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

October 5, 2018
Brenda Ekwurzel
Senior Climate Scientist, Director of Climate Science

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.

About the author

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Brenda Ekwurzel ensures that program analyses reflect robust and relevant climate science, and researches the influence of major carbon producers on rising global average temperatures and sea level. Dr. Ekwurzel is a co-author of the fourth National Climate Assessment (NCA4) Volume II. She presents frequently to a range of audiences on climate science, educating the public on practical, achievable solutions for climate change.