Three salient global mitigation pathways, assessed in light of the IPCC carbon budgets

This paper examines the levels of risk associated with three widely discussed global mitigation pathways: a Strong 2ºC pathway, a Weak 2°C pathway, and a G8 pathway. A very large number of analyses and debates refer to these or quite similar pathways. This paper assesses the three pathways in the light of Working Group I’s recently released contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC 2013), which provided three specific global carbon dioxide (CO2) budgets, and associated them with specific risks of a global surface temperature increase of more than 2°C by the end of this century, relative to the 1850–1900 average.

Figure 1 presents the three pathways.


Figure 1. Three politically salient mitigation pathways: G8 (red), Weak 2°C (blue), and Strong 2°C (green). Also shown (dotted lines) are three pathways consistent with the carbon budgets given by the IPCC, consistent with limiting warming to 2°C with 66%, 50%, and 33% probability, given non-CO2 emissions as per RCP2.6.[1]

The key features of these pathways and the findings of our analysis can be summarized as follows:

  • The Strong 2°C pathway is defined to be an extremely ambitious mitigation pathway that can still be defended as being techno-economically achievable (Höhne et. al. 2013). Emissions peak in 2014 and reach an annual peak reduction rate of about 6.1% per year (6.0% for fossil CO2 only). Cumulative carbon dioxide emissions after 2012 are 780 gigatonnes CO2 (Gt CO2), which is well within the IPCC’s budget of 1,010 GtCO2 for maintaining a 66% likelihood of keeping warming below 2°C.
  • The Weak 2°C pathway is fashioned after well-known and often-cited emissions pathways that are typically presented as having a “likely” (greater than 66%, in the IPCC’s terminology) chance of keeping warming below 2°C.[2] Emissions peak in 2014 and reach a maximum annual reduction rate of 3.3% per year (4.4% for fossil CO2 only). Cumulative carbon dioxide emissions from 2012 onward are 1,270 Gt CO2. This exceeds the IPCC’s budget of 1,120 GtCO2 for maintaining a 50% chance of keeping warming below 2°C, suggesting that this pathway carries substantially higher risks than previously believed.
  • The G8 pathway, a marker of the high-level political consensus in developed countries, is based on emissions targets given in an official declaration of the Group of Eight industrialized countries at its 2009 Summit in L’Aquila, Italy (G8 2009). This pathway is not precisely specified in this declaration, but is sufficiently well-defined that we can compare it with the IPCC budgets. Emissions peak in 2020, decline by a maximum of 4.9% per year (6.0% for fossil CO2 only). Its cumulative carbon dioxide budget of 1,610Gt CO2 considerably exceeds the IPCC’s budget of 1,410 GtCO2 for maintaining a 33% chance of keeping warming below 2°C[3]. We thus find that its chance of keeping warming below 2°C is far less than 33%.

Decision-makers face a choice among future pathways – a choice that will reflect political, economic and ethical considerations as much as science. This paper shows the consequences of choosing a less-ambitious pathway: a marked increase in climate risk. More specifically, according to the IPCC’s budget numbers, it shows that among the three politically salient pathways assessed here only the very ambitious Strong 2°C pathway is likely to hold warming below 2°C.


Table 1. Key data for the three pathways, and the IPCC carbon dioxide budgets against which to compare them.


In climate policy debates, there is broad agreement on the need to reduce greenhouse gas emissions to avoid dangerous climate change impacts – but not on how fast or how soon. This paper examines the levels of risk associated with three global mitigation pathways: a Strong 2°C pathway, a Weak 2°C pathway, and the G8 pathway. These pathways or very similar ones figure in a very large number of analyses and policy debates, as they correspond to three extremely important socio-political storylines.

This paper assesses the three pathways in the light of Working Group I’s recently released contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC 2013)which provided three specific global carbon dioxide (CO2) budgets, and associated them with specific risks of a global surface temperature increase of more than 2°C by the end of this century, relative to the 1850–1900 average.

