Abstract: Integrated assessment models (IAMs) form a prime tool in informing climate mitigation strategies. Diagnostic indicators that allow us to compare these models can help to describe and explain differences in model projections. This also increases transparency and comparability. Earlier, the IAM community developed an approach to diagnose models (Kriegler et al., 2015).

Here we build on this, by proposing a selected set of well-defined indicators as a community standard, similar to metrics used for other modeling communities such as climate models. These indicators are the relative abatement index (RAI), emission reduction type index (ERT), inertia timescale (IT), fossil fuel reduction (FFR), transformation index (TI) and cost per abatement value (CAV). We apply the approach to 17 IAMs, including both older version as well as their latest versions, as applied in the IPCC 6th Assessment Report (AR6).

The study shows that the approach can be easily applied and allows for comparison of model versions in time. Moreover, we demonstrate that this comparison helps to link model behavior to model characteristics and assumptions. We show that together, the set of six indicators can provide an useful indication of the main traits of the model and can roughly indicate the general model behavior. The results also show that there is often a considerable spread across the models. Interestingly, the diagnostic values often change for different model versions, but there does not seem to be a distinct trend across the different models. 

Abstract: Neither international treaties nor domestic policies control carbon dioxide (CO₂) emissions from international shipping. To enhance mitigation, a new multilateral mechanism could allocate these emissions to national carbon budgets, where different options could be used based on the location of industry actors and ships.

We analyze five allocation options, showing that a clear majority of CO₂ emissions would be distributed to ten countries under each option; however, the top ten countries vary across allocation options and the amount of CO₂ emissions allotted to individual countries could increase their carbon budgets thousand-fold or more.

We further examine how the different objectives, principles for decision-making, and geographical coverage of the United Nations Framework Convention on Climate Change (UNFCCC) and the International Maritime Organization influence the design and implementation of an allocation mechanism under each of these two bodies. We find that the allocation mechanism that best meets criteria related to effectiveness and equity would be one in which emissions are assigned to countries of ship owners, and which operates under the UNFCCC.