A growing number of reports warn that the world is crossing over the 1.5℃ threshold set out in the Paris Agreement and into overshoot. In some contexts, “overshoot” simply means surpassing 1.5℃. However, in the parlance of the Intergovernmental Panel on Climate Change (IPCC), it specifically means exceeding this limit temporarily, and returning to 1.5℃ warming or below later.
Climate models show that in such an overshoot scenario, even with aggressive decarbonization efforts in place, the world will need carbon removal to draw down excess atmospheric CO2 and restore global temperatures. Yet, the path to deploying CDR at climate-relevant scales remains uncertain, as does CDR’s role in climate policy under overshoot conditions. In fact, as a new report points out, “climate policy has yet to incorporate overshoot into its agenda,” and this ambiguity risks further delaying much-needed climate action.
The lack of robust policy discussion around overshoot scenarios stands in stark contrast to the urgency to plan for a world beyond 1.5℃ warming. The effects of warming are already felt globally, in the form of more severe droughts, more frequent heat waves, and more destructive storms. In a post-1.5℃ world, climate-induced stresses are likely to become more acute and frequent, with a higher chance of crossing over climate tipping points. The longer we stay above 1.5℃, and the more we exceed that threshold, the greater the social and environmental risks become. Importantly, there will be irreversible changes to ecosystems and society (distributed unequally across regions), even after temperatures dip back into safer bounds.
Considerations for CDR deployment under overshoot scenarios
Since CDR has a central role to play in restoring climate stability, it’s imperative to think about how the field intersects with overshoot scenarios. In this blog, we present some key considerations for further discussion.
First, we need to be clear eyed about the role of CDR in overshoot scenarios. CDR serves two distinct purposes: removing legacy emissions and offsetting residual emissions. The former draws down CO2 to rebalance the global climate, whereas the latter counteracts present and future hard-to-abate emissions. Although both ends use the same CDR processes, they call for fundamentally different strategies and scales. Achieving a return to 1.5°C requires mobilizing substantial CDR capacity towards removing legacy emissions. For reference, humanity has added over 1,500 gigatons of CO2 to the atmosphere since the start of the Industrial Revolution.
In contrast, applying CDR to offset residual emissions contributes towards net-zero targets, but does not lead to a net decrease in atmospheric CO2. We must also clearly define what constitutes residual emissions, as ambiguity allows CDR to be used as a crutch to offset emissions that actually can be avoided and delay decarbonization efforts. Importantly, deploying carbon removal to prolong oil and gas extractions, such as through enhanced oil recovery (EOR), undermines both intended outcomes (as Carbon180 has previously explained).
Second, the amount of carbon removal we will need is a dial rather than a switch, and is heavily dependent on the pace of decarbonization. In an overshoot scenario where we manage to stay below 1.7℃, today’s level of CDR capacity — which overwhelmingly comes from land use change and forestry activities — may be sufficient for a return to 1.5℃. However, beyond 1.7℃, a return to 1.5℃ relies on adding significant new carbon removal capacity. Estimates suggest that such a reduction in temperature could require between one to five gigatons of CDR per year, depending on the success (or failure) of collective climate actions, with roughly 220 gigatons of CDR needed for every 0.1℃ of cooling.
The magnitude of CDR needed to address overshoot should be seen as both a warning and a call to rapidly decarbonize, not a justification for the unchecked expansion of the carbon market. High-quality and durable CDR is expensive, and some portion will be allocated to offsetting residual emissions. We should view carbon removal as a valuable and limited resource, even as carbon removal moves towards gigaton scale.
Third, a warmer climate puts additional constraints on CDR capacity and efficacy. Additional warming necessitates more carbon removal (including land-based CDR) to cool the climate, potentially placing even greater pressure on already climate-stressed land and water systems. Complementing land-based approaches with technological CDR could help alleviate these pressures, yet most climate models have not yet adequately incorporated technological CDR in planning scenarios.
Moreover, new environmental and climatic factors under overshoot conditions could affect CDR efficiency itself. The interactions between climate stressors and CDR potential are especially complex for open-system CDR and deserve further research. For instance, while elevated atmospheric CO2 can boost carbon uptake in plants, higher temperatures and changing precipitation patterns could accelerate soil carbon loss. Looking at emerging ocean CDR pathways, the potential interactions are even less clear. For example, ocean warming is expected to alter ocean stratification and carbon transport across ocean layers, both of which could have consequential impacts on the efficiency of ocean CDR approaches.
Finally, we need holistic policy frameworks to guide the deployment of carbon removal in addressing overshoot. When included in climate strategies, CDR should be presented in all its diversity, beyond a narrow focus on bioenergy with carbon capture and storage (BECCS) and carbon capture and storage (CCS), especially given that the scale of BECCS presented in IPCC’s projections is unsustainable. Expanding the scope of CDR takes advantage of recent advances in the field, diversifies risks, and opens up future options. In addition, achieving the scale of removal needed to address overshoot is a massive undertaking, and calls for coordination between CDR deployment and other climate strategies, including decarbonization, climate adaptation, and efforts to remove non-CO2 greenhouse gases.
At this point, the science is clear: carbon removal is an essential element of any credible plan to address overshoot. The time is ripe for the conversation to shift from whether CDR is a viable climate solution, to questions about the appropriate scale, use cases, enabling conditions, and mix of CDR that can best support a return to 1.5℃ or lower. Acknowledging the possibility of overshoot is not conceding defeat. Rather, it can facilitate more effective responses and better planning. This is a critical discussion that both CDR practitioners and climate policymakers should engage in now.
Edited by Tracy Yu. Image by Ahmed.
