Extreme weather events are at the sharp end of climate change. As greenhouse gases continue to accumulate in Earth’s atmosphere, the resultant warming of the climate system changes the nature of extreme weather events. Significant changes in the severity and likelihood of extreme weather events have already been felt globally and in Aotearoa New Zealand.
Almost always, after some extreme weather event has occurred, the first question on the mind of the public, and one increasingly often asked by reporters, is “was this event caused by climate change?”. Answering that question is more difficult than might be expected since every extreme event has a contribution from natural variability as well as from human-induced climate change. Furthermore, it is almost always the case that the person being asked the question doesn’t have access to the quantitative information needed to answer it. The purpose of EWERAM is to semi-automatically generate the required data so that, within days after an extreme weather event has occurred somewhere in Aotearoa New Zealand, the attribution of the severity and/or likelihood of that extreme event to climate change can be quantitatively determined by a domestic expert team.
To achieve this goal we have brought together a team from five institutions across Aotearoa New Zealand (Bodeker Scientific, NIWA, MetService, Victoria University of Wellington, and University of Canterbury) whose combined skills and expertise have been used to develop EWERAM's workflow. We developed two different processes to detect changes in severity and likelihood of heatwaves and extreme rainfall events and thereby made use of the two different approaches to event attribution which are not commonly used together.
To make statements about changes in the severity of some extreme event that has just happened, we first make an important assumption that ‘frames’ the attribution statement in an important way - that assumption is that the event would happen in both preindustrial times and in modern times where climate change is underway. The question then becomes “Given that this event happened, how much worse did climate change make the event i.e. how did the severity of the event change?”. To address that question, we generate three large sets of numerical weather prediction (NWP) model simulations (a set of such simulations is referred to as an ‘ensemble’ of simulations). Acknowledging that we have imperfect information about the state of the atmosphere that triggered the event, each member of the ensemble is slightly different. The first set is called the anthropogenic (ALL) ensemble where the context for the simulations mirrors current conditions under which the extreme weather event evolved; often referred to as the ‘factual’ ensemble. The second is the natural (NAT) ensemble, where the context for the simulations is sourced from a similar synoptic meteorological state, but where sea surface temperatures, air temperatures, and the moisture holding capacity of the atmosphere have been modified to mimic pre-industrial conditions; often referred to as the ‘counterfactual’ ensemble. The extent to which the ALL and NAT ensembles of simulations differ provides a quantitative indication of the contribution of climate change to the severity of the event. A third ensemble, called ALL+ provides a consistency check by adding the anthropogenic change signal to the ALL conditions rather than subtracting them as it has been done to produce the NAT simulations.
The above is considered to be a highly ‘conditioned’ approach – it assumes that synoptic situations, identical to the target event, would have occurred in the past, and that may not necessarily be the case. Thus, while diagnosing the contribution of changes in the thermodynamic state (e.g. increases in ocean and air temperatures) to the severity of the observed event can be done as described above, changes in atmospheric dynamics (i.e. the weather systems themselves) also have an impact on the likelihood and location of extreme events in Aotearoa New Zealand. These changes in atmospheric dynamics are caused by the radiative forcing imposed by increased greenhouse gas loading of the global atmosphere. Diagnosing changes in the likelihood of events is much more difficult. However, within EWERAM we have access to thousands of years of ALL and NAT simulations for the Australia/New Zealand region generated through the weather@home project. For some target extreme weather event, we can search for, and count, the number of times that a similar event occurred in the ALL and NAT ensembles of simulations available from the weather@home archives. Differences between the ALL and NAT counts allow us to make quantitative statements about the changes in the likelihood of that type of extreme weather events.
The EWERAM project uses the combined skills and expertise of a team of researchers from five institutions across New Zealand, viz.: Bodeker Scientific (lead organisation), MetService, NIWA, Victoria University of Wellington, and the University of Canterbury.
A poster/one-page flyer that gives an overview of the project and its first results can be found here.