Kevin E Trenberth; 7 November 2016 At least in the present state of knowledge, in our view a more fruitful and robust approach to climate extreme-event attribution is to regard the circulation regime or weather event as a conditional state (whose change in likelihood is not assessed), and ask whether the impact of the particular event was affected by known changes in the climate system’s thermodynamic state (for example sea level, sea surface temperature or atmospheric moisture content), concerning which there is a reasonably high level of confidence. Examples There are several events whose attribution has not been addressed but that received an enormous amount of media attention, one example being Super Typhoon Haiyan/Yolanda, the strongest recorded storm ever to reach land. The ocean heat content (OHC) and sea level in the region had increased a great deal since 1993 and especially since 1998. Consequently, as the typhoon approached the Philippines, it was riding on very high sea surface temperatures (SSTs) with very deep support through the high OHC, and the strong winds and ocean mixing did not cause as much cooling as would normally be experienced, probably helping the storm to maintain its tremendous strength. Moreover, the storm surge was undoubtedly exacerbated considerably by the sea levels, which were some 30 cm above 1993 values. Although natural variability played a major role, there was also a global component through increased OHC from the Earth’s energy imbalance. Conflicting results for the Russian heat wave in 2010 were to some extent reconciled by recognizing that each study was about different aspects. One study focused on dynamical aspects whereas the other was much more about the record high temperatures and thermodynamic aspects. One study of the recent California drought found no significant trends in winter precipitation in recent decades, another pointed out the critical role of the record high annual mean temperatures in combination with record low annual mean precipitation for 2013 which led to increased evapotranspiration and more intense drought. The combination of these had impacts on water shortages, vegetation and agriculture, and increased wildfire risk. The odds of this combination have increased with humaninduced climate change and anthropogenic warming has increased drought risk. Again these two studies are consistent with the view that the atmospheric circulation changes are not the dominant factor as far as the climate change aspects are concerned. Hence the consequences are that things dry out quicker (stronger longer droughts) –as the atmosphere demands more evaporative moisture - and the extra moisture means heavier rains and greater risk of flooding, as observed in 2016 in hurricane Matthew and the earlier Louisiana floods. In fact it is the combination of natural variability (weather, El Niño etc.) and climate change when they go in the same direction that causes records to be broken. Hence there are more extremes of all sorts. The changes in extremes have huge impacts on society and on ecosystems and the environment. The costs of the total events are in the tens of billions of dollars each year, but there is no clean separation as to how much should be ascribed to human influences. Of course the human costs and loss of life are also enormous. In one sense, the extreme event would not have happened without global warming, because otherwise the event would have been well within previous experience, and so the whole cost might be so assigned. Every event is different. They occur in different places and evolve very differently, whether floods, wild fires, or heat waves, but they all have one aspect in common, they would not have been as severe without the human influence.