CO₂ would have been required to have the same effect as the additions of this forcing? ‘Effect’ is here defined in terms of radiative forcing (RF), which is (loosely) the change in the amount of incoming (to Earth) versus outgoing (to space) radiation/energy, measured in watts per square metre (w/m²). A positive RF warms, negative cools.

The other way of representing CO₂-e is by emission equivalence, which has units of the mass of CO₂-e per unit time. For instance, you could define the climate warming impact over a 100-year period of 1 million tonnes (Mt) of methane as being same as if you’d released 25 Mt of CO₂. If you shortened the time period to 20 years, that 1 Mt of methane would be the same as 72 Mt of CO₂. The difference between time periods is because methane is more powerful GHG than CO₂, but it breaks down more rapidly. This expression is also known as the global warming potential of a GHG.

Right. Now if you add up all of the GHGs (+100 ppm for CO₂, +1,000 ppb for CH₄, +50 ppm for N₂O, and so on), and multiply each gas by its concentration equivalence, then you come up with a number of 455 ppm CO₂-e for the atmosphere in 2005. Also, there is an uncertainty connected with the RF of each gas, due to incomplete scientific understanding. This means that the above quantity of 455 is actually just the average of a probability (statistical) distribution. I simulated this distribution in a computer, using the error bounds on individuals RFs given in AR4, and you can see the result in the figure at the top of this post (lower plot). 455 ppm CO₂-e is our best estimate, but it could be less than 400 or higher than 500 ppm CO₂-e. Hey, you ask, what warming does this commit us to? Well, you’ll have to wait for Part II for this answer!

But here’s where the rub comes. If you want to understand what we are doing to the climate system right now, or indeed have been doing for the past few centuries, you can’t just look at changes in GHG. You need to consider all the other things, besides GHG, which can and do ‘force’ the climate (i.e., cause warming or cooling across the planetary system). Changes in the sun, for instance can have a positive (+ve) or negative (-ve) forcing effect. As can melting polar ice and snow (+ve), land clearance (+ve for the extra GHG, -ve for exposed land), aerosols (+ve for black carbon, -ve for sulphates, +ve or -ve for clouds). And so on… (More on this in Part II – for now, check out the figure to the right for a summary). Climate models, of course, consider all of these, and moreover, attempt to evaluate the effect they have on each other.

If you make the necessary calculation to add and subtract all of these climate forcings, then you get the total current concentration equivalence of CO₂. This is 375 ppm CO₂-e. By a queer coincidence, this number (at least the best estimate) is very close to the actual concentration of just CO₂ alone (383 ppm). But the probability distribution is pretty wide – the true value of CO₂-e could be as low as 300 ppm or as high as 450 ppm. The simulated distribution, using the IPCC RF error bounds, is also shown in the figure at the top of the post (upper plot). This value of 375 CO₂-e is the actual forcing that is currently acting to warm the oceans, melt ice, and cause gradual upwards changes in average air temperature.

In brief then, we are NOT currently feeling the impact of 450 ppm CO₂-e. Because of aerosols and other cooling factors, we are most probably experiencing the partial result of the extra energy being trapped by about 375 ppm CO₂-e. Indeed, we are not even feeling all of that, at least in terms of changes in air temperature, because so much energy is currently going into heating large bodies of water and melting huge chunks of ice. But we will, given time, feel all of this and much more, if/when most of the cooling forcings start to go away (Part II)…

Of course, if you are technically minded or require more convincing beyond the few paragraphs of explanation I provide herein, I suggest you read chapter 2 of the IPCC Fourth Assessment Report (AR4), which has 106 pages on the topic of Changes in Atmospheric Constituents and in Radiative Forcing.

Or, for another take at this topic, BraveNewClimate reader Chris McGrath has made an excellent attempt to explain these concepts in simple terms, in comments posted here and here. I’ll reproduce them below (with some edited corrections), as they are very clear descriptions:

RealClimate gives a good explanation of carbon dioxide equivalents when used in terms of atmospheric concentrations rather than emissions at http://www.realclimate.org/index.php/archives/2007/10/co2-equivalents/.

Garnaut was referring to 455 ppm CO₂-e in terms of the effect of all GHGs currently in the atmosphere without any reduction for the cooling effects of aerosols.

The IPCC (2007: 102) summarised the effects of GHGs and aerosols as follows:

“Atmospheric CO₂ concentrations [reached] 379 ppm in 2005 … The direct effect of all the long-lived GHGs is substantial, with the total CO₂ equivalent concentration of these gases [in 2005] estimated to be around 455 ppm CO₂-eq (range: 433-477 ppm CO₂-eq). The effects of aerosols and landuse changes reduce radiative forcing so that the net forcing of human activities is in the range of 311 to 435 ppm CO₂-eq, with a central estimate of about 375 ppm CO₂-eq.”

