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Budgets of carbon emissions that are consistent with limiting warming to no more than 1.5 °C over preindustrial temperatures have been hotly debated, following the Paris Agreement on climate change targets. Here we present comments and primary research discussing the impacts of the debate on decision making processes, and the issues that the climate science community now needs to grapple with.
Upward estimates for carbon budgets are unlikely to lead to action-focused climate policy. Climate researchers need to understand processes and incentives in policymaking and politics to communicate effectively.
The remaining carbon budget consistent with limiting warming to 1.5 °C allows 20 more years of current emissions according to one study, but is already exhausted according to another. Both are defensible. We need to move on from a unique carbon budget, and face the nuances.
If CO2 emissions after 2015 do not exceed 200 GtC, climate warming after 2015 will fall below 0.6 °C in 66% of CMIP5 models, according to an analysis based on combining a simple climate–carbon-cycle model with estimated ranges for key climate system properties.
A 1.5 °C climate target implies total emissions of carbon from the start of 2017 must fall below 195 to 205 PgC, according to an observationally constrained very large ensemble of simulations with an efficient Earth system model.
A combination of the level and rate of human-induced warming allows estimation of remaining emission budgets to peak warming across a broad range of scenarios, suggests an analysis of emissions budgets expressed in terms of CO2-forcing-equivalent emissions.
If emissions continue at the present-day rate, about 22 years are left until global mean warming reaches the 1.5 °C Paris Agreement target, suggests a new metric based on the observed level and rate of anthropogenic warming.