Event attribution aims to determine the causes of a particular extreme weather event or short-duration climate event and quantify the contribution of particular drivers to the likelihood or severity of the event. The role of human influence is often of great interest. For many instances, the event is in the same direction as the underlying trends, forming one 'line of evidence' for the attribution. However, some extremes are in the opposite direction to the trend, does this discount anthropogenically-induced climate change as a cause?

Here we present cases where extreme events contrary to the trend may be able to be attributed to anthropogenic climate change. For temperature extremes (cold extremes against a warming trend) there may be changes in the circulation that have been attributed to increasing atmospheric greenhouse gases that then drive an extreme cold event (e.g. the 'wavy' Jetstream in the high latitudes of the Northern Hemisphere). Here we examine the circulation driver of south-west Australian frost (Grose et al. 2018). For extreme precipitation events, there are thermodynamic reasons why a short-duration wet extreme might be more likely to occur even on the background of a downward trend in mean rainfall. The definition of 'extreme' can be important in such attribution: for a multi-week rainfall extreme in south-east Australia in 2016, little influence from anthropogenic forcing was found (Hope et al. 2018; King 2018).

Thus there are some cases where extreme events occur in a direction contrary to the trend and there are good physical reasons why these can be attributed to anthropogenic forcing. However, this is not always the case and it highlights that event attribution studies can benefit from a number of lines of reasoning. In particular, the physical driver, and the timescale where the driver operates, are important to consider.