An essential component of Earth system science is therefore systematic comparison of “ensembles” of models.17 Efforts such as the Program for Climate Model Diagnosis and Intercomparison (PCMDI) co-ordinated at Lawrence Livermore National Laboratory have made headway in systematising the comparison process.18 This has enabled the use of the models in the Intergovernmental Panel on Climate Change (IPCC) assessment reports. In the most recent round of intercomparisons, CMIP6, researchers found that it was necessary to weighting some models more strongly than others to give a more accurate ensemble than a simple average:19 optimising such weightings is a significant research problem.
An ongoing challenge for Earth system modellers is to understand in which circumstances the Earth system is stable and/or resilient, and when it instead behaves chaotically or changes violently. Palaeoclimatologists have documented many sudden shifts in the climate: these include the 4.2ka BP event (a widespread east Mediterranean drought that may have lasted a century) and the rapid temperature shifts known as Dansgaard-Oeschger events that punctuated the last glacial period. Consequently, some climatologists have suggested the models are unrealistically stable, although a lack of suitably configured models means that these conclusions are premature. It remains uncertain just how much natural instability the system possesses and how to represent this in models.
All these uncertainties would be reduced by improved gathering of observational data, better anchoring the models in reality.