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  • 616 : The Number of the IPCC

    Posted on October 23rd, 2013 Jo 1 comment

    I have been looking into an anomaly in the recently published IPCC Fifth Assessment Report.

    No, I don’t think that global warming has paused, or that climate change is a hoax. We’re still on course for some very disturbing times, and the risks of rainfall change, sea level rise, and temperature and weather extremes are still gobsmackingly frightening.

    However, I’d like to ask why one particular figure from the Summary for Policymakers for Working Group 1 differs from the Chapters that underly it.

    Reference : Intergovernmental Panel on Climate Change, Fifth Assessment Report, Working Group 1

    Summary for Policymakers, 27 September 2013

    Section E.8
    “Climate Stabilization, Climate Change Commitment and Irreversibility”

    “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 [22], will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1560 GtC, 0 and about 1210 GtC, and 0 and about 1000 GtC since that period respectively [23]. These upper amounts are reduced to about 880 GtC, 840 GtC, and 800 GtC respectively, when accounting for non-CO2 forcings as in RCP2.6. An amount of 531 [446 to 616] GtC, was already emitted by 2011. {12.5}”

    This refers to Chapter 12, Section 12.5, so I looked that up :-

    Chapter 12

    Executive Summary

    “Climate Stabilization : The principal driver of long term warming is total emissions of CO2 and the two quantities are approximately linearly related. The global mean warming per 1000 PgC (transient climate response to cumulative carbon emissions, TCRE) is likely between 0.8°C–2.5°C per 1000 PgC, for cumulative emissions less than about 2000 PgC until the time at which temperatures peak. To limit the warming caused by anthropogenic CO2 emissions alone to be likely less than 2°C relative to preindustrial, total CO2 emissions from all anthropogenic sources would need to be limited to a cumulative budget of about 1000 PgC over the entire industrial era. About half [460–630 PgC] of this budget was already emitted by 2011.”

    Section 12.5
    “Climate Change Beyond 2100, Commitment, Stabilization and Irreversibility”

    12.5.4

    “Based on the evidence presented above, to limit warming caused by CO2 emissions alone to be likely less than 2°C, total CO2 emissions from all anthropogenic sources would need to be limited to a cumulative budget of about 1000 PgC over the entire industrial era, about half of which [460 to 630 PgC] (Section 6.3.1) have been emitted by 2011.”

    This refers to Chapter 6, Section 6.3.1, so I looked that up :-

    Chapter 6

    Executive Summary

    “Anthropogenic CO2 emissions to the atmosphere were 545 ± 85 PgC (1 PgC = 10^15 gC) between 1750 and 2011. Of this amount, fossil fuel combustion and cement production contributed 365 ± 30 PgC and land use change (including deforestation, afforestation and reforestation) contributed 180 ± 80 PgC. [6.3.1, Table 6.1]”

    Section 6.3
    “Evolution of Biogeochemical Cycles Since the Industrial Revolution”

    Section 6.3.1
    “CO2 Emissions and Their Fate Since 1750″

    “Prior to the Industrial Era, that began in 1750, the concentration of atmospheric CO2 fluctuated roughly between 180 ppm and 290 ppm for at least 2.1 million years (see Chapter 5, Section 5.2.2 and Hönisch et al., 2009; Lüthi et al., 2008; Petit et al., 1999). Between 1750 and 2011, the combustion of fossil fuels (coal, gas, oil, and gas flaring) and the production of cement have released 365 ± 30 PgC (1 Pg C = 10^15 gC) to the atmosphere (Table 6. 1; Boden et al., 2011). Land use change activities, mainly deforestation, has released an additional 180 ± 80 PgC (Table 6.1). This carbon released by human activities is called anthropogenic carbon.”

