Temporary overshoot as a concept

The physical climate system puts clear con­straints on human activities that are consist­ent with a peak and decline in the global average temperature. Global warming will be halted only when global net CO₂ emis­sions from human activities reach zero, with peak warming determined largely by cumu­lative net CO₂ emissions up to that point. A subsequent decline in global warming, which is needed to return to 1.5°C, relies on sustained net-negative CO₂ emissions from human activities, with total removals out­weighing residual emissions of all long-lived GHGs. Reducing emissions of short-lived climate forcers, in particular methane (CH₄), would also help limit peak warming. Further reductions beyond 2050 could con­tribute to the subsequent decline in tem­perature. In this sense, achieving net-zero GHG emissions globally, often dubbed “cli­mate neutrality” in climate policy circles, implies net-negative CO₂ emissions, leading to a slight decline in the global temperature.

“Magnitude” and “duration” of an overshoot thus depend on the emissions profile both up to and after exceeding 1.5°C. The faster the world reaches net-zero CO₂ emis­sions and the lower the rate of CH₄ emis­sions, the less the exceedance of 1.5°C. The deeper the level of subsequent net-negative CO₂ emissions, the faster the return to 1.5°C (Figure 1).

Current scientific understanding suggests that net-negative CO₂ emissions would re­duce global warming at roughly the same rate as ongoing emissions increase it. In quantitative terms, reducing the global aver­age temperature by just 0.1°C would require about 220 gigatonnes (Gt) of net-negative CO₂ emissions. Gross amounts of carbon dioxide removal (CDR) will need to be even greater, given that it is impossible to reduce CO₂ emissions fully to zero (hence the net in net-zero). For comparison: 220 Gt amount to five years of current annual emissions, and 100 years of current annual CDR levels (which are almost entirely in the form of con­ventional afforestation and reforestation). Novel methods, for example, bioenergy combined with carbon capture and storage (BECCS) and the direct capture of CO₂ from the ambient air and subsequent storage (DACCS), are both only in the early stages of development.

Risks and impacts

Exceeding 1.5°C will result in greater im­pacts on humans and ecosystems than if global warming had remained below that level, and it presents greater climate risks, including from extreme weather, ecosystem degradation, and socio-economic disruptions. Limiting overshoot – that is, limiting by how much and how long the world ex­ceeds 1.5°C in the first place – remains the critical step in addressing these threats.

It is much less clear at this point how much a subsequent decline in temperature would reverse different climate risks. In prin­ciple, reducing global warming will help limit all physical climate changes – either because they are reversed (e.g. the frequency and intensity of heatwaves), or because the changes will not be as significant as they would have been had the global average temperature remained elevated above 1.5°C (e.g. sea level rise). However, the risk from climate change arises not merely from phy­sical changes in climate, but from the inter­action of such changes with the exposure and vulnerability of human society and eco­systems. Regions that experience extreme climate events during the overshoot phase could face long-term social and economic consequences, since climate-related impacts such as malnutrition, poverty, and infrastruc­ture damage – along with weakened insti­tutions and strained financial capacity – are likely to persist well beyond peak warming.

For many ecosystems, a period of excess temperatures could disrupt species com­po­si­tion and food webs that may be irrevers­ible or take many human generations to recover. However, species extinction will be irreversible, even if restoring a former eco­system state were possible.

A world that returns to global warming of 1.5°C will be a significantly altered and more damaged world than if this level of warming had never been exceeded.

Political significance of overshoot

By now, relying on some form of overshoot trajectory has become the only way to “keep 1.5°C alive”. We do not know yet whether the UNFCCC will explicitly adopt this vision in future decisions. So far, there has been no serious discussion about climate system impacts, political responsibilities, and future policy responses resulting from exceeding – and potential returning to – 1.5°C. This is even though the term “overshoot” can be found in many UNFCCC documents – it is used to refer to the nomenclature of IPCC-assessed mitigation pathways for achieving 1.5°C by 2100 but does not deal with its sub­stance yet.

