The interface of the social and physical sciences is not unlike that of two fluids of different densities. People firmly self-identify in one category or the other, and rapidly sort themselves accordingly when given the opportunity; and yet, when mixing does occur, interesting phenomena (like gravity waves or hypothetical-hurricane-response research) can occur. I recently heard a talk by Rebecca Morss of NCAR who, in a paper that, due to the magic of Internet pre-publishing, is not coming out until the April edition of its journal, came up with several different warning messages based around the same hurricane scenario and posed them to residents of the South Florida coast. Some were told that 'Hurricane Julia' would cause a storm surge of 4 feet or higher; others, that there was a 55% chance of a direct strike on Miami; a third group, that "evacuation is the most effective way to protect" oneself; a fourth group, that food, water, and communication networks may be severed for weeks; and a final group, that the "storm surge will be extremely violent, destructive, and deadly. [...] You may die." Respondents were also shown a cone of uncertainty of the sort that the National Hurricane Center routinely produces. The authors found that the preconceptions and heuristics of human information-processing were the single best influence on people's reactions to the hurricane. All else being equal, Hispanics and women were more likely to evacuate, as were older people, those who had evacuated before, and those who had received the "4-ft storm surge" and "extremely violent and dangerous" warnings. Going to rhetorical extremes, while effective in increasing evacuation rates and making the warning memorable, also provoked the biggest backlash; in previous studies, some people have indicated a greater desire to shelter in place because they found the message rude, the source less reliable, or wanted to prove the sender wrong. Whether this increase in urgency of tone would have diminishing returns in future storms as its potency presumably wanes is an open question -- after all, there's only so far you can go towards annihilation before you reach annihilation itself. Relatedly, people with individualist personalities were less likely to evacuate, which the authors attribute to the large body of psychology research supporting the notion that people reject information that doesn't mesh with their worldview, and the government claiming that death and destruction will be visited upon those who disobey certainly clashes with individualism in a strict sense. It's an open question how to tailor these messages to still allow people their independence while encouraging the best decision-making on a society-wide level. Not everyone should evacuate even in a strong storm, for their own specific reasons, but the general consensus among scientists and emergency managers is that more caution would be wise. Whether or not it's, say, economically optimal is another question altogether -- and a very intriguing one at that. A yawning gap exists between "colloquial" and scientific risk assessment, traceable all the way down to the root of how a risk is described: in common parlance we use words like "dangerous" and "scary", whereas, traditionally, scientists (who write warnings) have described them purely in terms of their quantifiable qualities, for instance a 4-foot storm surge for a duration of 6 hours. As humans, we can't help but be influenced more by the appeal to emotions than to abstractions, perhaps because the abstraction requires an additional cognitive step to reach the conclusion of 'do something!' and our mental alarm bells start ringing. This was succinctly borne out by a study finding that the two biggest factors affecting people's perception of the threat posed by climate change were the elevation and distance from the coast of their own houses. Personal experience trumps vicarious experience and analytic knowledge, especially when life and property is on the line, according to this meticulous and careful review. The book "Thought Contagion" by Aaron Lynch notes more generally that the spread of many ideas can be attributed to either the "cognitive" or "motivational" advantage of adoption in the eyes of the adopter. The former occurs when we believe something to be right intrinsically, the latter when we have an objectively assessed outside motivation for believing it. Turning motivation on its head into risk is a tempting but (as far as I am aware) unproven way to explain these very personal effects on perceptions of global issues. And of course motivation can be derived equally well from conformity or non-conformity, depending on one's personality. Much more detail on this topic is surely warranted but this post is intended primarily as a meditative musing touching on some of the most interesting questions and findings that interdisciplinary social-physical research has thus far raised. Going beyond the trite 'why do some people not believe climate change is real' study is in my view essential for generating respect for the subfield, and for raising the bar so that a new generation of research questions, taller in stature, can fit underneath.
