Abstract
Global net-negative carbon emissions are prevalent in almost all emission pathways that meet the Paris temperature targets. In this paper, we generate and compare cost-effective emission pathways that satisfy two different types of climate targets. First, the common approach of a radiative forcing target that has to be met by the year 2100 (RF2100), and, second, a temperature ceiling target that has to be met over the entire period, avoiding any overshoot. Across two integrated assessment models (IAMs), we found that the amount of net-negative emissions—when global net emissions fall below zero—depends to a large extent on how the target is represented, i.e. implemented in the model. With a temperature ceiling (no temperature overshoot), net-negative emissions are limited and primarily a consequence of trade-offs with non-CO2 emissions, whereas net-negative emissions are significant for the RF2100 target (temperature overshoot). The difference becomes more pronounced with more stringent climate targets. This has important implications: more stringent near-term emission reductions are needed when a temperature ceiling is implemented compared to when an RF2100 target is implemented. Further, in one IAM, for our base case assumptions, the cost-effective negative carbon emissions (i.e. gross anthropogenic removals) do not depend to any significant extent on how the constraint is implemented, only, largely, on the ultimate stringency of the constraint. Hence, for a given climate target stringency in 2100, the RF2100 target and the temperature ceiling may result in essentially the same amount of negative carbon emissions. Finally, it is important that IAM demonstrate results for diverse ways of implementing a climate target, since the implementation has implications for the level of near-term emissions and the perceived need for net-negative emissions (beyond 2050).
Highlights
One objective of the Paris Agreement is to hold ‘the increase in the global average temperature to well below 2 ◦C above pre-industrial levels and pursue efforts to limit the temperature increase to 1.5 ◦C above pre-industrial levels’
We find that the temperature ceiling target corresponding to the RF targets in the year 2100 (RF2100) target of 1.9 W m−2 is too stringent in the DICE-REV model, and we set a target of 2.2 W m−2 as the most stringent RF2100 target, instead
We find that pathways generated under an RF2100 target have higher CO2 emissions in the near term and greater net-negative CO2 emissions towards the end of the century compared to pathways obtained under a temperature ceiling
Summary
One objective of the Paris Agreement is to hold ‘the increase in the global average temperature to well below 2 ◦C above pre-industrial levels and pursue efforts to limit the temperature increase to 1.5 ◦C above pre-industrial levels’. To represent the temperature goals, IAMs are often designed to meet a given radiative forcing (RF) level in the year 2100 (e.g. Riahi et al 2017), a concentration level (e.g. 450 ppm CO2eq) in 2100 (e.g. Kriegler et al 2013), or stay within a given amount of cumulative emissions by 2100 (e.g. Bauer et al 2018, Luderer et al 2018). These targets are often implemented as physical constraints in an optimization model, or as carbon price pathways selected to meet the constraint. One plausible explanation for the focus on RF levels in 2100 is that this approach has played a dominant role in the scenario framework based on the Representative Concentration Pathways (RCPs), the Shared Socioeconomic Pathways (SSPs), and the Shared Policy Assumptions (SPAs) (Moss et al 2010, van Vuuren, 2014, Kriegler et al 2014)
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