Law and Governance for Overshoot

---

---

Introduction

There is no option anymore to prevent climate change. Climate change is happening, and it is happening faster than science projected. The crucial questions therefore are: how much climate change — or how much warming — and when? 

The answers to these questions lie generally in the temperature thresholds contained in Article 2.1(a) of the Paris Agreement: holding the temperature increase to well below 2°C and pursuing efforts to limit it to 1.5°C, above pre-industrial levels.

How 1.5°C Became the Agreed Target

The need to hold warming below 2°C became apparent already at the Copenhagen climate COP in 2009, where the adoption of the Copenhagen Accord failed because it contained a temperature reference to 2°C. This was considered unacceptable by some small island states, especially Tuvalu, and sub-Saharan states, claiming that 2°C was disastrous for them as it carries the risk of wiping out their populations.

When the Paris Agreement was negotiated, the temperature target was one of the main issues of contention. During the negotiations, the draft text contained all options from ‘well below 1.5’, ‘below 1.5’, ‘1.5’, ‘well below 2’, ‘between 1.5 and 2’, ‘below 2’ and ‘2°C’.¹ In the end, a political compromise was achieved. This compromise was based on several considerations. First, that 2°C is unacceptable and disastrous. The words ‘well below’ were meant to establish a safety barrier in order to avoid the risk of temperatures spiking up to 2°C. Second, 1.5°C was included to satisfy AOSIS’ demands. Otherwise, there would not have been a treaty. The formulations are political compromises and reflect a global consensus. They were not scientific targets, yet they were based on science.

With the IPCC’s ‘1.5 Warming’ report in 2018,² called for in the decision which adopted the Paris Agreement, the pendulum started to swing. While in Paris it was unclear to many negotiators whether 1.5 was realistic, the IPCC clearly stated that it was. With this, the global focus on climate change shifted to 1.5°C and net-zero emissions by 2050.³

More profound recognition was later achieved in the Glasgow Climate Pact (1/CMA.3) in 2021 and in the Outcome of the First Global Stocktake (1/CMA.5) in 2023, where parties explicitly acknowledged the 1.5°C pathway. The GST decision from 2023 also reflects the IPCC 6th Assessment Report.⁴

Based on these recognitions, the ICJ found recently in its climate change advisory opinion that 1.5°C has become the scientifically based consensus target under the Paris Agreement.⁵ In the Court’s view, the CMA decisions express agreement in substance between the parties regarding the interpretation of Articles 2 and 4 of the Paris Agreement and thus constitute subsequent agreements within the meaning of Article 31, paragraph 3(a), of the Vienna Convention on the Law of Treaties. Accordingly, the Court considered 1.5°C to be the parties’ agreed primary temperature goal.

What 1.5°C Implies

A world warming up 1.5°C is not paradise. Even at warming of only 1.5 degrees, significant impacts will occur. At 1.5°C, about 14% of the world’s population will be exposed to severe heatwaves at least once every 5 years. Sea level rise is unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt. Over the next 2000 years, global mean sea level will rise by about 2–3 metres if warming is limited to 1.5. At 1.5°C, 70–90% of coral reefs are projected to be lost. At 1.5°C, approximately 350 million more people may experience water stress by 2100.⁶

These risks increase with every degree of warming, with every fragment of a degree, and with every tonne of GHG emissions. Risks and projected adverse impacts and related losses and damages from climate change escalate with every increment of global warming,⁷ and every increment will intensify multiple and concurrent hazards. So, 1.5°C of warming is not paradise, but anything permanently above it is worse.

Entering Overshoot: The Inevitable Exceedance of 1.5°C

The most recent projections show that exceeding global warming of 1.5°C is, by now, inevitable. Crossing the temperature threshold requires a two-decadal average of more than 1.5 warming — but all signs point to the fact that we have entered these two decades in 2024,⁸ and that the Earth is now most probably within the 20-year period that will reach the Paris Agreement temperature limit. There is no historic precedent or scenario anymore wherein emissions would reduce sufficiently rapidly to not exceed 1.5°C warming.

The question therefore is not whether global warming will exceed 1.5°C (it will) — but by how much and for how long. And this leads us to the concept of ‘overshoot’.

There are two fundamental options. One is to stabilize temperature increases above 1.5°C — between 1.7 and 1.8°C (but ‘well below 2°C’). This option is a farewell to ‘keeping 1.5 alive’ and implies significantly higher and irreversible risks.⁹ The second option is to ensure that the temperature excess over 1.5°C is as short and small as possible, and then to return to 1.5°C before the end of this century. The IPCC refers to this ‘peak and decline’ scenario as ‘overshoot’.