The Strong 2ºC pathway

The Strong 2ºC pathway is defined to be a challenging mitigation pathway that can still be defended as techno-economically achievable (Höhne et al. 2013). Cumulative emissions of all greenhouse gases for 2012–2100 are 1,390 Gt CO2. Non-CO2 emissions have a “floor” of 5 Gt CO2 annually, to account for the potentially irreducible requirements of agriculture.[4] Fossil-fuel emissions and emissions from land use change and forestry (LUCF) both decline asymptotically to zero, with budgets of 725 and 50 Gt CO2 respectively for 2012–2100; adding a small additional contribution of 5 GtCO2 to the post-2100 for continued exponential decline.

Key features of the Strong 2ºC pathway include the aggressive front-loading of mitigation, the above-mentioned floor on non-CO2 gases, and the absence of negative emissions of any kind.[5] This defines it as a highly precautionary pathway. Rather than allowing higher emissions in the early decades and assuming they’ll be offset by negative CO2 emissions in later decades (as in the IPCC’s rapid-mitigation pathway, RCP 2.6) or by allowing lower non-CO2 emissions, this pathway emphasizes the need for immediate and dramatic mitigation. It can fairly be characterized as an “emergency mobilization” pathway.

On this pathway, global emissions peak in 2014; the fastest rate of fossil CO2 reductions is 6.0% per year, and for all GHGs combined, it is 6.1%. Cumulative non-CO2 emissions are lower than those in the RCP2.6 pathway by about 15%, but annual non-CO2 emissions still remain above a fairly high minimum level (“floor”) of around 5 Gt CO2e. This plausible but optimistic estimate of the irreducible emissions of methane and N2O associated with agriculture limits the extent to which the overall target depends on a miracle in agricultural technology – or, in a less sanguine view, the sacrifice of adequate nutrition for the preservation of the climate.


Figure 2: Strong 2°C pathway, disaggregated into fossil CO2, LUCF CO2, and non-CO2 gases.


Table 2: Detailed budget by gas and source for Strong 2°C pathway. CO2e is calculated using 100-year global warming potentials per Forster et. al. (2007).

The Weak 2°C pathway

The Weak 2°C pathway is closely based on the Climate Action Tracker (CAT) 2°C pathway and essentially matches the reference pathway in the United Nations Environment Programme (UNEP) “emissions gap” reports (see footnote 1 and UNEP 2012). It has been corrected to match recent history, but otherwise has the same profile and cumulative CO2-equivalent emissions. Like the Strong 2°C pathway, it peaks immediately (2014). Its rate of COdecline then increases gradually, reaching 3.3% by 2020, and 4.4% by 2050; for all gases combined, the maximum rate of decline reaches 3.4%. Also like the CAT 2°C pathway, its emissions in 2020 are 44 Gt CO2e. Cumulative emissions (2012–2100) are about 1,160 Gt COfor fossil fuels, with an additional 110 Gt CO2 for LUCF, and 35 Gt CO2 after 2100 assuming continued exponential decline. We have adopted the RCP2.6 emissions pathway for the non-CO2 component of the 2°C pathway, contributing another 715 Gt CO2e from non-CO2 gases to the total budget.

Like the Strong 2°C pathway, this pathway front-loads mitigation, assumes no negative emissions, and has a substantial non-CO2 floor out to 2100 of approximately 6 GtCO2eq (consistent with the RCP2.6 pathway). However, and despite the fact that it would represent a dramatic change from business as usual, the Weak 2°C pathway, difficult though it may be, is widely considered achievable. For example, a variety of parties, including UNEP (2012 and previous publications), have identified technological options and policy measures that would close the gap between today’s mitigation pledges and the 44 Gt CO2 global emissions target for 2020. While the required policies would represent substantial transformations in energy policy, investment patterns and governance systems, they are well within reach.


Figure 3: Weak 2°C pathway, disaggregated into fossil CO2, LUCF CO2, and non-CO2 gases.


Table 3: Detailed budget by gas and source for 2°C pathway. CO2e is calculated using 100-year global warming potentials per Forster et al. (2007).