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If one were to suffer the misfortune of being sat down and forced to watch CNN nonstop 14 hours a day, it would certainly seem to unequivocally be the case that climate extremes are increasing in frequency and severity, and all around the globe — whether the (anomalous temporary southward shift of the circum-) polar vortex or the routine landfall of a hurricane. Less-breathless approaches to examining the world have also generally found this to be the case, though the rate of change is relatively modest compared to the frenzy of the 24-hour news cycle, and the overall picture is much more nuanced. For instance, high interannual variability means that one often-used climate-extremes index has only in the last 10 years began to routinely exceed maxima from earlier in the 20th century (see figure below; this is termed the 'emergence of a warming signal'). The index considers the percentage of the United States experiencing very high temperatures, very low temperatures, severe drought, and heavy precipitation. In another high-pitched, frenetic world this sort of gauge is known as the "fear index"; with that analogy in mind this is the first in a series of posts taking a measured look at world cities' exposure to climate extremes in the observational record up to the present, and how that exposure might change in the future. As discussed in the previous post, coasts are complicated, and only getting more so with sea-level rise and coastal flooding becoming of greater concern. But we don't abandon them and leave them to sort out their own problems because they are useful to us, and so we've built many of our most impressive accomplishments as a species along them. Therefore, landmark places from Shanghai to San Francisco to Sarawak regularly experience flooding (as can be discerned from the fact that these three spots, selected at random, all had disruptive flooding events within the past year). Even considering extreme events known more for their wind and waves coastal flooding causes the majority of the damage. A study of port cities focusing on sea-level rise and storm surges found about 39 million people worldwide live at elevations with an approximately 1% annual risk of being flooded, increasing to 150 million by 2075, and ballooning into the middle triple digits if more-minor events, like basement and street flooding, are included in the tally.
Using estimates from that study and another of future population and asset growth in flood zones, as well as of the future flood zones themselves, the value of global assets exposed to 100-year flooding in 2100 will be approximately 10% of global GDP — in other words, sea-level rise alone will exert an annual drag on global GDP on the order of 0.1% even with significant assumed investment in flood protection. This is an enormous sum in comparison with landmark disasters like Hurricane Katrina which caused about USD$100 billion in damages. (See figure below, where the largest values in individual urban areas are several trillion USD.) And this drag will of course not be spread out evenly around the globe but will be concentrated in those cities, many of which (such as Kolkata and Dhaka) are struggling to provide basic services even in the absence of this extra burden. Yet surely people will continue to pour into them, making the rational decision that, having next to nothing, a chance for upward economic mobility is worth the tradeoff of a still comparatively small risk of serious flooding. As economists know but scientists sometimes forget, people can know the environmental risks but still consciously take them in pursuit of other goals. On the bright side, the large majority of coastal urban areas would be minuscule dots in the figure because they are elevated enough to remain mostly dry in ~100-year flooding events even after a sea-level rise of several feet (an example for New York City from Philip Orton). But it is dangerous to take this as comfort, because 1. these are only estimates and the full spread of possibilities is much wider (due to model and climate-feedback uncertainty) even given the warming we're locked into based on our GHG emissions so far; 2. in the next 100 years about half of the coastline will experience events more intense than the 100-year benchmark and we don't know which half that will be; and 3. failure to react to a small change means a large one becomes more likely. Flood prevention is also a high-stakes game in the sense that much of the damage occurs in the first 50 or 100 cm of flooding, as well as being spatially concentrated and subject to the whims of Nature's cruel coincidences. A more-optimistic view comes courtesy of the Netherlands, which having engineered and maintained a successful large-scale flood-protection system has the exigency of needing to continue to protect the 60% of their land that's below present-day sea level, but concomitantly the luxury of being able to plan further ahead and dream more wildly, since big dreams are what's required. If Homo sapiens can conduct extraterrestrial colonization, we are certainly capable of constructing floating cities here in the otherwise friendly confines of Earth. Two recent events here in New York City have been clear reminders of the impact of the city's coastal location. An exceptionally warm December (warmer even than the average November) helped delay the first below-freezing temperature until January 4, for a growing season that clocked in at 271 days. Then the abbreviated winter did all it could to make up for lost time, producing a storm that dropped 24"-36" of snow from West Virginia through Long Island, in one moisture-laden shot bringing the region's total snowfall in line with much colder and typically snowier areas. These got me thinking about coastal climates — in particular, how they are milder than continental climates (e.g. continental interiors like Mongolia or Kansas) in some respects and places, but