Pathways to 1.5C: Carbon Budget in the IPCC Special Report

, senior climate scientist | October 5, 2018, 1:06 pm EDT
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This post is a part of a series on IPCC2018

The historic Paris Climate Agreement generated a request of the Intergovernmental Panel on Climate Change (IPCC) to prepare a Special Report on 1.5 degrees Celsius increase above pre-industrial temperatures. Scientists and government representatives are in the final stretch assessing that every word of the summary for policymakers (SPM) accurately conveys evidence presented in the report.  Policymakers, business leaders, and energy system planners will be paying close attention to what the SPM says about the carbon budget remaining to stay below 1.5 degrees Celsius.

Figure 1: Note the logarithmic scale on panels b) and c). Original Source: IPCC AR5 WG1 Fig 8-06 and reproduced in USGCRP NCA4 CSSR which can be consulted for full figure explanation here.

Why is the carbon budget so important?

This is largely because carbon in the atmosphere lead to the global temperature increase. The CO2 budget depends also on how much and how fast we reduce the other contributions to temperature rise.  Carbon dioxide (CO2) emissions, methane (CH4) emissions (which transform into COin a little over a decade after being emitted to the atmosphere), other well-mixed greenhouse gases and halocarbons from human activities all change the radiative forcing energy (watts per meter squared) which changes the surface temperature of Earth (Figure 1).   Or more precisely:

We already see that climate change influenced impacts are changing at a pace and scale that has surprised many.  Typically, climate change uncertainties tilt in the direction toward larger magnitude impacts due to the tendency to amplify different parts of the ocean, atmosphere, and biosphere system.  The special report will reflect the scientific advances regarding the greater risks that come with another half a degree Celsius temperature increase.   This represents a great advance for small island nations or species that depend on Arctic sea ice to last all year long.  Both are examples of being at risk of crossing perilous thresholds at lower temperatures. More extreme precipitation, dangerous heat waves, and other impacts occur at 1.5 C.  The incentives to stay within the carbon budget grow larger as we gain more evidence on the dangerous thresholds likely crossed with every half a degree additional temperature increase.

What if the carbon budget gives the world a little more time to reduce emissions?

Even if the carbon budget were found to be a bit larger, can we really take a break from our collective efforts to achieve carbon neutral human activities that alleviate poverty?  No.  Imagine a mountaineering expedition preparing to climb the largest mountain in North America – Denali.  Would they really take any less precautions or delay preparations after learning that a new more accurate scientific survey indicated the mountain was slightly shorter?  No. As climber, Nick Parker, put it, “It’s still high, it’s still hard, it’s still cold.”  Just as it is no walk in the park to stay within the carbon budget for remaining below 2 degrees Celsius global temperature increase and especially 1.5 degrees Celsius increase.

IPCC AR5 WG1 Technical Summary TFE.8-1

Figure 2. Source: IPCC fifth assessment report technical summary figure TFE.8-1

Yet it is possible because we know we still have some carbon budget remaining to work within as we transform our energy systems in equitable, sustainable ways toward net zero carbon emissions.  The scientific community has given the likelihood of achieving a temperature target based on a range of the carbon budget.  According to the IPCC AR5:

‘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 1861–188022, 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, respectively23. 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.’

The carbon budget range is likely an area of great discussion at Incheon, South Korea where everyone is gathered to decide upon the final approved language in the SPM.  To get a sense why, look at the green and yellow stacked bar chart at the bottom of figure 2.  It conveys the range of the carbon budgets that stem from the different models.  Not surprising since these approaches have to assess the natural and human influences on climate as well as the time frame of responses within every part of the system (surface temperature, ocean acidification, biological uptake and releases of carbon, land ice response, etc.).  One check on how well these approaches are doing when taken all together, is to compare the stacked bar chart for the temperature increase Earth already experienced above pre-industrial and compare that with the carbon we already emitted. Spot on. 90 percent of the models agree that Earth would have reached the temperature increase that actually occurred based on the cumulative emissions over the historical period (figure 2).

As soon as the special report is released – expected October 7 at 9 P.M. Eastern US time (October 8 at 10 A.M. local time (KST)) – I plan to tweet the carbon budget numbers for giving a greater than 66 percent probability of less than 1.5 degrees Celsius increase since the pre-industrial period.

IPCC AR5 WG1 Technical Summary

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  • Page Baird

    Is atomic energy a way to reduce the carbon budget quickly? (note: I am a non-scientist, just a concerned citizen interested in comments by individuals more knowledgeable about the subject)

    • Brenda Ekwurzel

      My colleague, Rachel Cleetus (, has shared her thoughts on your question:

      If nuclear power (esp new nuclear power plants) is to play a role in the transition to a net zero carbon world, it will be important to overcome the significant technical, economic and safety-related challenges that advanced nuclear technology currently faces. Meanwhile cost-effective solutions like renewable energy and energy efficiency must be quickly ramped up globally. For an overall perspective on global warming and nuclear power check out:

      The IPCC Special Report on 1.5 degrees Celsius report’s incudes figures SPM 3a and 3b ( with the range of the contribution from nuclear in 2030 (59 to 98% growth in 2030 relative to 2010). An illustrative set of modeled pathways in the IPCC report shows that renewable energy’s share of global electricity needs to be in the range of 48-60 percent by 2030 and 63 to 81% by 2050 if we are to limit temperature increase to 1.5C with no or low overshoot.

      Chapter 2 ( of the IPCC 1.5 C report states: “Nuclear power increases its share in most 1.5°C pathways by 2050, but in some pathways both the absolute capacity and share of power from nuclear generators declines (Table 2.15). There are large differences in nuclear power between models and across pathways (Kim et al., 2014; Rogelj et al., 2018). One of the reasons for this variation is that the future deployment of nuclear can be constrained by societal preferences assumed in narratives underlying the pathways (O’Neill et al., 2017; van Vuuren et al., 2017b). Some 1.5°C pathways no longer see a role for nuclear fission by the end of the century, while others project over 200 EJ yr–1 of nuclear power in 2100 (Figure 2.15).”

      Also, nuclear will likely not be the most cost-effective or practical option in many countries, especially in many places where the biggest challenge right now is simply access to modern forms of energy. Distributed renewable energy systems might be a much better way to meet energy needs.

      • Page Baird

        Thank you, Dr. Ekwurzel. Your information helps me understand that atomic energy may of be some help (but not as much as renewable energy). Anyhow, it’s not likely to be a “quick fix” to the situation. I will check out the additional information you suggest.