Evaluation of biospheric components in Earth system models using modern and palaeo observations: the state-of-the-art

Earth system models (ESMs) are increasing in complexity by incorporating more processes than their pre- decessors, making them potentially important tools for studying the evolution of climate and associated biogeo- chemical cycles. However, their coupled behaviour has only recently been examined in any detail, and has yielded a very wide range of outcomes. For example, coupled climate- carbon cycle models that represent land-use change simu- late total land carbon stores at 2100 that vary by as much as 600 Pg C, given the same emissions scenario. This large uncertainty is associated with differences in how key pro- cesses are simulated in different models, and illustrates the necessity of determining which models are most realistic us- ing rigorous methods of model evaluation. Here we assess the state-of-the-art in evaluation of ESMs, with a particular emphasis on the simulation of the carbon cycle and associ- ated biospheric processes. We examine some of the new ad- vances and remaining uncertainties relating to (i) modern and palaeodata and (ii) metrics for evaluation. We note that the practice of averaging results from many models is unreliable