The G8 pathway

The G8 pathway is based on the emissions goals expressed by the ministers of the G8 at their 2009 conference at L’Aquila, Italy (G8 2009). The L’Aquila text is ambiguous in several key ways, as it specifies only that the peak must be “as soon as possible” and omits the reference year against which goal the 50% reduction in “global emissions” by 2050 is to be calculated. We have optimistically interpreted this as a pathway that peaks in 2020, and we have taken 2005 as the base year for the 2050 reduction of 50%. The G8’s failure to explicitly state 1990 as the base year, and the fact that this comes from the G8 countries (some of which have used 2005 as a base year for their pledges) suggests that our use of 2005 as the base year is justifiable in the pathway. A more stringent interpretation of the de facto official pathway would arise if it were based on the 1990 base year proposed by the EU, but this proposal has not been accepted by the G8.[6]

Overall the G8 pathway has cumulative carbon dioxide emissions (2012–2100) of 1,485 Gt COfor fossil fuels, and 125 for LUCF, with an additional 45 GtCO2 after 2100 assuming continued exponential decline. (As with the Weak 2°C pathways, its non-CO2 emissions is adopted from RCP2.6, contributing another 715 Gt CO2e to the total budget.)

Interestingly, while this pathway has considerably greater overall emissions than the other pathways, it’s peak rate of emissions reductions is quite high. Fossil CO2 reductions reach 6.0%/year (4.9%/year for all GHGs), which is comparably strenuous to the Strong 2°C pathway. This is because the late peak (2020) necessitates very rapid reductions in order to meet the 50% reduction target for 2050.


Figure 4: The G8 pathway, disaggregated into Fossil CO2, LUCF CO2, and non-CO2 gases.


Table 4: Detailed budget by gas and source for the G8 pathway. CO2e is calculated using 100-year global warming potentials per Forster et al. (2007).

How we estimate climate risk for the three pathways

Calculating a temperature change from a given emissions pathway – even in probabilistic terms – is not a simple task. It was only with the widely cited publication of a landmark paper, Meinshausen et al. (2009), that 2°C risk probabilities were systematized in a widely accepted manner. The Meinshausen paper was supplemented by a calculator (giving results derived from the MAGICC climate model) that provided 2°C risk probabilities (including an often-cited “illustrative default”) based on 2000–2050 cumulative CO2 emissions. These estimates were very widely adopted – for example, to provide the figures in the influential Carbon Tracker Initiative (2011) study on “unburnable carbon,” and in’s “Do the Math” campaign, which used the Carbon Tracker numbers. As noted above, many of the 2°C pathways now in use in climate policy, such as the Climate Action Tracker 2°C pathway and the principal reference pathway in UNEP’s “emissions gap” reports, are similarly based on the Meinshausen analysis.

However, and critically, the Fifth Assessment Report (IPCC 2013) presents more conservative estimates of the budgets associated with various 2°C risk probabilities. In particular, it gives the 2°C-compliant carbon budgets as follows:

Limiting the warming caused by anthropogenic CO2 emissions alone with a probability of >33%, >50%, and >66% to less than 2°C since the period 1861–1880, will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1570 GtC (5760 GtCO2), 0 and about 1210 GtC (4440 GtCO2), and 0 and about 1000 GtC (3670 GtCO2) since that period, respectively. These upper amounts are reduced to about 900 GtC (3300 GtCO2), 820 GtC (3010 GtCO2), and 790 GtC (2900 GtCO2), respectively, when accounting for non-CO2 forcings as in RCP2.6. An amount of 515 [445 to 585] GtC (1890 [1630 to 2150] GtCO2), was already emitted by 2011.[7]

Converting these figures from gigatonnes of carbon (GtC) to gigatonnes of carbon-dioxide equivalent (Gt CO2e) and then partitioning those CO2e gigatonnes between COand non-CO2 and distributing them across time (as shown in Figure 1) yields the figures in Table 5.

For comparison with the IPCC budgets, Table 6 shows the cumulative budgets from 2012 forward for the three pathways examined in this paper. Figure 5 then displays all six budgets, allowing a direct comparison and enabling one to draw inferences about their relative likelihood of keeping warming below 2°C.


Table 5: The IPCC’s three carbon budgets, translated into more easily digestible form.


