Delayed and disorderly transition

Challenges to remaining within a 2°C carbon budget

Climate science suggests that average global temperature rises depend on cumulative emissions of greenhouse gases. In that context, the IPCC provides estimates of carbon budgets consistent with different probabilities of limiting average global temperature rises to different levels.

2°C carbon budget

The IPCC estimates that for a high (83%) probability of limiting global temperature rises to 2°C, the remaining carbon budget is around 900GtCO₂ (measured from the beginning of 2020). This budget estimate includes anthropogenic CO₂ emissions from agriculture, farming and other land use (AFOLU), but excludes the global warming effects from non-CO₂ emissions (such as methane) whose global warming effects are accounted for separately when estimating the CO₂ budget.¹

It should be recognized that the IPCC estimates of carbon budgets are uncertain. For example, if the assessed probability of temperature rises remaining within 2°C is reduced from 83% to 67%, the estimated remaining carbon budget increases from 900GtCO₂ to 1,150GtCO₂. This uncertainty, and the broader uncertainties around the links between emissions and average temperature rises, mean that any analysis based on estimates of carbon budgets should be viewed as only illustrative.

Adjusting the emission pathways

To compare the carbon emissions implied by Current Trajectory and Below 2° with the IPCC 2°C carbon budget, the emission pathways implied by the scenarios are adjusted to include IPCC estimates of AFOLU-related emissions and to exclude estimates of methane emissions associated with the production, transportation and distribution of fossil fuels and from the incomplete combustion of traditional biomass (see Modelling approach for Delayed Below 2° in the Annex).

The adjusted emissions pathway for Below 2° suggests that, given the low level of net emissions in 2050 and the pace at which they are declining, cumulative carbon emissions in Below 2° are likely to stay within the IPCC 2° carbon budget.

Risk of exceeding 2°C carbon budget

In contrast, cumulative carbon emissions in Current Trajectory exceed the IPCC 2°C carbon budget in the early 2040s.

This suggests that the longer the world remains on a pathway like Current Trajectory, the harder it would be to stay within a 2°C carbon budget. This raises the risk that an extended period of delay could increase the economic and social cost of remaining within a 2°C budget.

This risk is explored in an alternative Delayed and disorderly sensitivity.

Delaying the energy transition could lead to a costly and disorderly adjustment pathway

Delayed and disorderly

The Delayed and disorderly sensitivity assumes that the global energy system moves in line with Current Trajectory for a period, after which sufficient polices and actions are undertaken to bring about an accelerated fall in carbon emissions consistent with meeting a 2°C carbon budget.

Delayed and disorderly also assumes that there is a maximum speed at which the energy system can decarbonized in an ‘orderly’ way: that is, without having to resort to policies and actions that have substantial and widespread economic and social costs.

The maximum pace of an ‘orderly’ transition is uncertain and would depend on the circumstances that trigger the decision to pursue an accelerated energy transition, and on the technologies available at that time that might help enable a rapid decarbonization.

‘Orderly’ transition

For illustrative purposes, Delayed and disorderly assumes that the maximum pace of an ‘orderly’ transition can be approximated by the speed of decarbonization in Below 2°.

This stylised assumption suggests that the global energy system could not continue along the Current Trajectory pathway much beyond the early 2030s and still be able to bring about an orderly transition consistent with meeting a 2°C carbon budget.

‘Disorderly’ transition

This rough approximation suggests that if a move to an accelerated transition is delayed beyond the early 2030s, it would be increasingly hard to stay within a 2°C carbon budget without the subsequent need for costly – or ‘disorderly’ – measures.

These measures – which could take many different forms – would need to rapidly reduce or curtail the use of unabated fossil fuels and highly emitting activities, in order to further accelerate the speed of decarbonization.