As climate has moved more and more into the consciousness of the people and organizations who make political, investment, and development decisions that affect all of society, the vocabulary used to describe the various ways in which such decisions engage with climate or climate projections has increased commensurately. We now have 'impacts' on 'stakeholders' which we address with 'adaptation', 'mitigation', and 'resilience', but also with broader measures of 'sustainability' and reductions in 'vulnerability'. Even as someone who encounters these terms every day, I realized I might be hard-pressed to give an exact definition of them, much less to give a coherent explanation of why in a particular circumstance one would be preferable to another. Only once the terms are all on solid linguistic ground can the ultimate and inherently local question be taken up of when one type of reaction is preferable to another, given a suite of social, economic, and cultural factors.
The first of these terms is so common that I placed it in the title of this post without even noticing. The IPCC glossary defines impacts as "the effects of climate change on natural and human systems"; this is a fine definition for an organization that has the CC of climate change right in its name, but from a broader perspective natural climate variability (e.g. ENSO) should be included as well. After all it's the superposition of anthropogenic and natural climate forcings that will determine the future climate that planners must account for. 'Stakeholder' is similarly vaguely defined in terms of one's potential to be affected 'by an entity' -- in this case, the entity being the future climate in the region of the stakeholder's interest. In policy parlance 'stakeholders' and 'decision makers' tend to be strongly overlapping sets; in a typical region they might consist of various municipal-government departments, utilities, corporations, community boards, and the like. The concerns of individual households are assumed to be covered by this array of larger organizations, making it crucial that a wide variety of stakeholders are included in selecting research topics and deliberating on actions -- but not too wide, lest research efforts be spread too thin. This tradeoff follows the precepts of Arrow's impossibility theorem, where having fewer moving parts in a decision-making machine is often desirable despite thus less precisely matching the full distribution of societal preferences.
'Sustainability' is an overloaded station wagon of a word that even in the specific urban-climate context can take on a number of different meanings. Most broadly, it is used to refer to any system or action that reduces the consumption of some natural resource (energy, land, water, materials, etc) and by consequence the ecological footprint of the population. A full evaluation of it would require each person tracing back the resources consumed in the production of every commodity they use, wherever they came from and whatever steps were taken to get them into their final forms. It's typically the reduction in the resources that matters, as it's impossible to say exactly what can be sustained per se, and over what time period the accounting is done. When the flux of resources through our lives is reduced, we say that we are less vulnerable, meaning less susceptible to disruptions. This is a particularly salient point for urban residents the world over whose food, energy, clothes, devices, etc must be shipped in every day, even a brief halt to the system causing weeks-long ripple effects. Vulnerability is often broken down into sensitivity (e.g. the percent chance that someone will suffer heatstroke at a certain temperature) multiplied by exposure (the number of exposures per time period); to obtain the societal impact, multiply again by the population affected. Technological advances and transitions away from agricultural occupations may well reduce global exposure to climate extremes despite increasing numbers of extreme events overall.
In this solar system of vocabulary, where newly coined words hurtle around like asteroids, mitigation and adaptation are like the binary stars, at the center of most every conversation. Fittingly, neat definitions of the two are given on NASA's "Vital Signs of the Planet" website. In short, mitigation is mostly about greenhouse gases, while adaptation is about animals, plants, water, non-extreme temperatures, and all the other things that people need and climate affects. A decade or more ago mitigation was the preferred tack, via emissions cuts most prominently, but as the world has continued on a high-emissions pathway (see post "The Human Element: Time Dependence") adaptation has gone from being seen as a concession to something worth striving for -- indeed, that needs to be striven for in the continued absence of serious mitigation efforts. (Adaptation is sometimes rolled into the definition of vulnerability as well.) The talks in Paris this coming December will provide another chance for international reassessment of commitments to each vis-à-vis the other.
Recently, talk has moved beyond adaptation to resilience, and specifically the sense of the word suggesting that failure in the face of uncertain and changing conditions is not in and of itself disastrous, and that good preparations that happen are better than great ones that don't. The nuances of resilience are nicely analyzed in this blog post. The term "graceful failure" encapsulates the essence of this new concept, wherein a certain percentage and/or magnitude of failures are deemed acceptable. On the other hand, with distributions of climate variables often being inherently long-tailed, the merits of such an approach are debatable. It only takes one very extreme event to make irrelevant a lot of well-laid plans, so a reasonable case could be made that the absolute worst-case scenario should be at least considered, insofar as is possible. The insurance industry uses statistical models run hundreds of thousands of times to get estimates of the extreme tails of the distribution of whatever variable (wildfires, floods, heat, etc) they are financially exposed to. The burden of this computation makes greater sense at longer time horizons, so it doesn't make sense in every circumstance, but generally speaking knowing the bounds with reasonable certainty is an important prerequisite of resilience, which in turn backstops more-mundane adaptation and mitigation measures so that stakeholders can reduce their vulnerability in the near term while also keeping an eye on climate impacts over much longer time horizons.