Overshoot has different meanings. The ordinary, literal meaning is to exceed or breach a limit. But in the IPCC AR6, overshoot means to exceed the 1.5 limit and then to reverse and decline to it — it therefore includes, by definition, the required corrective action. A scenario that peaks at 1.8°C and stabilizes at 1.7 would not be called an overshoot scenario; rather, it would be a ‘permanent exceedance’ or ‘higher stabilization scenario’. An overshoot scenario in the understanding of the IPCC is a scenario that crosses the 1.5°C warming level, and after peaking, declines to or below 1.5°C by 2100.

Overshoot and the Paris Agreement

How does the overshoot scenario relate to the Paris Agreement? The 1.5°C temperature goal is central to the IPCC overshoot scenario — which now becomes the best possible scenario to hold warming to 1.5°C. Article 2 does not exclude temporary exceedance of 1.5°C. Meanwhile, the Agreement’s aim to reach net-zero GHG emissions in the second half of the century (Article 4.1) implies that warming should peak well before the end of the century but does not provide precise timing. Exceeding 1.5°C therefore does not render the goal irrelevant. Indeed, exceedance of 1.5°C, rather than rendering the goal irrelevant, is a wake-up call alerting Parties to redouble their efforts to bring emissions down and thereby halt or even reverse the increase in global warming.¹⁰

Exceeding 1.5°C strengthens rather than weakens States’ obligations to develop and communicate NDCs that reflect their highest possible ambition (PA, Article 4.3).

What Reversal Requires

The next pertinent question is: what is necessary to get temperatures back to 1.5°C after exceeding this limit? The answer depends on the extent of excess and the length of time — on magnitude and duration. But one thing is clear: the shorter and the less the 1.5°C limit is exceeded, the better — and the less risky.

In any case, reversing global warming back to 1.5°C requires even greater mitigation efforts than achieving net-zero emissions. Net-zero global CO₂ emissions would approximately stabilize CO₂-induced warming at around 1.7 or 1.8°C. Reversing such warming levels back to 1.5°C therefore requires net-negative global CO₂ emissions — a situation where anthropogenic, permanent atmospheric carbon dioxide removals (CDR) outweigh any residual CO₂ emissions. Net-zero is important, but not sufficient.

Reversing warming back to 1.5°C will require a combination of three options: (i) rapid and major global CO₂ emissions reduction to net-zero; (ii) global deployment of CDR at a scale of gigatons per year within the next few decades; and (iii) concurrent deep reductions in non-CO₂ emissions, especially short-lived climate forcers such as methane.

Each of these options comes with new and additional policy and legal challenges. Substantial further and more rapid reductions of gross CO₂ emissions to net-zero for hard-to-abate sectors could increase costs, particularly in aviation and long-distance heavy transport. The increased reliance on CDR is necessary not only to counterbalance residual CO₂ emissions to reach net-zero, but to achieve large-scale net-negative CO₂ emissions at a scale of about 20 Gt of atmospheric CO₂ per year. The known CDR technologies all have shortcomings: BECCS at scale bears major risks for biodiversity and food security; DACCS is energy-intensive and has high economic costs; ocean-based methods have limited scientific certainty and potential negative side effects; and none of the technological solutions have been tested or deployed at scale.

Reductions in residual non-CO₂ emissions — primarily methane from agricultural livestock — may require demand-side measures and novel technologies, such as methane inhibitors. Dietary changes to reduce methane emissions could free up grazing land, which in turn might enable land-based CDR without risking food security.

A decline in global warming to 1.5°C after a mid-century peak might have to involve a combination of these three factors, which are complementary. Yet, each one comes with distinct environmental, economic and/or technical barriers.¹¹

Does Overshoot Require New Governance and Legal Approaches?

There has not yet been a political or public debate about overshoot. Several governance and legal shifts are needed:

Moving beyond the net-zero narrative. Net-zero is important, but not sufficient. It is simply misleading as an end goal of climate policy. States and regions would need to start considering negative emission benchmarks from around mid-century. There might be an expectation that developed countries take the lead in exploring this ‘negative territory’ — with significant implications for global equity and the understanding of CBDR with respect to effort sharing and technology transfer.

