Models are an essential component of any assessment of ecosystem response to changes in global climate and elevated atmospheric carbon dioxide concentration. The problem with these models is that their long-term predictions are impossible to test unambiguously except by allowing enough time for the full ecosystem response to develop. Unfortunately, when one must assess potentially devastating changes in the global environment, time becomes a luxury. Therefore, confidence in these models has to be built through the accumulation of fairly weak corrobatin evidence rather than through a few crucial and unambiguous tests. The criteria employed to judge the value of these models are thus likely to differ greatly from those used to judge finer scale models, which are more amenable to the scientific tradition of hypothesis formulation and testing. This article looks at four categories of tests which could potentially be used to evaluate ERCC (ecosystem response to climate and carbon dioxide concentration) models and illustrates why they cannot be considered crucial tests. The the synthesis role of ERCC models are is discussed and why they are vital to any assessment of long-term responses of ecosystems to changes in global climate and carbon dioxide concentration. 49 refs., 2 figs.