Often discussed as an aspect of climate change is the idea that seasons will somehow be different than they have been in the recent past. How precisely they will differ, though, is exceptionally hard to pin down for analysis. For example, what defines 'spring'? Is it simply the transition between winter and summer? Is it a certain statistical distribution of temperature, precipitation, and cloudiness (not just their values but the combinations of them on certain days, the temporal sequences in which they occur, and so forth)? If May becomes substantially warmer, does that make it part of summer? How would we know?
It is these types of questions that have so far precluded meaningful seasonal-change analysis. One challenge is that seasons are so different from place to place -- both their first-order statistics, as noted, but also the experience of them. What would winter in New England be without abrupt cold fronts, or Great Plains summer with fewer thunderstorms? Such changes might not be readily apparent in temperature and precipitation averages, if compensated for example by increases in non-thunderstorm precipitation, but would lend the season a different feeling and influence impacts such as power outages. Similarly, particular combinations of weather patterns can be of immense cultural and/or agricultural significance. A moderate decrease in early-spring diurnal temperature variation may lead to a dramatically shorter maple-sugaring season in Vermont or Quebec, affecting sense of place. Crops that require chilling hours or moderate temperatures would necessarily see geographic shifts. Elsewhere, typical mid-latitude conceptions of distinct seasons do not apply, requiring a more careful multivariate depiction. Along the Southern California coast, for example, Santa Ana winds can cause the most intense heat of the year to occur in late autumn or even winter, leading to health impacts. This year, January, April, September, October, and November all saw higher temperatures in coastal Los Angeles than anything in June, July, or August.
Extreme (or indeed unprecedented) events can color impressions and shape impacts of an entire season, despite relatively short durations. The incredible Pacific Northwest heatwave this summer stands as a canonical example. Even if the rest of the summer conformed closely to expectations, the die of memory (for humans, vegetation, soil, etc.) was already cast. That raises the question of the proper scope and timescale for attribution analyses, which are coming fast and furious these days. Event-based attribution to anthropogenic climate change is the standard, but by omitting the seasonal context, important information may be being lost. This is particularly important for multi-season events such as summer drought preconditioned by spring heat, fire-following mudslides, or spatially propagating droughts.
One approach that holds promise for a more intuitive seasonal quantification involves a deeper investigation of 'weather types' -- in other words, reducing the dimensionality of weather by categorizing days into multivariate composite clusters. Changes in frequency of weather types could then indicate important shifts in seasonal behavior, perhaps concentrated at boundaries between seasons. But then here again several philosophical issues rear their head. First, whether to rely on actual values or values relative to an annual distribution -- warming, combined with a smaller winter/summer difference in a warmer world (continuing the pattern of greater winter warming that has occurred since the Last Glacial Maximum, also supported by longstanding model evidence), is the key underlying factor. Second, even an idealized conception of seasons has a 'ragged edge' where there is some alternation between weather typical of the preceding and the subsequent season.
Evidently, each of the Köppen climate 'types' contain multitudes, some more important than others but all posing complications to assumptions about what meaningfully distinguishes one season or weather type from another. The most material impact of warming-driven seasonal change will involve behavioral practices that will have to shift along with changing environmental suitability due to physiological constraints (those of humans, animals, and/or crops). A rigid climatic shift where all patterns are identical except warmer by 2 or 3 deg C would mean that a once-pleasantly-warm summer outing becomes uncomfortable or dangerous, potentially leading to large-scale societal changes in the timing of activities (avoiding the warmest part of the day and/or the warmest time of the year). Of course, the thermodynamics of a warmer and moister atmosphere also drive exponentially intensifying extreme events, which often have contributions from changes in dynamics as well.
Ultimately, the question of how to treat future seasons crystallizes a few of the principal issues in our collective understanding of a changing climate. Surely our ancestors of 15,000 years ago wrestled with some of the same problems, although with a different vocabulary and presumably less guilt, because there was little objective understanding of the situation (rapid glacial melting) and no perceived control over it. Despite (or even perhaps because of) reams of data, the world almost unavoidably requires narratives to imbue it with meaning. The concept of seasons is one such narrative. But they are a simplistic approximation of a complex system, and a threshold-based approach to a continuous problem. Both of these often lead to dramatic conclusions and misleading impressions. A relativistic definition of seasons across changing climates is without doubt the most basic approach, yet this conflicts with our closely held associations. Bringing our mindsets to a physical system is bound to get ourselves tied up in knots. As elegantly described in the essential climate-philosophy book "Climate Conundrums", Earth (and indeed life itself) has no values. *We* have values, but we shouldn't get confused into thinking these are objective or inevitable. Technology often outpaces principles, and following that idea, maybe it will turn out to be the case that changing the atmosphere, ocean, and land surface is the easy part, and adjusting our ideas and expectations to match the new reality that follows will prove significantly harder.