Abstract: Earth System models (ESMs) remain our best tool for process-level understanding of climate. It is well-known that ESMs are computationally expensive to run and require millions of person-hours to develop. Therefore, flexible reduced-complexity models have been created to efficiently emulate the large-scale behavior of ESMs in a fraction of the time. Key Earth System processes such as the carbon cycle, atmospheric chemistry, and the climate response to forcing are parameterised using simple functional relationships that are based on our knowledge of the underlying physics or the behavior of ESMs.
For reliable climate projections, reduced-complexity models need to be calibrated and constrained. Calibration involves tuning the model response to reproduce the global mean responses of existing ESMs, by appropriate selection of model parameters. This calibration provides the basis for a joint probability distribution of parameter sets from which a large Monte Carlo ensemble can be generated. The Monte Carlo ensemble is run in the reduced-complexity model, and then the constraint step rejects parameter combinations that generate simulated climates that are not in agreement with historical observed climate change and assessments of the distribution of key climate variables (e.g. the equilibrium climate sensitivity from the IPCC). This results in a posterior ensemble that can be used for making reliable climate projections whilst preserving observational and assessed uncertainty in our knowledge of the climate system.
Applications of reduced-complexity models include policy-relevant climate projections and coupling to economic models of climate change. The Working Group 3 contribution to the IPCC Sixth Assessment Report assessed nearly 2000 emissions scenarios using reduced-complexity models; work that would be impossible with ESMs. Reduced-complexity models are also an important step in the process chain of moving from emissions to climate impacts. A new initiative will annually update emissions scenarios and climate constraints and provide updated projections, again a task that would be difficult to achieve in ESMs. I demonstrate that updating the model constraints to take into account recent historical emissions and observed climate change (up to 2022) means that it is now very difficult for IPCC emissions scenarios to meet the Paris Agreement 1.5°C goal.
What is the Joint Oslo Seminar (JOS):
- Atmospheric and climate sciences have a stronghold in Oslo among the four institutions University of Oslo, the Meteorological Institute, CICERO and NILU.
- This joint seminar invites renowned international experts to contribute to an informal series of lectures, meant to create interaction with the Oslo atmospheric and climate science community on recent highlights and analysis in the field.
- Normally seminars will be held on Thursdays (12:15 pm -1pm).