Professor Myles Allen CBE FRS

Abstract:

The concept of net-zero carbon emissions has emerged from physical climate science. However, it is operationalized through social, political and economic systems. We identify seven attributes of net zero, which are important to make it a successful framework for climate action. The seven attributes highlight the urgency of emission reductions, which need to be front-loaded, and of coverage of all emission sources, including currently difficult ones. The attributes emphasize the need for social and environmental integrity. This means carbon dioxide removals should be used cautiously and the use of carbon offsets should be regulated effectively. Net zero must be aligned with broader sustainable development objectives, which implies an equitable net-zero transition, socio-ecological sustainability and the pursuit of broad economic opportunities.

Abstract:

Meeting the Paris Agreement temperature goal necessitates limiting methane (CH4)-induced warming, in addition to achieving net-zero or (net-negative) carbon dioxide (CO2) emissions. In our model, for the median 1.5°C scenario between 2020 and 2050, CH4 mitigation lowers temperatures by 0.1°C; CO2 increases it by 0.2°C. CO2 emissions continue increasing global mean temperature until net-zero emissions are reached, with potential for lowering temperatures with net-negative emissions. By contrast, reducing CH4 emissions starts to reverse CH4-induced warming within a few decades. These differences are hidden when framing climate mitigation using annual ‘CO2-equivalent’ emissions, including targets based on aggregated annual emission rates. We show how the different warming responses to CO2 and CH4 emissions can be accurately aggregated to estimate warming by using ‘warming-equivalent emissions', which provide a transparent and convenient method to inform policies and measures for mitigation, or demonstrate progress towards a temperature goal. The method presented (GWP*) uses well-established climate science concepts to relate GWP100 to temperature, as a simple proxy for a climate model. The use of warming-equivalent emissions for nationally determined contributions and long-term strategies would enhance the transparency of stocktakes of progress towards a long-term temperature goal, compared to the use of standard equivalence methods.
This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Abstract:

The question of how humans have influenced individual extreme weather events is both scientifically and socially important. However, deficiencies in climate models’ representations of key mechanisms within the process chains that drive weather reduce our confidence in estimates of the human influence on extreme events. We propose that using forecast models that successfully predicted the event in question could increase the robustness of such estimates. Using a successful forecast means we can be confident that the model is able to faithfully represent the characteristics of the specific extreme event. We use this forecast-based methodology to estimate the direct radiative impact of increased CO2 concentrations (one component, but not the entirety, of human influence) on the European heatwave of February 2019.

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.