Table 6: The 2012-2100 emissions budgets associated with the pathways, expressed in GtCO2 terms for comparison with the IPCC budgets (figures rounded to nearest 5 Gt CO2.) Note that the “2012 forward” budgets include emissions beyond 2100. The post-2100 budgets are calculated assuming a constant continuing exponential decline.


Figure 5: The remaining (post 2012, forward to exhaustion) CO2 budgets associated with the three mitigation pathways, as well as the IPCC’s three carbon budgets, shown in exactly the same terms.

From this, we can draw conclusions about risk probabilities that are consistent with the analysis in the IPCC’s AR5:

  • The Strong 2 °C pathway, with 780 Gt CO2 of emissions from 2012 forward, would have a considerably greater than 66% chance of keeping warming below 2°C, as both its CO2 and non-CO2 budgets are considerably lower than those characterizing the IPCC’s 66% pathway.
  • The Weak 2°C pathway would have a 33% to 50% probability of keeping warming below 2°C, as its carbon budget lies between the IPCC 33% and 50% budgets, while its non-CO2 emissions are equivalent to the RCP2.6 emissions assumed by the IPCC. Note that this estimate, based on the recent IPCC analysis, is quite a bit more pessimistic than Climate Action Tracker’s own description or other earlier assessments of equivalent pathways, such as in UNEP’s “Emissions Gap” report (UNEP 2012), which assign a 66% chance of staying below a 2°C increase in 2100 (“likely” in the IPCC’s terminology).
  • The G8 pathway has a CO2 budget well above the IPCC’s 33% budget. Given this, it cannot plausibly be considered to be a 2°C pathway, notwithstanding the fact that the G8 formally stated its emissions goals in the context of the 2°C objective.

Pathways, assumptions and risks

As the “2°C threshold” came to define the global climate effort, it became common to characterize any given emissions pathway or greenhouse gas budget by its probability of causing a temperature increase of 2°C or more. (More recently, as the risks of 2°C have become clearer, and stricter targets such as 350 ppm and 1.5°C have come to the fore, this standard practice has been applied to these targets as well.) However, it is important to note that there is no unique pathway associated with any given temperature or risk threshold. Many different pathways can yield the same likelihood of exceeding 2°C (or any other target).

Conversely, and notwithstanding the IPCC’s decision to specify 2°C emission budgets, an emission budget alone does not fully determine the probability of limiting warming to 2°C (or to any other given threshold). Other factors must also be specified. In general, once you have an emissions budget, there are three additional choices to make: how to distribute emissions reductions over time, how to allocate the budget between CO2 and non-CO2 greenhouse gases, and whether (and to what degree) “negative emissions” are considered.[8] In the next sections we consider these three important choices.

Front-loading vs. back-loading of emissions reductions

Any given emissions budget can be distributed in a way that favors near-term or far-term reductions (“front-loading” into the first decades, or “back-loading” to later decades). There are several common arguments for deferring mitigation, such as that it reduces immediate costs, making it easier to generate political support, and that it reduces total costs through discounting and technological change. The latter argument is particularly problematic, for as the International Energy Agency has noted, deferring mitigation is probably a “false economy” and can significantly increase total costs over time.[9] Back-loading also increases projected impacts and risks in a variety of ways, the most important being that it may cut off future options. For example, if impacts are greater than expected or if the effectiveness of mitigation measures is lower, more mitigation may be required than initially anticipated. And while it may be possible to compensate with additional investment or mitigation effort, it may not be – some goals may simply have slipped out of reach, or might require undesirable measures (e.g. geoengineering) that would not otherwise have been necessary.

This is not to trivialize the extreme difficulties posed by a dramatic transition from today’s world of steadily growing global emissions to a new world of rapid and sustained global reductions. But it does suggest that a precautionary pathway should front-load mitigation to the greatest extent possible.

CO2 vs. non-CO2 emissions

Another crucial element in the risk estimation of different emissions pathways is the set of assumptions made about non-CO2 greenhouse gases. In the IPCC’s pathways, non-CO2 forcings are taken to be consistent with RCP2.6, and we have done likewise for the Weak 2°C pathway and G8 pathway. Non-CO2 emissions in the Strong 2°C pathway, on the other hand, are taken to be about 15% lower cumulatively than in RCP2.6, and to level off toward the second half of the century at a slightly lower level. (See the detailed discussion of the Strong 2°C pathway.)

