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KLIMAFORSK-Stort program klima

Emit now, mitigate later? IMPlications of temperature OverShoots for the Earth system

Alternative title: Slippe ut nå, fikse det senere? Konsekvenser av temperatur overshoots i jordsystemet

Awarded: NOK 10.1 mill.

Project Number:

294930

Application Type:

Project Period:

2019 - 2023

Funding received from:

Location:

Partner countries:

IMPOSE has produced a large set of idealized overshoot scenarios with the Norwegian Earth System Model (NorESM2). In these scenarios, the CO2 emission are too large initially, and CO2 is removed from the atmosphere later in the simulations to meet the goal of staying "well below 2 degree warming". We have varied the overshoot intensity both in magnitude and in duration to be able to assess the risks and impacts of pathways that assume large scale carbon dioxide removal as a mitigation option. The analysis of our overshoot scenarios revealed a surprising response of the physical climate system in NorESM2. In our simulations the northern hemisphere temperatures decline much more than expected when negative emissions are applied. Here "more than expected" means a decline below the temperatures in the reference simulation without overshoot. This temporary cooling is strong north of 40 degrees north, and it increases with the size of the overshoot. The reason for this phenomenon has been identified to be a strong reduction in the strength of the Atlantic ocean circulation system in our model. This system of oceanic currents (called "Atlantic Meridional Overturning Circulation" or AMOC) transports large amounts of heat from the tropics into the northern high latitudes. This heat transport is strongly weakened when climate warms in our model. If negative emissions are applied in a state of weakened AMOC, the cooling effect of reduced CO2 concentrations amplifies the high latitude cooling effect of the weakened AMOC. The recovery of AMOC finally leads to a warming towards the "expected" level. The resulting warming-cooling-warming cycles have a pronounced effect on Arctic sea-ice cover, permafrost and ecosystems. These results have been published in Nature Communications (https://doi.org/10.1038/s41467-022-28573-5). To better assess the effectiveness and risks of mitigation pathways that rely on carbon dioxide removal, we have also used our scenarios to investigate whether the additional climate change during the overshoot period is reversible: will the climate be in the same or at least similar after a period of temperature overshoot compared to pathways where a temperature target is reached without overshoot? Are there critical limits to the duration and/or magnitude of an overshoot beyond which climate change become irreversible? Are there tipping points, beyond which self-accelerating feedbacks make a return to a safe climate state impossible or at least difficult? Three main results from this analysis can be highlighted. First, the atmospheric CO2 concentration is surprisingly insensitive to whether there was an overshoot or not. This is due to an almost perfect compensation of more carbon uptake by the ocean and less carbon uptake by the land during the overshoots. Second, the largest source of irreversibility is found for the permafrost carbon, which is lost during the overshoot periods. Third, in our model, we do not find any indication of tipping points or self-reinforcing feedbacks that would put the Earth system on a significantly different trajectory after an overshoot. However, our model is not well suited to assess tipping points in ecosystems, which are modeled in a very simplified fashion in Earth system models. These results have been published in Earth System Dynamics (https://doi.org/10.5194/esd-13-1641-2022). IMPOSE has looked at how ice sheets and vegetation dynamics (including fire activity and nutrient limitation) are affected by the overshoots. For fire activity and nutrient limitation, there are interesting long-term patterns, where shifts in vegetation under climate change play out differently in scenarios with and without overshoot. The effects of the overshoots on the simulated Greenland ice sheet remain relatively small in NorESM, except for very large overshoots. Nevertheless, it is important to stress that already the committed ice loss without overshoot might be large and irreversible. IMPOSE also contributed to improving Integrated Assessment Models (IAMs) regarding their representation of the carbon cycle and climate change. This is an important task, since these models are used to make policy decisions, and a skillful modeling of climate and carbon cycle under overshoot conditions is important to assess such pathways. Results of our work with the DICE model have been published in a paper (https://doi.org/10.1088/1748-9326/ac22c0) that describes the implementation of a more skillful climate and carbon cycle module into DICE. We decided to use the "Finite Amplitude Impulse Response" (FAIR) model to replace the original DICE climate and carbon cycle module, which is well known to have strong limitations. Results show that there are indeed large differences in climate policies (carbon prices) due to these updates if the DICE model is used to asses overshoot scenarios.

The Earth system response to phasing out emissions and to negative emissions has developed into a extremely important research topic during the past 5-10 years and IMPOSE was a very timely project. The project contributed significantly to closing some of the large fundamental knowledge gaps that existed (and still exist). It significantly contributed to positioning the NorESM community at the forefront of Earth system model-based research on these topics. The results of IMPOSE have been published in high level international journals (10 articles published, 4 under preparation), and we expect that these results will have an impact in future IPCC assessments. The activities and results in IMPOSE were instrumental in building up considerable expertise, which resulted in new international collaborations, for example, in two EU projects that deal with carbon neutrality and negative emission (OceanNETs and RESCUE). It is fair to say that without the expertise gained in IMPOSE, these collaborations would not have been possible. There is a growing concern about the lack of successful climate mitigation through rapid reduction of emissions. The feasibility of carbon dioxide removal therefore gains growing attention among policy makers and the general public. On the other hand, it is important to see that we are not on a road to a worst case emission future either. Therefore, climate sciences currently re-focuses on future scenarios where emissions gradually go to (or below) zero, and IMPOSE has greatly contributed to this development for the Norwegian climate research community. IMPOSE has contributed to numerous outreach event directed towards Norwegian policy makers and the general public, for example “Den store karbonløsningsdagen”, which was organized by the Bjerknes Centre and Norsk Klimastiftelse, and was well attended by actors from Norwegian politics, economics, and civil society.

It is now widely recognized that it might be inevitable to overshoot the 1.5 degree target temporarily, and to rely on net negative CO2 emissions over a later period to bring the Earth system back to a "safe" state. Obviously, less drastic short-term emission reductions facilitate a smoother transition of the global energy system and save social costs. However, these benefits need to be weighed against the impacts and costs associated with the overshoot period and the risk of missing the temperature target due to feedback processes in the Earth system. The realization of (net) negative emissions as a - currently largely hypothetical - climate mitigation measure can lead to a weakening and delay of climate mitigation actions, and it has been argued that clear limits to overshoots (e.g. magnitude and duration) need to be defined. However, Earth system science is currently unable to provide such limits since we know only little about the Earth systems' response to CO2 and temperature overshoots. Likewise, the integrated assessment models (IAMs) used to create emission scenarios and assess the economic implications have not been developed for overshoot conditions and might therefore show larger systematic errors under negative emissions. IMPOSE will address these issues by (1) assessing the reversibility for particularly vulnerable Earth system components (permafrost, high latitude ecosystems, Greenland Ice Sheet) under different overshoot scenarios, (2) quantifying feedbacks in the Earth system, particularly carbon cycle feedbacks, when negative emissions are imposed, and (3) improving IAMs for CO2 overshoot conditions based on results from full Earth system models. With these ambitious tasks IMPOSE is at the forefront of research. The project brings together experts in Earth system and integrated assessment modelling to make substantial progress towards a comprehensive assessment of climate mitigation policies that include overshoots and negative emissions.

Publications from Cristin

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Funding scheme:

KLIMAFORSK-Stort program klima