Kevin E Trenberth; 7 November 2016 More generally, there are perhaps two main kinds of attribution performed. The first relates the particular extreme event to the associated weather and weather patterns with statements such as ‘the drought was caused by a blocking anticyclone’; ‘the outbreak of tornadoes was caused by a displaced and active storm track and jet stream’ but although useful, they are really a description of the event, not a cause and do not ask the questions ‘Why did that weather phenomenon behave the way it did?’ In particular, ‘What influences external to the atmosphere were playing a role, and what climate factors were in play?’ The second kind of attribution relates to the objective of assessing the role of human activities, and especially of human-induced climate change, in the event. Results depend, however, upon how the questions are framed. In the past the traditional way of approaching attribution tried to deal with all aspects of the problem. But the changes in weather phenomena and weather systems, where they go, and so forth have infinite variety (called weather) and any climate change signal is tiny (except in the case of the ozone hole). This has confounded the results. In particular, the conventional approach to attribution of climate events is to characterize the event and ask (i) whether the likelihood or strength of such events has changed in the observational record, and (ii) whether this change is consistent with the anthropogenic influence as found in one or more climate models. This approach has had considerable success with extremes that are strongly governed by thermodynamic aspects of climate change, especially those related to temperature, each finding providing another independent line of evidence that anthropogenic climate change is affecting climate extremes. The conventional approach, however, is severely challenged when it comes to climate extremes that are strongly governed by atmospheric circulation, including local aspects of precipitation. It is inherently conservative and prone to false negatives, which underestimate the true likelihood of the human influence. More fruitful scientific questions In contrast to thermodynamic aspects of climate, forced circulation changes in climate models can be very non-robust, and physical understanding of the causes of these changes is generally lacking. Separating out the thermodynamic from dynamic effects may be a fruitful way forwards and result in a different set of questions to be addressed: • Given the weather pattern, how were the temperatures, precipitation and associated impacts influenced by climate change? • Given a drought, how was the drying (evapotranspiration) enhanced by climate change, and how did that influence the moisture deficits and dryness of soils, and the wildfire risk? Did it lead to a more intense and perhaps longer-lasting drought, as is likely? • Given a flood, where did the moisture come from? Was it enhanced by high ocean temperatures that might have had a climate change component? • Given a heat wave, how was that influenced by drought, changes in precipitation (absence of evaporative cooling from dry land) and extra heat from global warming? • Given extreme snow, where did the moisture come from? Was it related to higher than normal SSTs off the coast or farther afield? • Given an extreme storm, how was it influenced by anomalous SSTs and ocean heat content (OHC), anomalous moisture transports into the storm, and associated rainfall and latent heating? Was the storm surge worse because of high sea levels? In other words, given the change in atmospheric circulation that brought about the event, how did climate change alter its impacts?