Commentary: Networked Impacts and the Importance of Trailing-Edge Climate Risks

Abstract
            Extreme events with decreasing trends in frequency or severity can still cause major disruptions to human or natural systems — as vividly illustrated by February’s winter storm in the south-central United States, which may ultimately become the costliest natural disaster in U.S. history. These impacts underscore that the path to comprehensive climate resilience runs through assessments of the structure and interconnections of systems, as well as narratives that emphasize the large variability around trends in extreme events.

​Main
            In mid-February 2021, widespread extreme cold paralyzed power and water systems in Texas and neighboring states (Figure 1). Seeing areas known for hurricanes and summer heat shivering amid near-record cold and snow was especially striking in light of observed trends and future projections emphasizing increases in heat, drought, flood, and fire.(1) The concept of ‘global weirding’ has gained prominence for describing this surficial paradox of increasing extremes in both tails of a distribution, a paradox which is readily resolved by recognizing several critical factors, including the ongoing amplification of certain hydrological processes and the (possibly temporary) destabilization of the boreal-winter polar vortex.(2,3) Here, I argue that weirding — the variability component of climate change — and its potential impacts be investigated more closely and systematically, using the Texas event as a lens into some of the key issues and categories of potential solution.
            Global-mean warming is causing the ‘leading edge’ of various hazards to shift, decade by decade, into locally unprecedented territory.(4) Appropriately, much effort and funding is being expended to assess the growing risks these hazards pose, and to determine optimal adaptation and mitigation strategies for them.(5) In fact, catastrophic risk theory argues that they perhaps merit even more attention, so as to fully understand low-probability, very-high-risk scenarios for the remainder of the 21st century and beyond.(6)
            In this context, the disaster in Texas serves as a reminder that extreme winter cold — and other ‘trailing-edge’ hazards such as cold wet springs (7) with observed or projected decreasing trends in frequency or severity — remain a critical source of risk, more than capable of overwhelming fragile societal systems. That this is possible, despite rapid warming, is due in part to a lack of generalized resilience in the affected systems and in part to anthropogenic networks which scramble, and in some cases amplify, the direct link between hazard and impact which would exist in a simpler world.(8) As illustration, extreme cold between February 10th and 17th spiked power demand across Texas to levels higher than the state’s electric grid could handle, and its isolation from interstate network connections, along with the inadequate winterization of natural-gas power plants and other critical infrastructure, led to multi-day power outages. Poorly insulated water pipes indoors and outdoors then froze and burst, causing serious damage to buildings and leaving millions of people without running water, exacerbating a humanitarian crisis.
   

Figure 1: Icicles hang from frozen equipment at a natural-gas power plant in southeast Texas on February 15, 2021. Source: Entergy Texas (URL: https://www.entergynewsroom.com/article/entergy-texas-storm-update-2-15-21-5-p-m).

         The unfolding of this disaster points to two problems: a failure of knowledge (or awareness of that knowledge) and a failure of action. Much attention has focused on the latter issue, and justifiably, because recommendations following disruptive cold and snow events in Texas in 1989 and 2011 were mostly not followed through upon.(9) Designed to fit short-term climatologies and encompassing a spatial area too small to include meaningfully offsetting temperature anomalies, the state’s power grid has also seen strain from summer heat waves.(10) The issue of knowledge is somewhat more subtle but nonetheless crucial for optimal risk assessment and mitigation. Consider two episodes that affected Houston: the recent cold, and the flooding from 2017’s Hurricane Harvey. Both overwhelmed the city’s infrastructure, and both had impacts strongly shaped by investment, development, and policy decisions that often aggravated pre-existing socioeconomic inequalities.(11) However, although temperatures in Houston fall below -3C about annually, with -10C not uncommon across North Texas, freezing temperatures of sufficient duration and extent to cause serious impacts went unmentioned in a recent report on extreme-weather risks in the state (in contrast to tropical cyclones).(12) There are even indications that this ‘trailing-edge’ event could ultimately be the costliest natural disaster in US history, surpassing Harvey.(13)
            Beyond the recent headlines and regional anecdotes, the argument that a more complete picture of climate risk necessitates a deeper appreciation of trailing-edge-type events has several pillars. For one, there is evidence that the tails of the distribution of climate impacts are often thicker than models would suggest.(14) If true for the trailing edge as well as the leading edge, even large decreases in trailing-edge hazards might result in only small decreases in their impacts, for example due to downstream risk cascades.(15) At a more theoretical level, while the question of how much to value the future relative to the present (i.e., the economic discount rate) is a subjective one, this rate is nearly always negative — that is, avoiding harm now is worth the most. Skimming over events with decreasing trends in favor of preparation for the medium- and long-term future also requires a degree of presumption that societies are reasonably well-prepared for the set of extremes physically possible with (for example) 20th-century climate forcings. This may be true in isolated cases, but neglects the various resource impediments to optimal adaptation measures, as well as the crucial and dynamic role of societal modulation in shaping both events and their impacts.(16)
            The most direct way to ensure that trailing-edge risks are not underestimated lies in the framing of the narrative around extreme events, such as emphasizing how in many cases awareness and preparedness matter more than the magnitude of a hazard in determining its impacts. In both public-facing and intra-scientific discourse, this narrative could be furthered by portraying trailing-edge-type hazards over the next several decades as a threat that is very much present in most extratropical locations due to inherent climate-system variability. The emerging concept of multidimensional risk provides a helpful and widely applicable perspective, particularly relevant for its emphasis on the importance of specific combinations of extreme events rather than broad trends in any one variable. For example, the frequency of false springs (a period of warmth followed by a late frost) is expected to increase in some regions despite significant warming,(17) while rapid ‘whiplash’ between anomalous hydroclimatic states of opposite type is likely to accentuate both drought- and flood-related impacts in California.(18) Multi-hazard dependencies can also act to decrease risk compared with independent events.(19) Relative to a top-line emphasis on trends, this type of framing aids in placing into appropriate context the occurrence of any extreme events involving those variables, whether they are overall increasing or decreasing. A variety of complex relationships can be usefully explored through the framework of compound events (20), which considers the interaction of distinct variables or hazards as an integral driver of risk, and which can be extended to incorporate the substantial modulating effect of human systems like policies and transport networks.(8)
            Holistically evaluating climate risks also includes determining how much stands to be gained at the trailing edge in cases where risk is increasing at the leading edge. These benefits are often overshadowed, and rightly, by ramifications of the rapid and unrelenting increase in temperature to levels outside the range in which humans evolved.(21) But in a world where extreme cold still causes more mortality than extreme heat (22), the likely reduction of this harm by mean warming is worth exploring in greater detail. Similarly, that cities tend to be warmer than their surroundings is typically investigated as a summertime-heat problem, but it is equally valid to consider this urban-heat-island effect in moderating winter cold.(23)
            Many types of extreme events consistent with the current state of the climate system can lead to major harm when societies are not sufficiently resilient. In the recent Texas winter storm and cold spell, serious deficiencies in infrastructure were paired with critical socioeconomic vulnerabilities, as has been true for other recent disasters.(24) Although most focus deserves to remain on extreme events that are growing in prominence, due to their looming and poorly constrained impacts, trailing-edge-type risks cannot be neglected in refining scientific understanding or implementing resilience measures. Such a balance will help ensure that future climate extremes are not so disastrous, no matter what their location within statistical distributions or which combinations of systems they affect.
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