AERA-MIP: Emission pathways, remaining budgets and carbon cycle dynamics compatible with 1.5 ºC and 2 ºC global warming stabilization
Authors: Silvy Yona , Frölicher Thomas L. , Terhaar Jens , Joos Fortunat , Burger Friedrich A. , Lacroix Fabrice , Allen Myles , Bernadello Raffaele , Bopp Laurent , Brovkin Victor , Buzan Jonathan R. , Cadule Patricia , Dix Martin , Dunne John , Friedlingstein Pierre , Georgievski Goran , Hajima Tomohiro , Jenkins Stuart , Kawamiya Michio , Kiang Nancy Y. , Lapin Vladimir , Lee Donghyun , Lerner Paul , Mengis Nadine , Monteiro Estela A. , Paynter David , Peters Glen P. , Romanou Anastasia , Schwinger Jörg , Sparrow Sarah , Stofferahn Eric , Tjiputra Jerry , Tourigny Etienne , Ziehn Tilo 
Published: February 2024

Abstract. While international climate policies now focus on limiting global warming well below 2 °C, or pursuing 1.5 °C, the climate modeling community has not provided an experimental design in which all Earth System Models (ESMs) converge and stabilize at the same prescribed global warming levels. This gap hampers accurate estimations based on comprehensive ESMs of the carbon emission pathways needed to meet such agreed warming levels, and of the associated climate impacts under temperature stabilization. Here, we apply the Adaptive Emission Reduction Approach (AERA) with ESMs to provide such simulations in which all models converge at 1.5 °C and 2.0 °C warming levels by iteratively adjusting their emissions. These emission-driven simulations provide a wide range of emission pathways and resulting atmospheric CO2 projections for a given warming level, uncovering uncertainty ranges that were previously missing in the traditional CMIP scenarios with prescribed greenhouse gas concentration pathways. Meeting the 1.5°C warming level necessitates a 40 % (model full range: 7 to 76 %) reduction in multi-model mean CO2-forcing equivalent (CO2-fe) emissions from 2025 to 2030, a 98 % (57 to 127 %) reduction from 2025 to 2050, and a stabilization at 1.0 (-1.7 to 2.9) PgC yr-1 from 2100 onward after the 1.5 °C target is reached. For the 2.0 °C warming level, CO2-fe emissions require a 47 % (8 to 92 %) reduction until 2050 and a stabilization at 1.7 (-1.5 to 2.7) PgC yr-1 from 2100 onward. The on-average positive emissions under stabilized global temperatures are the result of a decreasing transient climate response to cumulative CO2-fe emissions. This evolution is consistent with a slightly negative zero emissions commitment – initially assumed zero – and leads to an increase in the post-2025 CO2-fe emission budget by a factor 2.2 (-0.8 to 6.9) by 2150 for the 1.5 °C warming level and a factor 1.4 (0.9 to 2.4) for the 2.0 °C warming level compared to its first estimate in 2025. Our simulations highlight shifts in carbon uptake dynamics under stabilized temperature, such as a cessation of the carbon sinks in the North Atlantic and in tropical forests. On the other hand, the Southern Ocean and the northern high-latitude land remain carbon sinks over centuries after temperatures stabilize. Overall, this new type of target-based emission-driven simulations offers a more coherent assessment across climate models and opens up a wide range of possibilities for studying both the carbon cycle and climate impacts, such as extreme events, under climate stabilization.