The weather risk attribution forecast for December 2012

Whenever an unusual weather event occurs these days, the question is immediately asked: "Are our emissions to blame for this event?" Unfortunately, real-time or near-real-time assessments so far have all been subjective, amounting to contradictory samples of expert opinion which place different weights on various indirect sources of evidence and different interpretations of the question. Here we will present the "attribution forecast" for December 2012 from an on-going systematic real-time system for examining how anthropogenic greenhouse gas emissions have contributed to weather risk in our current climate. By comparing real seasonal forecasts against parallel counterfactual seasonal forecasts of the climate that might have been had human activities never emitted greenhouse gases, this service responds proactively to the question: "Has this event been made more or less frequent by our emissions?" In presenting this information for December 2012, we will discuss what we have learned from three years of operation of this service, most particularly what does not work. As a balance between confidence in the model data, reduction of "look-at-the-neighbour" bias, and relevance, we find that pre-defined regions around 2 million km2 work best. We also find that information is more easily conveyed if we present results in terms of what can be said with confidence concerning exceedance of a set of risk ratio thresholds, rather than estimates of what is most likely. Although this product is produced in parallel with a seasonal forecast, it has been quite clear that presentation of the two must be kept distinct.