    Table 6.1 has :-

    “Fossil fuel combustion and cement production (b) 365 ± 30 (f)”
    “Net land use change (d) 180 ± 80 (f), (g)”
    “(b) Estimated by the Carbon Dioxide Information Analysis Center (CDIAC) based on UN energy statistics for fossil fuel combustion (up to 2009) and US Geological Survey for cement production (Boden et al., 2011), and updated to
    2011 using BP energy statistics.”
    “(d) Based on the “bookkeeping” land use change flux accounting model of Houghton et al. (2012) until 2010, and assuming constant LUC emissions for 2011, consistent with satellite-based fire emissions (Le Quéré et al., 2013; see 6.3.2.2. and Table 6.2).”
    “(f) “The 1750–2011 estimate and its uncertainty is rounded to the nearest 5 PgC.”
    “(g) Estimated from the cumulative net land use change emissions of Houghton et al. (2012) during 1850–2011 and the average of four publications (Pongratz at al., 2009; van Minnen et al., 2009; Shevliakova et al., 2009; and Zaehle et al., 2011) during 1750–1850.”

    So how come the Summary for Policymakers gives total carbon emissions as :-

    “An amount of 531 [446 to 616] GtC, was already emitted by 2011

    but Chapter 6 and Chapter 12 have :-

    “Anthropogenic CO2 emissions to the atmosphere were 545 ± 85 PgC (1 PgC = 10^15 gC) between 1750 and 2011.” [ This equates to a range of 460 to 630 PgC. ]

    About half [460–630 PgC] of this budget was already emitted by 2011.

    With the help of Aubrey Meyer of the Global Commons Institute, I have tracked down the source of the 531 PgC figure from the Summary for Policymakers – it’s the sum total of columns B and C of the “Harmonized Emissions” spreadsheet for the RCP 2.6 (also known as RCP 3-PD) scenario between 1765 and 2011 :-

    http://www.pik-potsdam.de/~mmalte/rcps/

    Go to the link at the row :-
    “RCP3-PD: Low RCP with Peak & Decline (2005-2500)”
    in the column :-
    “For information : Harmonized Emissions”

    http://www.pik-potsdam.de/~mmalte/rcps/data/RCP3PD_EMISSIONS.xls

    The 531 PgC (equivalent to GtC) comes from summing the Harmonized Emissions from the year 1765 to 2011 by applying the formula :-

    =SUM(B39:B285)+SUM(C39:C285)

    and you get :-

    531.40626 GtC

    This still doesn’t explain where the 616 PgC number comes from…but I’d guess they come from the run of the MAGICC programme.

    What disturbs me is that this change from using the workbook methods to using the harmonized emissions from the CMIP5 computer modelling runs is not noted or explained (or defended) in the Summary for Policymakers.

    They need to get more sleep at UNFCCC and IPCC conferences, clearly.

     

    One response to “616 : The Number of the IPCC”

    1. Hi Jo

      Well spotted (although you don’t need to be disturbed!)

      The figure of 531 [446 to 616] was an error in the draft of the SPM released after the plenary, which has now been corrected in the version published last month for COP, with the correction documented in the errata dated 11th Nov 2013.

      The published version says “since the period 1861–1880 … An amount of 515 [445 to 585] GtC … was already emitted by 2011.”

      NB The figure of 515 GtC in the SPM is relative to 1861-1880, for consistency with the model projections of cumulative emissions for the different RCPs (these use a baseline of 1861-1880). The figure of 545 ± 85 GtC (460 to 630 GtC) in Ch6 and Ch 12 is relative to 1750, as you note above. This is the date conventionally used to mark the start of the industrial era – but the CMIP5 climate models didn’t start that early (it would have taken more computing power and time).

      The First Order Draft of the SPM did include the figure of 545 ± 85 GtC (see page SPM-17 lines 30-31) but I guess somebody realised that this was inconsistent with the rest of the paragraph, as the baseline for the past emissions from Ch6 was 1750 whereas that for total past and future emissions from the CMIP5 models was 1861-1880. They changed it as some point between the FOD and the version released after the plenary, to present the past cumulative emissions relative to the 1861-1880 baseline, but made a mistake. This has now been corrected.

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