Overshoot: What does it mean to exceed and return to global warming of 1.5°C?

People take refuge on the roofs of buildings following flooding caused by Cyclone Idai in Mozambique. March 2019. Credit: World Vision/DFID

Richard Betts, https://experts.exeter.ac.uk/22431-richard-betts-mbe https://www.metoffice.gov.uk/research/people/richard-betts,  has been collaborating with us on Net Zero+ Project,  for some years now,  and shared this latest publication, which was published on 14th April.

Many national and corporate climate targets seek to limit global warming to 1.5°C. Yet, global warming continues to increase. This makes it almost inevitable that 1.5°C of global warming will be exceeded.

Scientists, policymakers, and society will increasingly engage in a new discussion about what it now means to “keep 1.5°C alive”: is it possible to exceed 1.5°C but to return to or below this level within a given timeframe? What would be needed to at least keep that option on the table, and what would be its consequences?

A first comprehensive conceptual review explores the implications of such “overshoot” pathways. Authored by an international team of scientists and design experts involved in assessments by the Intergovernmental Panel on Climate Change, the review considers climate-related damages and risks, adaptation and vulnerability, global emission requirements, and issues arising for global and national climate policy. The review sets out evidence and knowledge gaps that illustrate the choices and challenges that decision-makers are now facing.

A key conclusion is that accelerated emission reductions in the near term are critical to limit peak warming and to keep the option of a return to 1.5°C on the table, but that effective and equitable adaptation will remain critical to limit damages along the way. Many more decisions and choices will determine what risks such a return would avoid, what damages would nonetheless be irreversible, and the implications for equity and climate justice.

The review focuses on the “overshoot”: global warming trajectories that exceed a specified warming limit for some time, but return to or below that limit within a specified time period.

Co-author Professor Richard Betts MBE, from the University of Exeter and the Met Office Hadley Centre, said: “Global warming is now approaching 1.4°C and global emissions are not yet decreasing, so we urgently need to consider responses to exceeding 1.5°C. If we can reduce warming back to this level, this will still be better than staying at higher warming – but ‘overshooting’ will still have led to long-term consequences.”

Risks, adaptation, and the path back to 1.5°C

The new paper provides a clear framework for understanding climate risks in the context of overshoot pathways. It examines how risks evolve when global temperatures exceed 1.5°C, explores adaptation options, and assesses their feasibility from a geophysical and Earth system response perspective. It also highlights key barriers, challenges, and knowledge gaps that need urgent attention.

The study applies this framework to risks aggregated according to the IPCC’s “reasons for concern”, which include: irreversible losses to ecosystems and cultural heritage, increasing extreme weather events, shifting regional vulnerabilities that could worsen global inequalities, complex global-scale impacts, and the risk of triggering irreversible tipping points like ice sheet collapse or Amazon forest dieback.

A key insight is that a world that exceeds 1.5°C even only temporarily will be a more damaged world than if we had avoided exceeding 1.5°C – but bringing warming back to or below 1.5°C would generally result in lower risks than if warming stabilised and remained permanently above 1.5°C.

To reverse global warming after overshoot, climate policy could pursue different strategies, described in the paper by three illustrative and complementary strategies: further scaling up carbon dioxide removal (CDR), further reducing residual CO2 emissions, and further reducing short-lived climate forcers especially methane. To achieve a reduction in global temperature, these actions would need to go beyond what is currently envisaged in most national climate targets.

The feasibility of returning to 1.5°C depends on how high peak warming reaches – lower peaking reduces environmental, technological, and economic barriers to recovery. Accelerating near-term actions to reduce emissions is shown to be a key prerequisite to keep a return to 1.5°C on the table, at least in principle.

Alongside the paper, the Euro-Mediterranean Center on Climate Change (CMCC) has launched a digital platform to provide accessible, science-based insight on the concept of overshooting 1.5°C global warming and its implications for climate risks, adaptation, and mitigation.

The overshoot journey is not a straightforward process but a complex path shaped by multiple competing motivations and pressures. Decision-makers will need to navigate a variety of competing drivers, trade-offs, and synergies when considering whether and how quickly to reduce global temperatures back below 1.5°C after exceeding that threshold.

The journey along an overshoot pathway will require simultaneous, integrated decisions on adaptation, mitigation, and resilience, all while accounting for differing preferences, capacities, and responsibilities for action.