Technical Note: Are large error bars desirable? A note on quantitative model-proxy comparison

Abstract. The combined use of proxy records and climate modelling is invaluable for obtaining a better understanding of past climates. However, many methods of model-proxy comparison in the literature are fundamentally problematic because larger errors in the proxy tend to yield a "better" agreement with the model. Here we quantify model-proxy agreement as a function to proxy uncertainty using the overlapping coefficient OVL, which measures the similarity between two probability distributions. We found that the model-proxy agreement is poor (OVL < 50%) if the proxy uncertainty (σ_p) is greater than three times the model variability (σ_m), even if the model and proxy have similar mean estimates. Hence only proxies that fulfil the condition σ_p < 3σ_m should be used for detailed quantitative evaluation of the model performance.