Attribution of extreme weather and climate-related events

2016

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Nature Climate Change, 2015

There is a tremendous desire to attribute causes to weather and climate events 37 that is often challenging from a physical standpoint. Headlines attributing an 38 event solely to either human-induced climate change or natural variability can be 39 misleading when both are invariably in play. The conventional attribution 40 framework struggles with dynamically-driven extremes because of the small 41 signal-to-noise and often uncertain nature of the forced changes. Here, we 42 suggest that a different framing is desirable, which asks why such extremes 43 unfold the way they do. Specifically, that it is more useful to regard the extreme 44 circulation regime or weather event as being largely unaffected by climate 45 change, and question whether known changes in the climate system's 46 thermodynamic state affected the impact of the particular event. Some examples 47 briefly illustrated include "snowmaggedon" in February 2010, super storm Sandy 48 in October 2012, and super typhoon Haiyan in November 2013 and, in more detail, 49 the Boulder floods of September 2013, all of which were influenced by high sea 50 surface temperatures that had a discernible human component. 51 52 Weather and climate extremes happen all of the time, even in an unchanging climate. Yet 53 there is a justifiably strong sense that some of these extremes are changing to be more 54 frequent, and that the main reason is because of human-induced climate change. Indeed, 55 the main way climate change is likely to be manifested on societies around the world is 56 through changes in extremes. As a result, the scientific community faces an increasing 57 demand for regularly updated appraisals of evolving climate conditions and extreme 58 weather. Such information would be immensely beneficial for adaptation planning. 59 60 The large-scale atmospheric circulation determines where it is dry, where it is wet, where 61 it is hot, and so on. A recent example is documented for the Pacific Northwest 1. Yet as 62 discussed below, in contrast to thermodynamic aspects of climate, forced circulation 63 changes in climate models can be very non-robust, and physical understanding of the 64 causes of these changes is generally lacking 2. Therefore, we suggest that separating out the 65 thermodynamic from dynamic effects may be a very fruitful way forward and result in a 66 different set of questions to be addressed. In turn, these provide a better basis for 67 communication of climate change to the public. 68 69 Complexities associated with extreme event attribution 70 71 The climate community has responded to the demand for timely information by attempting 72 to perform attribution of climate extremes, both through the Intergovernmental Panel on 73 Climate Change (IPCC) 3 reports but most evident in closer to real time through an annual 74 report, the most recent of which-"Explaining extreme events of 2013 from a climate 75 perspective"-was published by the American Meteorological Society in September 2014 4 76 and reported on several events from 2013. The question posed in each case was whether 77 the likelihood or strength of the event was affected by anthropogenic climate change. 78 79 More generally, there are perhaps two main kinds of attribution performed. The first 80 relates the particular extreme event to the associated weather and weather patterns, and 81 this has been a useful and long-standing activity in climate science. We hear statements 82

Progress in Physical Geography (submitted version) 38(4), 499-511 doi: 10.1177/0309133314538644 , 2014

Over the last 30 years scientific research has increasingly implicated human activities in contemporary regional- to global-scale climatic change. It is hardly surprising then that interest has risen in the possibility of detecting the fingerprint of human activities, not just on such broad-scale changes in climate, but on individual extreme weather (and short-term climate) events. Yet this possibility raises many difficult philosophical, epistemological and political questions. What does it mean for something to be caused by something else – especially in complex systems? Is the sought-after cause of extreme weather deterministic (‘this caused that’) or stochastic (‘this made that more likely’)? And in what ways are answers to the question of causation shaped by political or moral considerations rather than by scientific inquiry? My two previous reviews of climate change published in this journal were concerned with the nature and status of the IPCC (Hulme & Mahony, 2010) and with the possibility of global climate engineering through stratospheric aerosol injection (Hulme, 2012). In this third and final review I survey the nascent science of extreme weather event attribution. The article proceeds by examining the field in four stages: motivations for extreme weather attribution, methods of attribution, some example case studies and the politics of weather event attribution.

Climatic Change, 2015

Extreme weather events are a significant cause of loss of life and livelihoods, particularly in vulnerable countries and communities in Africa. Such events or their probability of occurring may be, or are, changing due to climate change with consequent changes in the associated risks. To adapt to, or to address loss and damage from, this changing risk we need to understand the effects of climate change on extreme weather events and their impacts. The emerging science of probabilistic event attribution can provide scientific evidence about the contribution of anthropogenic climate change to changes in risk of extreme events. This research has the potential to be useful for climate change adaptation, but there is a need to explore its application in vulnerable developing countries, particularly those in Africa, since the majority of existing event attribution studies have focused on mid-latitude events. Here we explain the methods of, and implications of, different approaches to attributing extreme weather events in an African context. The analysis demonstrates that different ways of framing attribution questions can lead to very different assessments of change in risk. Crucially, defining the most appropriate attribution question to ask is not a science decision but one that needs to be made in dialogue with those stakeholders who will use the answers. This is true of all attribution studies but may be particularly relevant in a tropical context, suggesting that collaboration between scientists and policy-makers is a priority for Africa.

