Overshoot will remake climate politics as we know it
By Harry Smith, Ingrid Sundvor and Petra Bistricic
In the coming years, almost certainly by the early 2030s, we will exceed the 1.5˚C target stipulated in the Paris Agreement (Betts et al., 2023; Copernicus). Recent climate negotiations have been accompanied by increasingly panicked metaphors, as to whether 1.5˚C is ‘alive’ or ‘dead’, or in a liminal state in between. Soon, we will know for certain (New Scientist, 2024). Once we cross beyond the 1.5˚C target, we will enter a new era in climate politics, that of ‘overshoot’ (Reisinger, 2025).
The near-linear relationship between cumulative CO2 emissions and global temperatures means there is no pass or fail threshold when it comes to the climate, every tenth of a degree of warming matters (Allen et al., 2009). In climate politics, however, exceeding 1.5˚C will likely be treated as the first failure of the Paris Agreement. This failure may be used to undermine ambition and question the value of climate targets altogether.
In the language of the IPCC, overshoot does not only represent an exceedance of a temperature limit, as implied by the common use of the term, but also corrective action to return below it (Reisinger, 2025). In other words, for 1.5˚C, overshoot involves temporarily exceeding the limit before returning below 1.5˚C by the end of the century (Reisinger, 2025). In the IPCC’s latest assessment report, ‘C2’ scenarios allow for ‘high overshoot’, reaching around 1.6 - 1.7 ˚C before returning below 1.5˚C by 2100, whilst ‘C1’ scenarios aim to largely stay below 1.5˚C across the century. These scenarios illustrate the challenges of overshoot. For example, scenarios with high overshoot (C2) rely much more on carbon dioxide removal methods, such as direct air carbon capture and storage (DACCS), a technology that is still nascent compared to the scales reached in scenarios assessed by the IPCC. High overshoot scenarios similarly entail greater climate impacts than scenarios that stay below 1.5˚C (Johansson, 2021).
Figure 1 - C1 and C2 scenarios compared. Sources, scenarios from the AR6 Database (Byers et al., 2022). The historical timeseries is annual, sourced from ‘Indicators of Global Climate Change 2023’ (Smith, C. et al. 2024)
Within climate politics, it will be tempting to claim that overshoot has not changed the sequence of climate action laid out in Article 4.1 of the Paris Agreement: countries must still peak their emissions, rapidly reduce them thereafter, soon reaching a balance of emissions and removals, or ‘net zero’ (Rogelj, 2023). But overshoot adds an additional step: countries must go beyond net zero and attain net-negative emissions.
Net-negative requires either scaling carbon dioxide removal beyond the extent of the remaining, or ‘residual’ emissions, at the point of net zero, or further shrinking these residual emissions (Reisinger, 2025). Both will likely be challenging. Further scaling carbon dioxide removal may put too much pressure on land and ecosystems (Deprez et al., 2024), whilst deeper residual emission cuts may rely on large lifestyle changes to reduce demand, such as diet shifts away from animal agriculture (Humpenöder et al., 2024).
Net-negative targets may also require a new commitment within the UNFCCC, a shared political goal to return to 1.5˚C, but this is far from guaranteed. Current national net zero targets put the world on track for around 1.9˚C of warming (Climate Action Tracker, 2024). Many large economies are already off track for meeting their 2030 targets, and political backsliding is already apparent (Nascimento et al. 2023). In the US, for example, the current administration has withdrawn from the Paris Agreement altogether (White House, 2025).
In this context, a new international consensus to stabilise warming at a level above 1.5˚C seems to be the more likely outcome (Reisinger, 2025). But such a consensus would require relitigating the Paris Agreement’s temperature goals. Article 2 of the agreement currently commits parties to hold global average temperatures to ‘well below 2˚C’. What ‘well below’ means, however, has never been formally clarified (Schleussner et al. 2022). Clarifying ‘well below’ can serve to place an upper limit on overshoot, and should be an urgent agenda item for upcoming COPs (Geden and Löschel, 2017).
Is there hope?
There might be. A handful of governments have already signaled net-negative ambitions (Dunne, 2024) but they will face growing pressures to justify the fairness of their targets, especially in light of their historical contribution to warming. This is where the concept of ‘carbon debt’ re-enters the conversation.
Carbon debt has long been proposed in the UNFCCC. The first proposals of using carbon debt to inform climate targets date back to 1997, but were sidelined during the run-up to the Kyoto Protocol, the predecessor to Paris (Müller, Höhne and Ellermann, 2009). Since then, the case for carbon debt has only grown stronger: half of all human-caused CO2 was emitted after 1995, leaving little room to hide behind different baselines (Figure 2).
Still, carbon debt remains politically difficult. The Paris Agreement’s bottom-up structure, where countries set their own targets, conflicts with top-down formulas of responsibility. Net-negative targets are therefore largely dependent on enlightened governments acknowledging their own carbon debts.
Figure 2 - Cumulative fossil CO2 emissions since 1750. Based on the Global Carbon Budget, processed by Our World in Data.
Achieving not only net-negative requires setting a new target, it also requires ‘net-negative ready’ climate policy (Schenuit, Geden and Peters, 2024). One emerging idea is that large emitters would accrue a carbon debt before net zero, and then repay the debt by deploying carbon dioxide removal (Bednar et al. 2021; Bednar et al. 2024). But for this to work, these schemes need to be put in place now. Outside of this, few policy proposed exist, beside large public subsidies funded by more general taxation (Bednar, Obersteiner and Wagner, 2019). Both options may prove politically challenging, but both require urgent research and debate.
Overshoot is the first real test of the Paris Agreement. It will reshape global climate politics, revisiting long-standing debates around responsibility and fairness, increasing the need for net-negative targets, and intensifying the scrutiny of carbon dioxide removal. The familiar metaphors of 1.5˚C being ‘alive’ or ‘dead’ will reach a logical end, to be replaced by calls for 1.5˚C to be ‘resuscitated’.
At Carbon Balance Initiative, we are working with academics and civil society to develop net-negative ready policy, informing the debate on overshoot and net-negative targets. Governments should act now to prepare for what is coming, by:
Pushing for an upper limit on overshoot in international climate negotiations,
Setting clear net-negative targets beyond vague ambitions, extending the time horizon of climate policy beyond 2050,
Starting to develop net-negative ready policy today.