How we made the Emissions Peaking Tool

Creating 2°C emissions curves for different peak years

The aim of the tool is to illustrate the implications of different peaking years for global carbon dioxide emissions. Basically, the later emissions peak, the faster emissions have to come down afterwards in order that cumulative emissions are compatible with the 2 Celsius global warming target.

We created emission scenarios that have a 50% chance of keeping the eventual rise in global temperature below 2°C. They use five possible years in which global carbon emissions may peak (2015-2035).

General approach and constraints:

Following the conclusions of the recent Intergovernmental Panel on Climate Change (IPCC) report, we use a cumulative emissions budget of 1300 Gt CO2 from 2011-2100. The likelihood of staying below 2°C for this cumulative budget is 50%. Global CO2 emissions are used rather than CO2-equivalent on the assumption that non-CO2 emissions cancel each other out (Hassol, 2011): as aerosol emissions decrease, methane emissions will also have to decrease to stay within the emissions budget.

The maximum realistic possible rate of annual decrease in emissions has been calculated as 3.5% (Den Elzen et al. 2010). In constructing our scenarios, this rate is used as a cap until no emissions pathways can stay below it for that peak year (for the 2015 and 2020 peak). For a peak year of 2025 and above, the cap on the annual rate of decrease had to be lifted to enable the pathways to stay below 2°C.

Negative emissions, for example using BECCS (Bio-Energy with Carbon Capture and Storage), are only considered when no emissions paths exist that can fit the 2°C criterion without using them. We include an assumption that annual negative emissions from BECCS would be at most -5Gt CO2 by 2050 and -10Gt CO2 by 2100 (Van Vuuren et al, 2013). This assumes that all bioenergy crops would be used in BECCS plants with a capture efficiency of 90%, and assumes a land-use related emission factor of 15 kg CO2 emitted per GJ. Using the assumptions from Van Vuuren et al (2013), 10GtCO2/year of BECCS would use around 0.15-0.3 Gha (0.15-0.3 billion hectares), which is about 10-15% of the area used for crop production (1.4-1.7Gha). Similarly, a maximum rate of CO2 removal of 20GtCO2/year was estimated by Edmonds et al (2013) to take up 490–530MHa of land in 2100, which could result in higher food prices (Tavoni and Socolow, 2013).

The online tool shows the average of the scenarios for each peak year as a solid line. When you choose a certain peak year, the shaded area also shows the extent of the range of scenarios created in our calculations that meet the 2°C criterion.

Method of curve creation:

The general method used is that of the Equal-Quantile-Walk, as outlined in the supplementary material of Meinhausen et al. (2009). For this method the initial CO2 data is taken from the Emissions Database for Global Atmospheric Research (EDGAR), and then five branching years are selected [2015, 2020, 2030, 2040, 2050] and 19 possible emission changes (%) are possible at each branching point [-15 to 3]. Higher negative changes in emissions were used than in Meinhausen et al (2009) to allow scenarios that peak after 2020 to stay under 2°C. This creates a bank of 100,000 emissions scenarios, which are sub-sampled for pathways that would produce warming of less than 2°C and peak at the appropriate year and have no negative emissions (i.e. BECCS). For the peak year of 2015, we also restricted the maximum reduction rate to 3.5% of the peak emissions. This adapted method produced 350 curves that peaked in 2014-2016. As few or no curves could stay under 2°C after 2015, there was no restriction on the decrease rate and more curves were used.

To achieve emissions curves that peaked after 2035 and to allow deep negative curves, the branching points were changed to [2015, 2030, 2050, 2065, 2080].

Conclusions:

• The earlier that emissions peak, the lower the rate of emission reduction needed thereafter to give a 50% chance of keeping global warming below 2°C

• Only peaking global emissions in 2015 (now) allows the use of emission reduction rates thought to be the maximum realistic (below 3.5% per year) with confidence

• If global emissions do not peak before 2030, a negative emissions technology such as BECCS will have to be used to keep global warming below 2°C. This could result in higher food prices. Additionally, emissions would need to be reduced at a rate considerably higher than that currently thought feasible

• If global emissions do not peak by 2030, then according to our analysis and current expectations of maximum negative emissions, it will be impossible to stop global warming rising above the 2°C threshold.

The scenarios were created by Dr Luke Sheldon. Additional research and analysis was undertaken by Dr Helena Wright

References:

Den Elzen, M.G.J., Van Vuuren, D.P., Van Vliet, J (2010) Postponing emission reductions from 2020 to 2030 increases climate risks and long-term costs. Climatic Change (2010) 99:313–320

Edmonds J, P Luckow, P, K Calvin, M Wise, J Dooley, P Kyle, L Clarke (2012) Can radiative forcing be limited to 2.6 W/m2 without negative emissions from bioenergy AND CO2 capture and storage? Clim Chang. doi:10.1007/s10584-012-0678-z

Hassol, S.J (2011) Emissions Reductions Needed to Stabilize Climate. Presidential Climate Action Project. https://www.climatecommunicati...

IPCC (2014) Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

Meinshausen, M., Meinshausen, N., Hare, W., Raper, S.C.B., Frieler, K., Knutti, R., Frame, D. J., & Allen, M.R (2009) Greenhouse-gas emission targets for limiting global warming to 2oC, Nature, Vol 458:30, April 2009.

Tavoni, M. and Socolow, R. (2013) Modeling meets science and technology: An introduction to a special issue on negative emissions, Climatic Change, 118(1), 1-14

Van Vuuren, D.P., Deetman, S., Van Vliet, J., Van den Berg, M., Van Ruijven, B.J., Koelbl, B (2013) The role of negative CO2 emissions for reaching
2 °C-insights from integrated assessment modeling, Climatic Change, 118:15–27