Guarding the Paris Agreement. The Agreement is central for the IPCC overshoot scenario and remains at highest relevance in providing the necessary tools and guidelines to govern temperature reversal. Yet, it is under pressure. There are already voices claiming that the temperature threshold has been breached and therefore the Agreement is redundant. This is dangerous. Often, these voices were of those that never wanted or accepted the Agreement and find now a channel to further undermine it. Moreover, the current geo-political situation suggests that any negotiation of a new climate treaty would be protracted and complicated. It is necessary to create a shared understanding in the context of the Paris Agreement discussions that 1.5 remains the core target, also for returning after peak temperature. One possible place for such discussions could be the 2028 Second Global Stocktake, whose processes already start in 2026.

Recalibrating policies and measures. Carbon markets make sense in reducing GHG emissions. However, in a carbon-neutral world, they may lose their relevance. When the global net sum of emissions needs to go down to net-zero, there may not be much supply of credits as states will need to exhaust their own mitigation potential domestically. But what about carbon markets in a negative-emission world? For negative emissions, the polluter-pays principle — which underlies carbon markets — does not apply. In the context of Article 6.4 of the Paris Agreement, there is an underlying assumption that it will be primarily developing countries who are the host countries. But in a net-negative world, there will be a need to set and reach ambitious domestic climate targets.

Courts and climate litigation. Many courts have considered carbon neutrality or net-zero an end goal of climate policy. In KlimaSeniorinnen¹², the ECtHR stated that Article 8 requires each Contracting State to undertake measures for the substantial and progressive reduction of GHG emission levels, with a view to reaching net neutrality within, in principle, the next three decades. ITLOS in its advisory opinion stated that States must take all necessary measures to prevent, reduce and control pollution in light of the 1.5°C temperature threshold and timelines of the Paris Agreement,¹³ which it interpreted as meaning net-zero GHG emissions by 2050. The quantification of what corresponds to 1.5°C by these courts may already be outdated — but the judgments and the ‘legalisation’ of net-zero as the end goal stand. It is therefore a pertinent question how to take these judgments forward and ‘overshoot-proof’ them.

Balancing adaptation, mitigation and losses and damages. Overshoot bears risks. The higher and longer the temperature excess, the higher the risks. The level and nature of risk in a world that returns to 1.5°C after an overshoot period will depend not only on the reversal of climate hazards but also on how the exposure of society and ecosystems and their capacity to cope have been affected by the period of temperature excess. There are significant knowledge gaps on risk transmission pathways, especially for the most vulnerable communities and ecosystems.

Solar radiation modification. With the focus on temperature, discussions about solar geoengineering or solar radiation modification (SRM) have increased. SRM is a high-risk response to climate change — one which only addresses consequences rather than causes. There are significant differences between CDR and SRM: while CDR enhances a global public good, SRM creates spatial differences in heating/cooling and can create local ‘hotspots’ or ‘cold spots’. The challenge is the absence of an international framework to govern SRM.¹⁴ Under customary international law, States have the obligation not to cause significant damage to the environment of other states or areas beyond national jurisdiction. The more the discussion addresses peak temperature, excess and overshoot, the more SRM will surface — and we need to be prepared for this.

Conclusion

Exceeding 1.5°C strengthens rather than weakens States’ obligations to develop and communicate ambitious NDCs that reflect their highest possible ambition (PA, Article 4.3). Even in a world that has exceeded 1.5°C of global warming, States must continue to pursue 1.5°C-aligned pathways.¹⁵

It is therefore necessary to bring net-negative emissions into geopolitical as well as national policy conversations — and to adjust regulations and law to these requirements. States need to consider including net-negative targets in their long-term policy goals, rather than treating net-zero as a natural end point for climate policy. The content of NDCs is a legal obligation of conduct informed by stringent due diligence. Stringent due diligence requires alignment with the 1.5°C goal, more demanding climate targets progressively, and the reflection of each party’s highest possible ambition.¹⁶ Accordingly, building in long-term trajectories towards net-negative emissions, based on scaling up of emission reductions, deployment of CDR and reductions of residual non-CO₂ emissions, must become part of every party’s climate governance.

Eventually, it is all about addressing an ‘existential problem of planetary proportions that imperils all forms of life and the very health of our planet’.¹⁷ In a world that exceeds 1.5°C, this holds a truth of unparalleled and unprecedented dimensions, including for the legal profession.

---

Conflict of Interest

The author declares no conflicts of interest.

---

References