Taking overshoot seriously would catalyse two paradigm shifts in global climate policy. Initially, the more obvious one seems to be the conceptual introduction of a phase of managed temperature decline, enabled by aiming for net-negative CO₂ emis­sions globally, potentially to be fol­lowed by net-negative GHG emissions. This would immediately raise the question of who is in charge to lead the way into net-negative territory. Under the UNFCCC’s core principle of Common But Differentiated Respon­sibilities (CBDR), the answer can only be that developed countries will have to continue to be the frontrunners by adopting and pur­suing national net-negative targets, which entails stringent emissions reductions while rapidly upscaling CDR. If 1.5°C is to remain the core temperature goal, then net-zero can no longer be seen as an end point but only as a transition point in climate policy.

However, the more pressing and maybe more disruptive shift will come with the inevitable, yet contentious introduction of a new interim – and perhaps only implicit – temperature target: peak warming. Although there has not been any discussion so far, the global climate debate is likely to focus on the 1.7–1.8°C range. Everything below 1.7°C does not seem feasible any­more. Everything above 1.8°C would be too high a level to achieve a return to 1.5°C in this cen­tury and also increase the potential for warming to exceed even 2°C, once scientific uncertainties are taken into account.

Least-developed countries can be expected to call for enhanced financial flows to cover additional damages linked to the exceedance of 1.5°C, even if only thought to be tem­porary. Since countries such as China, India, and Saudi Arabia seem to favour “well below 2°C” as the focal point for global cli­mate policy, the UNFCCC may never be able to reach consensus on an exact peak warm­ing target. But even with ambiguity con­cerning the intended peak level, reaching net-zero CO₂ emissions would remain the global focal point for climate action as a pre­requisite for halting further warming.

Challenges for EU climate policy

If the EU really wants “to keep 1.5°C alive”, it will need to adopt and elevate the con­cept of a temperature overshoot target and proactively deal with its consequences for long-term emissions targets, both in multi­lateral fora and within Europe. Failing to do so will inevitably lead to the impression that “1.5°C overshoot” is nothing more than an interesting thought experiment, and that sta­bilising global warming well above 1.5°C is by now the best the world can still hope for.

At the global level – whether it is the UNFCCC, the G20, or the G7 – the over­shoot concept can only gain political cred­ibility if developed countries underscore it by first committing to reaching and sustain­ing net-negative emissions. The requirement for frontrunners to enter net-negative terri­tory holds, even if global ambition remains limited to net-zero GHG emissions, given the uneven distribution of responsibilities and capacities. This will inevitably trigger a renewed debate about global burden-shar­ing (including the role of emerging econo­mies such as China) and international col­lective efforts to drastically upscale CDR in the medium- to long-term while managing risks from adverse side-effects.

Within the EU, this would first and fore­most entail setting a quantified net-negative GHG emissions target for 2060, as already implicitly foreseen in the European Climate Law, but currently only applied by a single member state (Denmark, with -110 % by 2050). This will refuel burden-sharing con­flicts within the EU, where economically less advanced member states may demand that countries such as Germany, France, and Denmark take the lead and commit to deep levels of net-negative emissions, while Central and Eastern European countries – and those with high shares of hard-to-abate residual emissions – may follow only later. The same applies to economic sectors, where mitigation pathway modelling indicates that the European power sector will deliver net-negative emissions early on while agri­culture stays net-positive (see SWP Research Paper 8/2020). Furthermore, the EU will be forced to develop viable macroeconomic and policy frameworks that advance beyond the standard “polluter pays” principles and simple win-win narratives. Going deeply net-negative will turn carbon pricing from a source of income into a significant financial burden; so far, no convincing vision has been presented that illustrates how entering net-negative territory will give frontrunners a competitive advantage in the long run.

The EU has been a pioneering force in international climate policy for more than three decades now, even within developed-country groupings such as the G7. Reaching net-negative emissions will be the next frontier that multiplies many current policy challenges. Making serious plans to enter this territory will provide a crucial example for the rest of the world.