Professor Myles Allen CBE FRS

Abstract:

In the absence of immediate, rapid, and unprecedented reduction in global demand for carbon-intensive energy and products, the capture and permanent storage of billions of tons of carbon dioxide (CO2) annually will be needed before mid-century to meet Paris Agreement goals. Yet the focus on absolute emission reductions and cheaper, more temporary forms of carbon storage means that permanent CO2 disposal remains starved of investment, currently deployed to capture only about 0.1% of global Energy and Industrial Process (EIP) emissions. This stored fraction, the percentage of fossil EIP emissions that are captured and permanently stored, must reach 100% to stop EIP emissions causing further global warming. Here, we show that a cost-effective transition can occur by mandating an increasing stored fraction through a progressive carbon takeback obligation (CTBO) on fossil carbon producers and importers. By emulating the behavior of an integrated assessment model (IAM) and employing conservative assumptions for the costs of permanent carbon storage, we show that projected economy-wide costs of a CTBO policy are comparable to the costs associated with achieving similarly ambitious climate goals in IAMs employing a global carbon price, or potentially lower if the perceived policy risk cost associated with a CTBO is lower than that associated with a politically determined carbon price. Compared with a global carbon price, an upstream CTBO has advantages of simple governance, speed, and controllability: equivalent carbon prices under a CTBO are reliably capped by the cost of direct air capture and storage, by ensuring deployment keeps pace with continued fossil fuel use, reducing the risk of punitive carbon prices or more draconian measures being needed to drive out the final tranche of emissions. When combined with measures to reduce CO2 production in the near-term, a CTBO could deliver a viable pathway to achieving net-zero emissions consistent with 1.5°C by mid-century.

Abstract:

The IPCC Special Report on 1.5 °C concluded that anthropogenic global warming is determined by cumulative anthropogenic CO2 emissions and the non-CO2 radiative forcing level in the decades prior to peak warming. We quantify this using CO2-forcing-equivalent (CO2-fe) emissions. We produce an observationally constrained estimate of the Transient Climate Response to cumulative carbon Emissions (TCRE), giving a 90% confidence interval of 0.26–0.78 °C/TtCO2, implying a remaining total CO2-fe budget from 2020 to 1.5 °C of 350–1040 GtCO2-fe, where non-CO2 forcing changes take up 50 to 300 GtCO2-fe. Using a central non-CO2 forcing estimate, the remaining CO2 budgets are 640, 545, 455 GtCO2 for a 33, 50 or 66% chance of limiting warming to 1.5 °C. We discuss the impact of GMST revisions and the contribution of non-CO2 mitigation to remaining budgets, determining that reporting budgets in CO2-fe for alternative definitions of GMST, displaying CO2 and non-CO2 contributions using a two-dimensional presentation, offers the most transparent approach.

Abstract:

Abstract Growth in aviation contributes more to global warming than is generally appreciated because of the mix of climate pollutants it generates. Here, we model the CO2 and non-CO2 effects like nitrogen oxide emissions and contrail formation to analyse aviation’s total warming footprint. Aviation contributed approximately 4% to observed human-induced global warming to date, despite being responsible for only 2.4% of global annual emissions of CO2. Aviation is projected to cause a total of about 0.1 °C of warming by 2050, half of it to date and the other half over the next three decades, should aviation’s pre-COVID growth resume. The industry would then contribute a 6%–17% share to the remaining 0.3 °C–0.8 °C to not exceed 1.5 °C–2 °C of global warming. Under this scenario, the reduction due to COVID-19 to date is small and is projected to only delay aviation’s warming contribution by about five years. But the leveraging impact of growth also represents an opportunity: aviation’s contribution to further warming would be immediately halted by either a sustained annual 2.5% decrease in air traffic under the existing fuel mix, or a transition to a 90% carbon-neutral fuel mix by 2050.

Abstract:

The remaining carbon budget for limiting global warming to 1.5 degrees Celsius will probably be exhausted within this decade1,2. Carbon debt3 generated thereafter will need to be compensated by net-negative emissions4. However, economic policy instruments to guarantee potentially very costly net carbon dioxide removal (CDR) have not yet been devised. Here we propose intertemporal instruments to provide the basis for widely applied carbon taxes and emission trading systems to finance a net-negative carbon economy5. We investigate an idealized market approach to incentivize the repayment of previously accrued carbon debt by establishing the responsibility of emitters for the net removal of carbon dioxide through ‘carbon removal obligations’ (CROs). Inherent risks, such as the risk of default by carbon debtors, are addressed by pricing atmospheric CO2 storage through interest on carbon debt. In contrast to the prevailing literature on emission pathways, we find that interest payments for CROs induce substantially more-ambitious near-term decarbonization that is complemented by earlier and less-aggressive deployment of CDR. We conclude that CROs will need to become an integral part of the global climate policy mix if we are to ensure the viability of ambitious climate targets and an equitable distribution of mitigation efforts across generations.