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Since the adoption of the 2015 Paris Agreement and the publication of the 2018 Special Report on Global Warming of 1.5°C of the Intergovernmental Panel on Climate Change, net zero targets have become a central feature in climate policy. This Perspective looks back at the scientific foundations of this recent policy development, the current state of play, and next frontiers for research on this topic.

To limit warming, action plans from countries and companies must be fair, rigorous and transparent. To limit warming, action plans from countries and companies must be fair, rigorous and transparent.

Emissions inevitably approach zero with a “carbon law” Although the Paris Agreement's goals ( 1 ) are aligned with science ( 2 ) and can, in principle, be technically and economically achieved ( 3 ), alarming inconsistencies remain between science-based targets and national commitments. Despite progress during the 2016 Marrakech climate negotiations, long-term goals can be trumped by political short-termism. Following the Agreement, which became international law earlier than expected, several countries published mid-century decarbonization strategies, with more due soon. Model-based decarbonization assessments ( 4 ) and scenarios often struggle to capture transformative change and the dynamics associated with it: disruption, innovation, and nonlinear change in human behavior. For example, in just 2 years, China's coal use swung from 3.7% growth in 2013 to a decline of 3.7% in 2015 ( 5 ). To harness these dynamics and to calibrate for short-term realpolitik, we propose framing the decarbonization challenge in terms of a global decadal roadmap based on a simple heuristic—a “carbon law”—of halving gross anthropogenic carbon-dioxide (CO 2 ) emissions every decade. Complemented by immediately instigated, scalable carbon removal and efforts to ramp down land-use CO 2 emissions, this can lead to net-zero emissions around mid-century, a path necessary to limit warming to well below 2°C.

Committed warming describes how much future warming can be expected from historical emissions due to inertia in the climate system. It is usually defined in terms of the level of warming above the present for an abrupt halt of emissions. Owing to socioeconomic constraints, this situation is unlikely, so we focus on the committed warming from present-day fossil fuel assets. Here we show that if carbon-intensive infrastructure is phased out at the end of its design lifetime from the end of 2018, there is a 64% chance that peak global mean temperature rise remains below 1.5 °C. Delaying mitigation until 2030 considerably reduces the likelihood that 1.5 °C would be attainable even if the rate of fossil fuel retirement was accelerated. Although the challenges laid out by the Paris Agreement are daunting, we indicate 1.5 °C remains possible and is attainable with ambitious and immediate emission reduction across all sectors. Power plants, vehicles and industry will continue to produce emissions for as long as they are used. Here, the authors show that retiring existing fossil fuel infrastructure at the end of its expected lifetime provides a good chance that the 1.5 °C Paris Agreement target can still be met.

Of the carbon dioxide that we emit, a substantial fraction remains in the atmosphere for thousands of years. Combined with the slow response of the climate system, this results in the global temperature increase resulting from CO₂ being nearly proportional to the total emitted amount of CO₂ since preindustrial times. This has a number of simple but far-reaching consequences that raise important questions for climate change mitigation, policy and ethics. Even if anthropogenic emissions of CO₂ were stopped, most of the realized climate change would persist for centuries and thus be irreversible on human timescales, yet standard economic thinking largely discounts these long-term intergenerational effects. Countries and generations to first order contribute to both past and future climate change in proportion to their total emissions. A global temperature target implies a CO₂ “budget” or “quota”, a finite amount of CO₂ that society is allowed to emit to stay below the target. Distributing that budget over time and between countries is an ethical challenge that our world has so far failed to address. Despite the simple relationship between CO₂ emissions and temperature, the consequences for climate policy and for sharing the responsibility of reducing global CO₂ emissions can only be drawn in combination with judgments about equity, fairness, the value of future generations and our attitude towards risk.

To understand how global warming can be kept well below 2 degrees Celsius and even 1.5 degrees Celsius, climate policy uses scenarios that describe how society could reduce its greenhouse gas emissions. However, current scenarios have a key weakness: they typically focus on reaching specific climate goals in 2100. This choice may encourage risky pathways that delay action, reach higher-than-acceptable mid-century warming, and rely on net removal of carbon dioxide thereafter to undo their initial shortfall in reductions of emissions. Here we draw on insights from physical science to propose a scenario framework that focuses on capping global warming at a specific maximum level with either temperature stabilization or reversal thereafter. The ambition of climate action until carbon neutrality determines peak warming, and can be followed by a variety of long-term states with different sustainability implications. The approach proposed here closely mirrors the intentions of the United Nations Paris Agreement, and makes questions of intergenerational equity into explicit design choices. Fundamental value judgments about acceptable maximum levels of climate change and future reliance on controversial technologies can be made explicitly in climate scenarios, thereby addressing the intergenerational bias present in the scenario literature.

