COP23 Realizing the Paris Agreement: A rapidly closing window of opportunity

Using advanced Earth system models to deliver reliable estimates of future global change

Human activity, notably the emission of carbon dioxide (CO2), is changing the climate. Presently, only ~50% of emitted CO2 remains in the atmosphere causing the climate to warm. The remaining 50% is absorbed by the Earth’s natural carbon sinks, on land and in the ocean. If the efficiency of these sinks decreases in the future in response to climate change, then for a given emission of CO2 a larger fraction of the gas will remain in the atmosphere, causing greater warming than occurs today. For low warming targets the CO2 emission space available to realize these targets is already very small. Climate–carbon cycle feedbacks may reduce this space even further. To provide guidance on the level of CO2 mitigation required to achieve the Paris targets, realistic estimates of future CO2 emissions must be combined with models capable of simulating the coupled climate-carbon cycle response.

At this event, the CRESCENDO project presented the latest research in this field, exploring the likelihood of different CO2 emission trajectories realizing the Paris agreement. Findings show that very large and sustained reductions in CO2 emissions, very soon, are the only credible route for staying below 1.5°C or 2°C global warming. Furthermore, staying below 1.5°C warming by 2100 will likely also require massive deployment of technologies for removing CO2 from the atmosphere. Climate – carbon cycle feedbacks will very likely further reduce the CO2 emission space available for achieving Paris. If triggered, such feedbacks may exhibit significant inertia, overshooting a warming target and then returning to it will therefore be a challenge. Earth system models, as used and further developed in CRESCENDO, are the best tools for quantifying such feedbacks and the risks they pose.