These are approaches that allocate targets to countries based on the annual flow of emissions (as opposed to the cumulative stock of emissions). The contract and convergence (C&C) approach to mitigation is one of the
oldest methods within this set of approaches. In this approach, equity is operationalised by considering the equality of per capita emission flows. What this means is that developed nations must reduce, and developing
nations can increase their per capita emissions flows, and thus converge to a common target at a predetermined year. Some variants of this method utilise a country’s per capita GDP or other economic criteria to
determine the reduction rates in the annual emission flows.
Such flow-based understanding is common and seen in official documents of the UNFCCC, e.g.Nationally Determined Contributions (NDCs) of developed countries, targets in the Kyoto Protocol, among others. But as easy as
they are to formulate or implement, these approaches have 3 fundamental problems:
- They do not include historical emissions and by their own definition consider themselves to be ‘forward looking’ in nature.
- The flow trajectories have implied cumulative emissions associated with them which are ignored. This may mean that flow-based targets of the developed nations allow them to capture more carbon space into the future
even though they have exceeded their fair share in the past. This is unfair to historically low emitting nations.
- There are versions of C&C which have an arbitrary clause that developing nations cannot exceed the emissions of developed countries. This limit on developing country emissions may severely restrict them from utilizing
resources for their development.
Another approach in this category is the burden sharing or ‘Bending the curve’ approach and its variants. This approach considers the mitigation burden to be the effort required to move from a flow-based baseline emissions
trajectory to a stabilization trajectory. The baseline scenarios are typically constructed using results from Integrated Assessment Models.
The steps to distribute the mitigation burden are varied. One approach that comes closest to including equity is the Greenhouse Development Rights approach or its newer version, the Equity Reference Framework. This
allocates emissions rights to countries based on the proportion of the country’s population living below the poverty line. CBDR is implemented by allocating a share of the burden, i.e., the difference between the
business-as-usual trajectory and the stabilisation trajectory, to countries, based on a formula. This formula includes both responsibility (in terms of cumulative emissions from a base year, or per capita emissions)
and capability (i.e., per capita GDP, non-income HDI).
The advantages of this approach include:
- The explicit acknowledgment of both responsibility and capability, the two key elements of Article 3.1 of the Convention.
- It uses cumulative historical emissions as an indicator of responsibility, but the actual calculations of mitigation burden are emissions flow based. This makes it amenable to the five-year review process that is
part of the Paris Agreement’s Article 14 on the Global Stock Take.
- It factors in equity without having to invoke ‘entitlements’ which makes it potentially acceptable to all parties, especially the developed countries.
The drawbacks of this approach are the following:
- The mitigation burden is the difference between the global stabilization trajectory and the baseline trajectory for each region.
- For every temperature target there is a range of associated cumulative emissions and so multiple stabilisation trajectories are possible for the same value of the carbon budget. However, only one such trajectory
is chosen in this approach. This choice influences the estimation of mitigation burdens.
- Baseline trajectories are highly speculative. They are essentially counter-factual as very often, they are constructed based on assumptions that do not include current trajectories (encompassing stated mitigation
policies of nations). Additionally, for developing countries with rapidly changing economic structures there is a much higher chance that any given baseline will not hold for the long term. This implies
that mitigation burdens that are calculated using both these two key criteria are highly uncertain.
- Mitigation burden is calculated progressively. So, any economic growth will automatically lead to an increase in the share of the mitigation burden that a country has to undertake. In this approach ‘fair share’
is not the ‘fair share of the carbon space’, but a ‘fair share of the mitigation burden’. So progressive estimation of the burden means that with economic growth the share of the carbon space for the country
changes, and hence any steps taken by the country to improve their circumstances will be penalized with a higher mitigation burden. This is the cost of not considering an explicit allocation of entitlements.