more-extreme in others. Thermal stabilization comes first to mind, because it's so intuitive and ubiquitously observable: water has four times the specific heat and about 800 times the density of air, so 3200 times the heat capacity. If 'everything in moderation' is a good mantra for life, then water is the greatest life-enabler of all. Without it transporting massive amounts of heat from the tropics to the poles, the latter would be completely inhospitable; the striking warmth of the North Atlantic relative to continental regions at the same latitude bears that out. The same map also indicates regions where the ocean has a cooling effect, due to upwelling of cold deep waters or to equatorward transport of cold waters via surface currents like the Labrador. Also of note is that, following Newton's Law of Cooling, water's moderating effect gets stronger as the air temperature gets more extreme. This can play out directly, with sea/lake breezes, or indirectly, such as when cold air triggers lake-effect clouds (and snow) that keep temperatures from dropping extremely low in the onshore areas. In the case of the freeze-less November/December in NYC, an analysis of mean growing-season length shows that the signature of the coast is very strong, lengthening the season by something like 25 or 50 days relative to inland locations (see lefthand figure below), a significant effect even with some uncertainty for elevation- and urban-based differences. Water is a very abundant source of water — this truism means that oceans tend to be cloudier than land, and this contrast is particularly striking in subtropical areas where cool, moist marine-influenced air and warm, dry continental air abut each other, such as in southwestern Africa, western South America, and southwestern North America. The difference leads to stratification, copious layers of low stratus clouds, and, very often, to seasonally imperturbable levels of stability that limit the mixture of the two air masses. For southwestern North America the effect of this stability is clearly manifest in the summertime mean-temperature field shown in the other two figures above. [I was a bit incredulous at the coolness of the coast even for extreme events during boreal summer, but independent data searching backs it up.] Any lapse in protection by the cool water (e.g. when a high-pressure system results in easterly winds and prevents the cold moist air from seeping inland) is immediately felt. Warm water, on the other hand, is a powerful destabilizing force, as when moist Gulf of Mexico air comes rolling across the Plains and interacts with air descending from the heights of the Rockies. Again, the Persian Gulf and Red Sea emerge as prominent unique areas. Several weeks ago I had the chance to speak with Jeremy Pal, who recently wrote about this, and asked him about the dynamics of the coastal environments surrounding them; he remarked that their shallow connections to the Indian Ocean (through the Straits of Hormuz and Mandeb) combined with their high surface-area-to-volume ratios allow them to heat up dramatically in the summer, so much so that the sea breezes they generate could hardly be considered refreshing — not when the air above them has a dewpoint close to the sea-surface temperature of 33 C (thereby supporting wet-bulb temperatures of the same magnitude). One less-appreciated benefit expanses of water provide comes from what they are not — namely, with the exception of ship traffic, they are not places that people have yet figured out how to pollute from. As a result, cities like Seattle have some of the cleanest air in the U.S., their only imported pollutants being those that are long-lived-enough to make it across the Pacific from Asia. A last curiosity on this tour are the ice-formation zones off of Antarctica and in certain areas of the Arctic, where the offshore, downslope katabatic wind from the continental interior is so strong as to continually blow newly formed ice out to sea and leave coastal areas of open water called polynyas. These zones are biologically rich from phytoplankton all the way up the food chain to seals and penguins, despite being from a human perspective incredibly harsh environments.
Needless to say, no major cities are located near any of them. In the tropics and the mid-latitudes, where the weather is not normally so vicious, coastal environments above the tidal zone are typically more moderate than nearby inland areas when looking at averages, which is one reason (along with trade) why coasts are much more densely populated than continental interiors — but an alarming number of hazards do come from the ocean: tsunamis, tropical cyclones, severe thunderstorms, indeed storms of all types fed by its moisture supply (as in the snowstorm mentioned to lead off this post), not to mention the natural swings in ocean sea-surface temperature that reverberate around the globe in the form of various climate oscillations. Which may be why, in terms of conduciveness for migrations and other landmark events in human history, maritime archaeologists have posited that coasts are much more unstable and unfavorable than has previously been assumed. Perhaps instead of eliminating the speculative mythical creatures that inhabited the oceans on old maps, we should replace them with the dangers that we know are actually out there. After the last post, I couldn't get the topic out of my head, and so decided to dive a little deeper into the mechanisms (climatic or otherwise) behind the projected spatial shifts in climate suitability of various kinds. Arable land The effects of projected climate changes on crops can be fairly neatly separated into two groupings: those due to increases in temperature, and those due to increases in CO2. On the whole the former are expected to cause decreases in yields, while the latter will cause increases (though with many interactions and variations by species and region). The temperature effect, at least to