Tracing the Basis for a Claim about Near-Term Emission Reduction Targets

for Industrialized Countries on a Path to Climate Stabilization at 2 degC

Holmes Hummel, PhD

January 2008

Since the IPCC’s Fourth Assessment Report was released in 2007, there have been recurring claims based on the science that “to have a 50% chance to stabilize at 2ºC (global average temperature above pre-industrial) industrialized countries need to reduce their emissions 25-40% below 1990 by 2020.”

Based on my prior work on ways to interpret implications of emissions scenarios, I have fielded several inquiries about the source of that claim and its implications.

Here I will discuss only the source. The fact-checking requires multiple steps – and it is a fascinating trail to follow.

Use of the science in policy

First, check paragraph 3 of the work plan negotiated under Conclusions adopted by the Ad Hoc Working Group on Further Commitments for Annex I Countries under the Kyoto Protocol to see what all Annex I signatories (that’s all industrialized countries except the United States and Turkey) agreed regarding this tight near-term target range.

The U.S. eliminated a specific acknowledgment that conclusion by the Kyoto Parties in a statement proposed for the pre-amble of the Bali Action Plan (negotiated in parallel): “Recognizing that much deeper emissions cuts by developed countries will be required and that Parties to the Kyoto Protocol are considering the indicative range of emissions reductions of Annex I Parties as a group of 25-40% below 1990 by 2020,”… Though this refusal to accept the language was covered in the press as a point of contention (whether or not the U.S. would accept targets), reference to the scientific basis for the main claim was still included in a footnote.

Altough this target range is challenging, the E.U. committed unilaterally in February 2007 to “reducing its overall emissions to at least 20% below 1990 levels by 2020,” adding that it “is ready to scale up this reduction to as much as 30% under a new global climate agreement if other developed countries make comparable efforts.”

Origins of the Indicative Target Ranges

To continue tracing the basis for this indicative target range, paragraph 4 of the Kyoto Parties Work Plan and the fourth pre-ambulatory statement of the Bali Action Plan each provide a footnote for the source of information in paragraph 3, referencing pages 39 and 90 of the Technical Summary of Mitigation of Climate Change (IPCC Working Group III).

Start with page 39: Table TS.2 here is the same as Table SPM.6 on page 21 of the more widely-read Synthesis Report Summary for Policymakers.

Category I in the Table SPM.6 (Table TS.2) shows figures that indicate a stabilization target of 2.0-2.4ºC would be in the range of 350-400ppm CO₂ or 445-495ppm CO₂-equivalent. For reference, the atmospheric concentration of CO₂ is approximately 380ppm CO₂ now. That same Category I row also indicates that to meet the target, global emissions would need to peak between 2000-2015.

In the Summary for Policymakers, also look at the following page (22) to see Figure SPM.11, which gives a better sense of what the numbers in the table mean. The green-shaded areas represent Category I, the 2.0-2.4ºC target range.

Here, it is also important to point out that just below those graphs on page 22 of the Synthesis Report Summary for Policymakers there is a sentence in bold that was negotiated and approved line by line by every government participating in the IPCC, including the U.S.:

“There is high agreement and much evidence that all stabilization levels assessed can be achieved by deployment of a portfolio of technologies that are either currently available or expected to be commercialized in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers.” [italics in the original]

To pick up the distinction between “developed” nations (Annex I) and all other nations, return to the Technical Summary of Mitigation of Climate Change (IPCC Working Group III) to look up the second reference to page 90, which points to the first sentence under Regime Stringency:

“Under most equity interpretations, developed countries as a group would need to reduce their emissions significantly by 2020 (10-40% below 1990 levels) and to still lower levels by 2050 (40-95% below 1990 levels) for low to medium stabilization levels (450-550 ppm CO₂-eq).”

Note that the page 90 passage combines “low and medium stabilization” targets, which results in a wider indicative range of reduction for developed countries: 10-40% below 1990 by 2020 (instead of 25-40%).  To distinguish between low and medium stabilization targets, refer to the original text cited in the Technical Summary as well as the Bali Action Plan footnote: Chapter 13, Section 3, Box 13.7 on page 776 of the full Working Group III report:

In Box 13.7, we see the key figures “25-40%” reduction below 1990 by 2020 target range for Annex I countries.  It is in the first row, labeled Category A, which is basically the same as Category I on page 39 of the Technical Summary.  And the combination of Category A & B here in Box 13.7 is what yields the “10-40%” reduction figure on page 90 of the Technical Summary.

Factoring in Uncertainty

Now, about the “50% chance” - check the footnote (b) under the Table SPM 6 on page 39 of the Technical Summary of Mitigation of Climate Change (IPCC Working Group III).

It says the figures in the table are based on a “best estimate” climate sensitivity of 3ºC.  [Climate sensitivity is a key variable in a calculation of the global average temperature increase anticipated as a result of a doubling of CO₂ in the atmosphere.] Then it goes on to say that the term “best estimate” means the mode of the probability density function (PDF) for climate sensitivity used in the calculation. 

Mode means the value with the highest frequency in the function, and that is not the same as the median, which would be in the middle.  However, because the PDF for this “best estimate” is not printed, it has been reasonably interpreted as a distribution for which the mode and median are sufficiently close that a mode of 3ºC is essentially the same as a saying there’s a 50% chance it could be higher, or lower than 3ºC.

Altogether, this is what leads to the core claim:  

To have a 50% chance of making a 2ºC stabilization target, global emissions would need to peak by 2015 and Annex I countries would need to be 25-40% below 1990 by 2020.