Weather and Climate Extremes

Attributing the change in likelihood of extreme weather events, particularly those occurring at small spatiotemporal scales, to anthropogenic forcing is a key challenge in climate science. While a warmer world is associated with an increase in atmospheric moisture on a global scale, the impact on the magnitude of extreme precipitation episodes has substantial regional variability. Analysis of individual cases is important in understanding the extent of these changes on spatial scales relevant to stakeholders. Here, we present a probabilistic attribution analysis of the extreme precipitation that fell in large parts of the Netherlands on 28 July 2014. Using a step-by-step approach, we aim to identify changes in intensity and likelihood of such an event as a result of anthropogenic global warming while highlighting the challenges in performing robust event attribution on high-impact precipitation events that occur at small scales. A method based on extreme value theory is applied to observational data in addition to global and regional climate model ensembles that pass a robust model evaluation process. Results based on observations suggest a strong and significant increase in the intensity and frequency of a 2014-type event as a result of anthropogenic climate change but trends in the model ensembles used are considerably smaller. Our results are communicated alongside considerable uncertainty, highlighting the difficulty in attributing events of this nature. Application of our approach to convection-resolving models may produce a more robust attribution.

Bulletin of the American Meteorological Society, 2014

Current Climate Change Reports

The extent to which a given extreme weather or climate event is attributable to anthropogenic climate change is a question of considerable public interest. From a scientific perspective, the question can be framed in various ways, and the answer depends very much on the framing. One such framing is a risk-based approach, which answers the question probabilistically, in terms of a change in likelihood of a class of event similar to the one in question, and natural variability is treated as noise. A rather different framing is a storyline approach, which examines the role of the various factors contributing to the event as it unfolded, including the anomalous aspects of natural variability, and answers the question deterministically. It is argued that these two apparently irreconcilable approaches can be viewed within a common framework, where the most useful level of conditioning will depend on the question being asked and the uncertainties involved.

Climatic Change, 2018

Since [Allen, 2003]'s seminal article, the community of extreme event attribution (EEA) has grown. Several approaches have been developed: the main ones being the "risk-based approach"-estimating how the probability of event occurrence correlates with climate change-and the "storyline approach"-evaluating the influence of climate change on thermodynamic processes leading to the event. In this article, we map the different ways to frame attribution used in a collection of 105 case studies from 5 BAMS (Bulletin of American Meteoro-1 Manuscript Click here to download Manuscript Revision_AJ_20180412.tex Click here to view linked References logical Society) special issues on extreme events. In order to do so, we propose to define EEA, based on corpora of interviews conducted with researchers working in the field as follows: EEA is the ensemble of scientific ways to interpret the question "was this event influenced by climate change?" and answer it. In order to break down the subtleties of EEA, we decompose this initial question into three main problems a researcher has to deal with when framing an EEA case study. First, one needs to define the event of interest. Then, one has to propose a way to link the extreme event with climate change, and the subsequent level of conditioning to parameters of interest. Finally, one has to determine how to represent climate change. We provide a complete classification of BAMS case studies regarding those three problems.

Advances in Statistical Climatology, Meteorology and Oceanography, 2020

Over the last few years, methods have been developed to answer questions on the effect of global warming on recent extreme events. Many "event attribution" studies have now been performed, a sizeable fraction even within a few weeks of the event, to increase the usefulness of the results. In doing these analyses, it has become apparent that the attribution itself is only one step of an extended process that leads from the observation of an extreme event to a successfully communicated attribution statement. In this paper we detail the protocol that was developed by the World Weather Attribution group over the course of the last 4 years and about two dozen rapid and slow attribution studies covering warm, cold, wet, dry, and stormy extremes. It starts from the choice of which events to analyse and proceeds with the event definition, observational analysis, model evaluation, multi-model multi-method attribution, hazard synthesis, vulnerability and exposure analysis and ends with the communication procedures. This article documents this protocol. It is hoped that our protocol will be useful in designing future event attribution studies and as a starting point of a protocol for an operational attribution service.

Bulletin of the American Meteorological Society, 2013

Attribution of extreme events is a challenging science and one that is currently undergoing considerable evolution. In this paper are 19 analyses by 18 different research groups, often using quite different methodologies, of 12 extreme events that occurred in 2012. In addition to investigating the causes of these extreme events, the multiple analyses of four of the events, the high temperatures in the United States, the record low levels of Arctic sea ice, and the heavy rain in northern Europe and eastern Australia, provide an opportunity to compare and contrast the strengths and weaknesses of the various methodologies.