As noted previously, assumptions about non-CO2 emissions are extremely important, and yet the role and dynamics of non-CO2 gases and other forcings (e.g., black carbon and sulfate aerosols) are complex and often confusing. For example, the influential papers by Rogelj et al. (2011; 2012), which use the same model calibration as Meinshausen et al. (2009) but the non-CO2 emissions specified in RCP2.6, produce considerably higher CO2 emissions budgets than Meinshausen et al. (2009). Similarly, the IPCC’s budgets give a 33% budget (33% chance of not exceeding 2°C) that is 2,090 Gt COlarger than its 66% budget when excluding non-CO2 forcings. This difference (which is ultimately related to the fact that different greenhouse gases have different radiative implications over time,[10] drops to a mere 400 Gt CO2 when RCP2.6 non-CO2 forcings are included in the comparison (see Table 4).

For all these reasons, the relationship between CO2 emissions, non-CO2 emissions and climate risk is quite uncertain – which further justifies precaution when making non-CO2-related target / mitigation-pathway tradeoffs.

Negative emissions

The term “negative emissions” designates CO2 that is removed from the atmosphere, and can refer to either techno-industrial processes (e.g., Biomass Energy with Carbon Capture and Sequestration, or BECCS) or changes in land-use practices that yield substantial enhancement of carbon sinks (e.g. afforestation and low-carbon agro-ecological techniques).[11] While the economic practicality of large-scale negative-emissions programs remain undemonstrated, some of the possibilities are easily imaginable – for example, we could plant great numbers of trees[12] – and a wide range of published scenarios do consider the large-scale removal of CO2 from the atmosphere in the second half of the century. Indeed, this is argued by many to be a necessity.[13]

However, although practical negative emission options would prove a great boon, it is risky to assume their future availability at large scale. Doing so tempts decision-makers to defer ambitious near-term mitigation while claiming to be adhering to a 2°C target, even though, in fact, it is slowly drifting out of reach. As noted above, back-loading emission reductions increases risk in dangerous ways.

In sum, the assumptions that are made in these three areas – front-loading vs. back-loading, CO2 vs. non-CO2 mitigation, and negative emissions – have the direct result of specifying mitigation pathways that allow for larger or smaller fossil-fuel CO2 budgets. Critically, they also allow larger or smaller emissions in the near future.


The three pathways presented in this paper have significantly different risks associated with them. By making certain plausible assumptions with respect to front-loading, non-CO2 mitigation, and negative emissions, we can compare the emissions budgets associated with these pathways to those presented in the IPCC’s AR5. By so doing, we can conclude the following:

  • The Strong 2°C pathway has a considerably greater than 66% chance of keeping the warming below 2°C. This being the case, it is alone among the three pathways examined here in having a high probability of holding the 2°C line.
  • The Weak 2°C pathway has between a 33% and 50% chance of keeping warming below 2°C. The very influential family of pathways that it represents carries substantially higher risks of exceeding 2°C than was previously estimated.
  • The G8 pathway has much less than a 33% chance of keeping warming below 2°C. In fact, it cannot plausibly be taken as a 2°C pathway, and this notwithstanding the fact that the G8 Declaration stated its emissions goals in the context of the 2°C objective.

Decision-makers now face a choice among future pathways. The temptation to choose a pathway that allows us to defer action will be great, but deferral has consequences. It increases the reliance on future technological breakthroughs (e.g., negative emission technologies) that may not prove available. As AR5 explains, for example, there are risks of carbon-cycle feedbacks that would accelerate non-anthropogenic emissions (e.g., the release of methane hydrates, or increased wildfires or the accelerated deterioration of the Greenland ice sheet). And it increases the risk of truly catastrophic impacts, such as several meters of sea-level rise. It is helpful, in this context, to remember that 2°C – once considered the plausible margin of “dangerous” climate change – is now widely understood among climate scientists to mark the approximate point of transition from “dangerous” to “extremely dangerous” climate change, and possibly to altogether unmanageable levels of warming (see, e.g., Anderson and Bows 2011).