me, seemed counterintuitive — though this is perhaps an artifact of hailing from a place that's always limited by too little heat rather than too much. But major crops, even ones famous for their love of heat like corn and cotton, have yields that fall off precipitously when temperatures climb above 30 or 32 deg C (see left panel below). [Not coincidentally, this maximum tolerance of about 90 deg F matches up well with the climate of the Tehuacan Valley of Mexico, where corn is believed to have been domesticated.] This susceptibility to extremes contributes to an optimal growing temperature somewhat cooler than might be expected, and one that is already surpassed for many of the world's staple crops (see right panel below, and first link). Hidden in the below charts are a multitude of effects: for example, high temperatures contribute to a higher vapor-pressure deficit of leaves relative to air — causing water stress that reduces photosynthesis rates — as well as to more agricultural pests, more ozone, and greater direct danger from the heat itself. There are also fewer cold extremes, but this factor is apparently outweighed, at least when considering the world's current agricultural regions. Meanwhile, higher CO2 has a fertilization effect on "C3" plants (e.g. the majority which are better adapted to high-CO2 conditions, including wheat, rice, and vegetables; the C4 pathway evolved fairly recently in semi-arid climates as global CO2 levels fell). Therefore, C4 plants like corn and sorghum that pump in CO2 will see relatively less CO2 benefit and more temperature damage. The CO2 benefit will also likely be weaker in the tropics, where nutrients and fertilizer usage are both currently low, because increased CO2 will tend to decrease protein levels without additional nitrogen fertilization. Finally, changes in consistency and/or timing of precipitation will strike the tropics the hardest, both because it will require the most adjustment of practice, and because the agricultural sector there is least-equipped to adapt to the change. Water stress Almost all climate projections, including some I cited in the previous post, suggest that the semi-arid and arid areas of the subtropics will become hotter and drier over the course of this century. But it's not self-evident that this is a major factor driving latitudinal shifts in suitability on a global scale, considering that this may be a minor factor of importance only in sparsely populated regions, and that water is routinely moved hundreds of miles from mountain or lake to city. A recent article from a member of my research group succinctly dispels that notion by first observing that about 1.9 billion people live in areas that depend to some extent on snowmelt runoff to meet their water needs, and then mapping the projected change in runoff by 2080 (see figure below) — the decreases coincide in a number of locations with likely demand growth due to temperature and population increases. The large-scale spatial correlation between basins means that in many cases solutions won't be as simple as pumping water from a neighboring watershed. The impressive water-conservation figures coming out of California fortunately show that this projection is not destiny for those who dream of life in warm sunny locales, and technology from pipelines to desalination always has a role to play, but it does help illustrate the flat or increasing suitability of the high latitudes juxtaposed with the decreasing overall suitability of the expanding subtropics. Temperature Extremes One may wonder why cold extremes are expected to retreat at a faster rate than warm ones will expand (see figure below) — effectively increasing the latitudinal band spared from conditions outside of the range of human comfort. For one thing, atmospheric moisture increases at about 7%/K, meaning that in the already moist deep tropics it will even more strongly limit temperature extremes, leading to only modest gains in wet-bulb temperature. The strong high-latitude warming on the coldest night of the year is generally thought to be related to a combination of weakening inversions and the albedo effect of retreating snow cover/sea ice. That study found the primary cause of "Arctic amplification" to be the near-constant high-latitude inversion, meaning that warming is distributed over a shallower layer than in the tropics where heat is quickly mixed up to the tropopause. Summertime humidity and cold meltwater in the North Atlantic will further help hold down the warming of the hottest day at high latitudes. This warming, while more spatially uniform, will be strongest in subtropical and mid-latitude grassland regions that presently have just enough moisture to moderate potential high extremes vis-à-vis those in deserts at the same latitudes, but for which that will not be the case 75 years hence. At the forefront of the disparity between trends in cold and warm extremes, and consequently the increasing frequency of the latter relative to the former, are urban areas, where heat waves are presently more intense, and cold waves less intense, than in nearby rural areas, with a widening gap going into the future. This widening can be further ascribed, in part, to decreases in windiness in urban areas, which tends to exacerbate the urban-heat-island effect while simultaneously making high-latitude cities more livable by lessening the windchill. So perhaps certain cities that are currently quite inhospitable and quiet should begin preparing to gain new cachet as the poleward fringe of the zone for pleasant human habitation accelerates their way. These benefits, of course, are far outweighed by the negatives of habitat loss, ecosystem shifts, and the accompanying extinction of local traditions. The air may feel mild in Nunavut circa 2100, but what would be the point of visiting if there were no polar bears to see, or sealskins to decorate, or walrus tusks to carve?
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