Models and scenarios on which climate projection are based vary between IPCC reports. To facilitate meaningful comparison, this study provides probabilistic climate projections for different scenarios in a single consistent framework, incorporating the overall consensus understanding of the uncertainty in climate sensitivity, and constrained by the observed historical warming. Climate projections for the fourth assessment report 1 (AR4) of the Intergovernmental Panel on Climate Change (IPCC) were based on scenarios from the Special Report on Emissions Scenarios 2 (SRES) and simulations of the third phase of the Coupled Model Intercomparison Project 3 (CMIP3). Since then, a new set of four scenarios (the representative concentration pathways or RCPs) was designed 4 . Climate projections in the IPCC fifth assessment report (AR5) will be based on the fifth phase of the Coupled Model Intercomparison Project 5 (CMIP5), which incorporates the latest versions of climate models and focuses on RCPs. This implies that by AR5 both models and scenarios will have changed, making a comparison with earlier literature challenging. To facilitate this comparison, we provide probabilistic climate projections of both SRES scenarios and RCPs in a single consistent framework. These estimates are based on a model set-up that probabilistically takes into account the overall consensus understanding of climate sensitivity uncertainty, synthesizes the understanding of climate system and carbon-cycle behaviour, and is at the same time constrained by the observed historical warming.

Research reported during the past decade has shown that global warming is roughly proportional to the total amount of carbon dioxide released into the atmosphere. This makes it possible to estimate the remaining carbon budget: the total amount of anthropogenic carbon dioxide that can still be emitted into the atmosphere while holding the global average temperature increase to the limit set by the Paris Agreement. However, a wide range of estimates for the remaining carbon budget has been reported, reducing the effectiveness of the remaining carbon budget as a means of setting emission reduction targets that are consistent with the Paris Agreement. Here we present a framework that enables us to track estimates of the remaining carbon budget and to understand how these estimates can improve over time as scientific knowledge advances. We propose that application of this framework may help to reconcile differences between estimates of the remaining carbon budget and may provide a basis for reducing uncertainty in the range of future estimates. A method of tracking changes in estimates of the remaining carbon budget over time should help to reconcile differences between these estimates and clarify their usefulness for setting emission reduction targets.

The Paris climate agreement aims at holding global warming to well below 2 degrees Celsius and to “pursue efforts” to limit it to 1.5 degrees Celsius. To accomplish this, countries have submitted Intended Nationally Determined Contributions (INDCs) outlining their post-2020 climate action. Here we assess the effect of current INDCs on reducing aggregate greenhouse gas emissions, its implications for achieving the temperature objective of the Paris climate agreement, and potential options for overachievement. The INDCs collectively lower greenhouse gas emissions compared to where current policies stand, but still imply a median warming of 2.6–3.1 degrees Celsius by 2100. More can be achieved, because the agreement stipulates that targets for reducing greenhouse gas emissions are strengthened over time, both in ambition and scope. Substantial enhancement or over-delivery on current INDCs by additional national, sub-national and non-state actions is required to maintain a reasonable chance of meeting the target of keeping warming well below 2 degrees Celsius. The objective of the Paris climate agreement is to limit global-average temperature increase to well below 2 degrees Celsius above pre-industrial levels and to further pursue limiting it to 1.5 degrees Celsius; here, the adequacy of the national plans submitted in preparation for this agreement is assessed, and it is concluded that substantial enhancement or over-delivery on these plans is required to have a reasonable chance of achieving the Paris climate objective. Paris climate action plans assessed The principal climate goal of the Paris Agreement of December 2015 is to hold the increase in the global average temperature to well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 degrees above pre-industrial levels. This Perspective assesses the national plans submitted to the Paris meeting for post-2020 action to reduce global greenhouse gas emission by 2030. It also provides projections for global mean temperature increase over the twenty-first century that would be consistent with the present national plans and discusses options that may help to reduce greenhouse gas emissions to levels that are more consistent with maintaining a reasonable chance of meeting the well below 2 degrees Celsius climate target.

Policy, business, finance and civil society stakeholders are increasingly looking to compare future emissions pathways across both their associated physical climate risks stemming from increasing temperatures and their transition climate risks stemming from the shift to a low-carbon economy. Here, we present an integrated framework to explore near-term (to 2030) transition risks and longer-term (to 2050) physical risks, globally and in specific regions, for a range of plausible greenhouse gas emissions and associated temperature pathways, spanning 1.5–4 °C levels of long-term warming. By 2050, physical risks deriving from major heatwaves, agricultural drought, heat stress and crop duration reductions depend greatly on the temperature pathway. By 2030, transition risks most sensitive to temperature pathways stem from economy-wide mitigation costs, carbon price increases, fossil fuel demand reductions and coal plant capacity reductions. Considering several pathways with a 2 °C target demonstrates that transition risks also depend on technological, policy and socio-economic factors.There is a balance in mitigation pathway design between economic transition cost and physical climate threats. This study provides a comprehensive framework to assess the near- and long-term risks under various warming scenarios globally and in particular regions.