Ultimately, the choice a global mitigation pathway reflects political, economic and ethical considerations as much as scientific ones. These decisions are inseparable from the assignment of newly-scarce emissions rights across countries and classes and generations, and choices about who will bear the associated costs and risks. And this, of course, is why the subject of ambitious mitigation pathways is so fraught, and so crucial. An “emergency transition” like the one implied by the Strong 2°C pathway (and arguably the Weak 2°C pathway as well) will be neither cheap nor easy, and this despite the vast flowering of low-emissions energy technology that’s now on the near horizon. This comparison of budgets makes it obvious that, despite the difficulties, such a transition is necessary if warming is to be kept to manageable levels.


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Bowerman, N. H. A., Frame, D. J., Huntingford, C., Lowe, J. A. and Allen, M. R. (2011). Cumulative carbon emissions, emissions floors and short-term rates of warming: implications for policy. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 369(1934). 45–66. DOI:10.1098/rsta.2010.0288.

Carbon Tracker Initiative (2011). Unburnable Carbon – Are the World’s Financial Markets Carrying a Carbon Bubble? London.

Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., et al. (2007). Changes in Atmospheric Constituents and in Radiative Forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, et al. (eds.). Cambridge University Press, Cambridge, UK, and New York.

G8 (2009). Responsible Leadership for a Sustainable Future. Declaration by the Group of Eight at the 2009 Summit in L’Aquila, Italy.,1.pdf.

Hansen, J., Kharecha, P., Sato, M., Ackerman, F., Hearty, P. J., et al. (2011). Scientific Case for Avoiding Dangerous Climate Change to Protect Young People and Nature. arXiv:1110.1365v3 [].

Höhne, N., van Breevoort, P., Deng, Y., Larkin, J. and Hänsel, G. (2013). Feasibility of GHG Emissions Phase-out by Mid-century. Project No. CLIDE14075. Prepared by Ecofys for Global Call for Climate Action, Cologne, Germany.

IEA (2011). World Energy Outlook 2011. International Energy Agency, Paris.

IPCC (2013). Climate Change 2013: The Physical Science Basis – Summary for Policymakers. Contribution of Working Group I to the Intergovernmental Panel on Climate Change Fifth Assessment Report, Stockholm, Sweden.

Meinshausen, M., Meinshausen, N., Hare, W., Raper, S. C. B., Frieler, K., et al. (2009). Greenhouse-gas emission targets for limiting global warming to 2°C. Nature, 458. 1158–63.

Rogelj, J., Hare, W., Lowe, J., van Vuuren, D. P., Riahi, K., et al. (2011). Emission pathways consistent with a 2 °C global temperature limit. Nature Climate Change, 1(8). 413–18. DOI:10.1038/nclimate1258.

Rogelj, J., Meinshausen, M. and Knutti, R. (2012). Global warming under old and new scenarios using IPCC climate sensitivity range estimates. Nature Climate Change, 2(4). 248–53. DOI:10.1038/nclimate1385.

Schaeffer, M., Hare, B., Rocha, M. and Rogelj, J. (2013). Adequacy and Feasibility of the 1.5°C Long-term Global Limit. Prepared by Climate Analytics for the Climate Action Network (CAN) Europe, Brussels, Belgium.

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This brief was written by Paul Baer and Tom Athanasiou of EcoEquity and Sivan Kartha of the Stockholm Environment Institute.

[1] For a detailed discussion of the RCPs, see van Vuuren et al. (2011), or for a “beginner’s guide”, see Wayne (2013).

[2] In particular, the 2ºC pathway is based on a Climate Action Tracker pathway developed by Climate Analytics, EcoFys and the Potsdam Institute for Climate Impact Research. For more information, see See also UNEP (2012).

[3] This high maximum rate of decline could be reduced by requiring an earlier peak.

