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Bioenergy is booming as nations seek to cut their greenhouse gas emissions. The European Union declared biofuels to be carbon-neutral, triggering a surge in wood use. But do biofuels actually reduce emissions? A molecule of CO 2 emitted today has the same impact on radiative forcing whether it comes from coal or biomass. Biofuels can only reduce atmospheric CO 2 over time through post-harvest increases in net primary production (NPP). The climate impact of biofuels therefore depends on CO 2 emissions from combustion of biofuels versus fossil fuels, the fate of the harvested land and dynamics of NPP. Here we develop a model for dynamic bioenergy lifecycle analysis. The model tracks carbon stocks and fluxes among the atmosphere, biomass, and soils, is extensible to multiple land types and regions, and runs in ≈1s, enabling rapid, interactive policy design and sensitivity testing. We simulate substitution of wood for coal in power generation, estimating the parameters governing NPP and other fluxes using data for forests in the eastern US and using published estimates for supply chain emissions. Because combustion and processing efficiencies for wood are less than coal, the immediate impact of substituting wood for coal is an increase in atmospheric CO 2 relative to coal. The payback time for this carbon debt ranges from 44-104 years after clearcut, depending on forest type-assuming the land remains forest. Surprisingly, replanting hardwood forests with fast-growing pine plantations raises the CO 2 impact of wood because the equilibrium carbon density of plantations is lower than natural forests. Further, projected growth in wood harvest for bioenergy would increase atmospheric CO 2 for at least a century because new carbon debt continuously exceeds NPP. Assuming biofuels are carbon neutral may worsen irreversible impacts of climate change before benefits accrue. Instead, explicit dynamic models should be used to assess the climate impacts of biofuels.

Mitigation scenarios to limit global warming to 1.5 • C or less in 2100 often rely on large amounts of carbon dioxide removal (CDR), which carry significant potential social, environmental, political and economic risks. A precautionary approach to scenario creation is therefore indicated. This letter presents the results of such a precautionary modelling exercise in which the models C-ROADS and En-ROADS were used to generate a series of 1.5 • C mitigation scenarios that apply increasingly stringent constraints on the scale and type of CDR available. This allows us to explore the trade-offs between near-term stringency of emission reductions and assumptions about future availability of CDR. In particular, we find that regardless of CDR assumptions, near-term ambition increase ('ratcheting') is required for any 1.5 • C pathway, making this letter timely for the facilitative, or Talanoa, dialogue to be conducted by the UNFCCC in 2018. By highlighting the difference between net and gross reduction rates, often obscured in scenarios, we find that mid-term gross CO 2 emission reduction rates in scenarios with CDR constraints increase to levels without historical precedence. This in turn highlights, in addition to the need to substantially increase CO 2 reduction rates, the need to improve emission reductions for non-CO 2 greenhouse gases. Further, scenarios in which all or part of the CDR is implemented as non-permanent storage exhibit storage loss emissions, which partly offset CDR, highlighting the importance of differentiating between net and gross CDR in scenarios. We find in some scenarios storage loss trending to similar values as gross CDR, indicating that gross CDR would have to be maintained simply to offset the storage losses of CO 2 sequestered earlier, without any additional net climate benefit.

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Global negotiations to reduce greenhouse gas (GHG) emissions have so far failed to produce an agreement. Even if negotiations succeeded, however, a binding treaty could not be ratified or implemented in many nations due to inadequate public support for emissions reductions. The scientific consensus on the reality and risks of anthropogenic climate change has never been stronger, yet public support for action in many nations remains weak. Policymakers, educators, the media, civic and business leaders and citizens need tools to understand the dynamics and geopolitical implications of climate change. The WORLD CLIMATE simulation provides an interactive role-play experience through which participants explore these issues using a scientifically sound climate policy simulation model. Participants playing the roles of major nations and regions negotiate proposals to reduce GHG emissions. Participants then receive immediate feedback on the implications of their proposals for atmospheric GHG concentrations, global mean surface temperature, sea level rise and other impacts through the C-ROADS (Climate Rapid Overview and Decision Support) policy simulation model used by negotiators and policymakers. The role-play enables participants to explore the dynamics of the climate and impacts of proposed policies using a model consistent with the best available peer-reviewed science. WORLD CLIMATE has been used successfully with students, teachers, business executives and political leaders around the world. Here we describe protocols for the role-play and the resources available to run it, including C-ROADS and all needed materials, all freely available at climateinteractive.org. We also present evaluations of the impact of WORLD CLIMATE with diverse groups.