Overconfidence in Climate Overshoot: A Critical Perspective
The idea that humanity might exceed dangerous global warming limits and later return to safer levels has been discussed for decades, with large-scale carbon dioxide removal (CDR) often seen as a potential solution. The Paris Agreement, adopted in 2015, has brought this issue into sharper focus, particularly its goal of limiting global warming to 1.5°C above pre-industrial levels. However, this limit leaves room for "overshoot"—where warming temporarily exceeds 1.5°C before eventually falling back below it. This raises critical questions about the feasibility and risks of such an approach, as well as overconfidence in our ability to control the situation.
The Overshoot Pathways
The Paris Agreement outlines 1.5°C as the upper limit for global temperature rise, but it does not demand immediate stabilization at this level. Instead, the goal is to peak and then reduce temperatures below 1.5°C. The Intergovernmental Panel on Climate Change (IPCC) has developed emission pathways aligned with the Paris Agreement, which are categorized based on their peak temperature and subsequent decline. One of these pathways, called the "temperature overshoot" pathway, describes scenarios where the 1.5°C threshold is temporarily surpassed.
Within this framework, overshoot is divided into two categories: limited overshoot (exceeding by up to 0.1°C) and high overshoot (up to 0.3°C). While these categories suggest that overshoot will be limited, they are based on median outcomes and do not account for higher, more extreme warming scenarios, which remain plausible. This reliance on median projections can lead to overconfidence in our ability to manage the overshoot effectively.
Risks of Overconfidence
Focusing solely on median outcomes in overshoot pathways carries several risks. While achieving net-negative CO2 emissions (NNCE) may reduce global temperatures over the long term, uncertainties surrounding how the climate will respond to overshoot remain significant. For example, climate feedback loops could amplify warming, making it difficult to control temperature rise even after achieving net-zero CO2 emissions. The long-term impacts of overshoot, such as sea-level rise or irreversible climate effects, could persist for centuries, even if global temperatures eventually decline.
Mitigation and Adaptation Challenges
Even if humanity can reduce temperatures after an overshoot, the question remains whether climate impacts—like regional climate changes or sea-level rise—will also reverse. Research indicates that while temperatures may decrease, certain climate impacts may be irreversible, particularly in vulnerable regions like coastlines and polar zones.
The focus on climate overshoot may also divert attention from more immediate and necessary actions to mitigate climate risks. Instead of assuming that future technologies will allow us to reverse global warming, we should prioritize reducing emissions now to prevent overshoot from occurring in the first place. Overconfidence in our ability to manage and reverse overshoot could lead to delayed action, exacerbating climate risks for future generations.
Conclusion
In addressing climate overshoot, the international community must avoid overconfidence in technological solutions like CDR. While these tools may help lower global temperatures after overshoot, uncertainties in the climate system and long-term impacts pose significant risks. The primary focus should remain on reducing greenhouse gas emissions immediately, minimizing the need for overshoot and its associated risks. Failure to do so could lock in irreversible climate changes, undermining the very goals of the Paris Agreement.