[4] The appropriate “floor” for emissions from agriculture remains an open question. Even basic categories are vague, since a CO2 component of agriculture that isn’t from land clearing is typically grouped with industrial CO2 emissions. Nonetheless, current agricultural emissions of CH4 and N20 are estimated at 5 to 6 Gt CO2e annually, and we use that range as our floor. It seems plausible that both population growth and more equitable access to food can be offset by improvements in agricultural practices and technology. Anderson and Bows (2011) discuss the various contributors to a non-CO2 floor, and adopt a value of 6 GtCO2eq annually. Bowerman et al. (2011) test the significance of alternative floors and find that they matter a lot, and CO2 budgets would be reduced if higher floors were assumed.

[5] Because the emissions budgets that define these pathways are net budgets, negative emissions (whether from afforestation, biochar, BECCS, or even free air capture) can still legitimately be part of a mitigation scenario consistent with this Strong 2ºC pathway. If negative emissions become practical, this would simply allow larger positive emissions while holding to the same net budget and same risk of exceeding 2ºC. Alternatively, these technologies could make possible a more rapid reduction of net radiative forcing and a corresponding reduction in the risk of adverse impacts and the costs of adaptation.

[6] The EU’s goal requires a peak in 2020 and a 50% reduction in 2050 below 1990 levels, rather than 2005 levels. This would lead to a somewhat smaller CO2 budget (roughly 1,430 Gt CO2 for 2012–2100 instead of 1,610 Gt CO2) and all GHG budget (2,145 Gt CO2e instead of 2,325 Gt CO2e). This is gives it a better chance than the G8 pathway of keeping warming below 2°C, but still less than a 33% chance.

[7] Note that the numbers in this paragraph for budgets taking non-CO2 forcings into account have changed slightly since the first version of this document was published in October of 2013, to match the corrected version of the IPCC’s WGI Summary for Policymakers published on 11 November 2013. Specifically, the three budgets have changed from 880, 840 and 800 GtCO2 to 900, 820 and 790 GtCO2, and the emissions through 2011 has changed from 531 [446 to 616] GtCO2 to 515 [445 to 585] GtCO2.

 [8] It is differences in these types of assumptions about emissions that lead to the very different estimates of the emissions budgets associated with particular risk levels made by recent papers – most prominently by Rogelj et al. (2011; 2012) – relative to the estimates made by Meinshausen et. al. (2009). The critical point here is that the Rogelj et al. papers use the same parameterizations of the MAGICC model. For example, Meinshausen (2009) estimates the CO2 budget for a 67% chance of staying below 2ºC to be ~1160 Gt CO2 between 2000 and 2050 and ~1680 Gt CO2e, while Rogelj et al. (2011) estimates emissions of the “median” pathway with a >66% chance of staying below 2ºC to be ~1880 Gt CO2e between 2000 and 2050. The changed assumptions come from the incorporation of assumptions about non-CO2 gases from the RCP2.6 emissions pathway, as opposed to the “Equal Quantile Walk” method used in Meinshausen et al. (2009).

[9] For example, the 2011 World Energy Outlook (IEA 2011) states: “Delaying action is a false economy: for every $1 of investment avoided in the power sector before 2020 and additional $4.3 would need to be spent after 2020 to compensate for the increased emissions.”

[10] In particular, methane has a short atmospheric lifetime, whereas N2O and many F-gases have lifetimes comparable to CO2 or longer.

[11] The particular confusion here is that a broad category of non-land-use CO2 emissions is frequently called “fossil fuel emissions”, even though it usually includes modern biofuels used for energy, and often non-energy industrial CO2 emissions (e.g. from cement manufacturing) as well.

[12] For example, Hansen et. al. (2011) estimate the total 21st century biosequestration potential at 100 GtC, which is equivalent to about 370 Gt CO2, based entirely on afforestation and reforestation.

[13] Often, mitigation deferral is wearily accepted as necessary. For example, Adequacy and feasibility of the 1.5C long-term global limit (Schaeffer et al. 2013) notes: “Constrained by real emissions until 2010 and energy-economic reduction potential until the 2020s, the 1.5°C scenarios necessarily require net-negative CO2 emissions in the second half of the 21st Century. The later the emissions peak, the more CO2 needs to be removed starting around the 2050s.”