Here is my explanation of the background to these figures and the term “carbon dioxide equivalents” (Nb. I am lawyer, not a climate-scientist, so take this with a grain of salt. It comes from an article I wrote in a law journal last year):

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For ease of comparison and modelling greenhouse gas emissions and atmospheric concentrations are commonly measured in a standard unit known as “carbon dioxide equivalents” (CO₂-e or CO₂-eq). This term is defined and used in slightly different ways in the context of emissions and atmospheric concentrations of greenhouse gases. The unifying theme for the different uses is that they allow the effect of different greenhouse gases to be compared using carbon dioxide as a standard unit for reference. It may be noted also that some authors and inventories refer to “carbon equivalents” when discussing quantities or atmospheric concentrations of greenhouse gases. Figures for “carbon equivalents” can be converted to “carbon dioxide equivalents” by multiplying by 44/12 to take account of the different molecular weights. Carbon equivalents can be a more meaningful term when considering carbon not held in the form of CO₂, such as coal. However, the IPCC generally uses “carbon dioxide equivalents”.

When referring to greenhouse gas emissions, “carbon dioxide equivalent” 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. Article 3 of the Kyoto Protocol states targets for emissions reductions in terms of “aggregate anthropogenic carbon dioxide equivalent emissions of the greenhouse gases listed in Annex A.” Using this approach, Australia’s net greenhouse gas emissions across all sectors in 2004 totalled 564.7 million tonnes of carbon dioxide equivalent. The expected carbon dioxide equivalent emissions from burning different fuels can also be calculated using a standard methodology (see http://www.climatechange.gov.au/workbook/pubs/workbook-feb2008.pdf).

When referring to atmospheric concentrations of greenhouse gases, “carbon dioxide equivalent” refers 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. Put in a more technical way, this means the atmospheric concentration of carbon dioxide that gives a radiative forcing equal to the sum of the forcings from all of the individual greenhouse gas in the atmosphere.

Houghton (2004: 259) explains that when converting from carbon dioxide only concentrations to carbon dioxide equivalent concentrations, the amount that needs to be added varies with different concentrations of greenhouse gases as the relationship between radiative forcing and concentration is non-linear. For example, setting stabilisation targets of atmospheric carbon dioxide at 450 or 550 ppm would become about 520 or 640 ppm carbon dioxide equivalents, respectively, due to the additional warming effect of other greenhouse gases. Stern (2007) used the term in this manner. These approaches exclude the cooling effect of aerosols.

However, the use of this term is not uniform when discussing stabilisation targets as 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. Hare and Meinshausen (2006) is an example of this approach. The inclusion of aerosols alters the meaning considerably. As noted earlier, the IPCC’s latest report indicates that the current radiative forcing of non-carbon dioxide greenhouse gases and aerosols effectively cancel each other, so that the net effect of all radiative forcing components is currently roughly equal to the effect of carbon dioxide alone. However, this offsetting effect is unlikely to remain in the future as improved pollution controls are expected to significantly reduce the cooling effect of aerosols over the course of coming decades: Meinshausen et al (2006).

With this context explained, it is understandable why Hansen et al (2008) prefer to use CO₂ only targets and avoid the use of CO₂-eq targets but for non-climate scientists (such as myself) we have to largely work with the approach adopted by the IPCC and international framework so we cannot avoid using CO₂-eq. It is important to understand exactly what people mean when they refer to the term.

References:

Hansen et al (2008) “Target CO2 – Where should humanity aim?” (in review – see draft at http://arxiv.org/abs/0804.1126).

Hare B and Meinshausen M, “How much warming are we committed to and how much can be avoided?” (2006) Climatic Change 75: 111.

Houghton J (2004), Global Warming: The Complete Briefing (3rd ed, Cambridge University Press, Cambridge)

IPCC (2007), Climate change 2007: Mitigation. Contribution of Working group III to the Fourth Assessment Report of the IPCC (Cambridge University Press, Cambridge), http://www.ipcc.ch/ipccreports/ar4-wg3.htm

Meinshausen M, Hare B, Wigley TML, van Vuuren D, den Elsen MGJ, and Swart R (2006), “Multi-gas emissions pathways to meet climate targets” Climatic Change 75: 151.

Stern N (2007), The Stern Review on the Economics of Climate Change (Cambridge University Press, Cambrige).

And…

Tony states that, “Garnaut says Australia should establish its emissions reduction framework within an agreed global target to stabilise atmospheric carbon at between 450 and 550 parts per million (ppm): the present level is 387 ppm.”

Garnaut recommended aiming between 450 and 550 ppm carbon dioxide equivalents (CO2-e) and he states in his supplementary draft report (at page 29) “Today, the atmospheric concentration of greenhouse gases is about 455 CO2-e ppm (2005)”.

Garnaut is using CO₂-e in a particular way. He is not talking about atmospheric carbon dioxide levels but the combined effects of all greenhouse gases and excluding the cooling effects of aerosols and land use changes.

Anyone who does not understand the difference between targets based on atmospheric carbon dioxide concentrations (which are currently at 387 ppm) and carbon dioxide equivalent concentrations needs to learn about these important terms if they are going to follow the policy debate on emissions reduction and stabilisation targets.

Carbon dioxide equivalents is a term used in different ways for emissions and atmospheric levels of greenhouse gases. Atmospheric carbon dioxide equivalent levels were around 455 ppm CO₂-e in 2005 if you ignore the cooling effects of aerosols but around 375 ppm CO₂-e in 2005 if you include the cooling effects of aerosols and landuse changes: see the IPCC (2007) Working Group III report at page 102, available at http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter1.pdf.

Well said Chris!

Part II of this series will look at a new paper published recently in PNAS on committed warming, which I think, once explained, will blow your mind…