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Relationship between global emissions and global temperature rise Reto Knutti CLA chapter 12 © Yann Arthus-Bertrand / Altitude Climate change commitment • Stable CO2 concentration will result in further warming over centuries. Warming will persist for centuries • Zero CO2 emissions lead to near constant surface temperature. A large fraction of climate change persists for many centuries. • Depending on the scenario, about 15-40% of the emitted carbon remains in the atmosphere for 1000 yrs. • This represents a substantial multicentury climate change commitment created by past, present and future emissions of CO2. But there is no commitment to further surface warming in the climate system, it’s only in society. Cumulative carbon determines warming • Peak warming is approximately proportional to cumulative (total) emissions. • Transient climate response to cumulative carbon emissions TCRE = Warming per 1000 PgC: likely 0.8-2.5°C Cumulative carbon determines warming 1%/yr CO2 RCP8.5 • Evidence from observations, and from simple to complex models for many scenarios. • Near linear in all models, but the slope is uncertain. • Any temperature target implies a maximum amount of carbon that can be emitted. • Due to non CO2, RCP warming is larger than from CO2 only. Fig. 12.45 Controls on the carbon budget • Higher likelihood to achieve target implies lower budget • Lower temperature target implies lower budget • 2°C budgets for CO2 only are (90%/66%/50%/33%/10%): 730/1000/1212/1567/3567 GtC • 1.5°C budgets for CO2 only are (90%/66%/50%/33%/10%): 548/750/ 909/1176/2675 GtC • Non-CO2 implies lower budget: likely <2°C budget from 1000 PgC down to about 790 GtC for RCP2.6, 515 GtC emitted. Controls on the carbon budget • Higher likelihood to achieve target implies lower budget • Lower temperature target implies lower budget • 2°C budgets for CO2 only are (90%/66%/50%/33%/10%): 730/1000/1212/1567/3567 GtC • 1.5°C budgets for CO2 only are (90%/66%/50%/33%/10%): 548/750/ 909/1176/2675 GtC • Non-CO2 implies lower budget: likely <2°C budget from 1000 PgC down to about 790 GtC for RCP2.6, 515 GtC emitted. Cumulative carbon determines warming RCP2.6 • Warming is largely independent of the emission profile. Only the total matters. • More emissions or delay early imply stronger reductions later. • Any temperature target implies a maximum in cumulative CO2 emissions. This is purely a physical and carbon cycle problem. • Overshooting the budget will overshoot the target. • Allocation over time is a economic and policy problem. Fig. 12.46 Implications • Taking into account historical carbon emissions, a maximum of about 270 GtC remains if warming is to likely be kept below 2°c.. • With emissions at current levels, that remaining budget would be used in less than three decades. • Higher emissions earlier imply lower emissions later on to stay within the budget. Exceeding the budget implies actively removing emissions at a later point to return to the target. • Without a large-scale technology to actively remove carbon dioxide from the atmosphere, exceeding the budget in the coming years implies a that one actually commits the world to more warming than the chosen threshold, and higher associated climate-related risks for decades to centuries. Cumulative carbon determines warming • Every ton of CO2 causes about the same amount of warming, no matter when and where it is emitted. SPM.10 Temperature targets and the RCP scenarios • Global surface temperature change for the end of the 21st century is likely to exceed 1.5°C relative to 1850 to 1900 for all RCP scenarios except RCP2.6. It is likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5. • Warming will continue beyond 2100 under all RCP scenarios except RCP2.6, which will likely stay below 2°C above 1850 to 1900. Fig. 12.5 Summary • Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2. • Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions. Halting global-mean temperature rise at any level requires near zero carbon emissions at some point in the future. • Every ton of CO2 causes about the same amount of warming, no matter when and where it is emitted. • Cost optimal pathways for low targets show peak emissions in the next decade. Delay will commit future generations to stronger reductions, with implications on cost, technology, etc. • The long term goal can be defined as a threshold, or as an eventual target after overshooting. Targets other than temperature are possible. Further Information www.climatechange2013.org © Yann Arthus-Bertrand / Altitude