Two major features stand out with regards to recent population changes: the dramatic increase in the last 100 years, and the worldwide ongoing migration from rural to urban areas. Both of these demographic factors affect the ultimate climate impacts, just as much as any climate oscillation or long-term change itself would. In this post I'll focus on how current and projected interregional population shifts (due to migration and to natural population increase/decrease) amplify or curb human exposure to climatic hazards. A more-complete accounting of societal impacts would also consider non-population-dependent regional effects, such as those on agriculture, fisheries, or water resources.
A pioneering study in this branch of demography quantified the surprisingly large percentage of the world population living in low-elevation areas vulnerable to future sea-level rise. Their main findings: the majority of megacities are on coasts, and the median person lives at an elevation of only 194 m. Thus the economic effects of even a modest amount of sea-level rise would be considerable, in addition to the disruptive migrations and political headaches. Estimates for these regional economic impacts (taking into account all aspects of climate change) indicate that the spatial and temporal benefits and costs will continuously vary, creating a complex mosaic of 'winners' and 'losers' as climatic averages and probabilities of extreme events shift simultaneously. For example, a simple review piece shows that cool temperate areas will gain comfortable mild days, while much of the tropics and subtropics will see formerly pleasant dry-season days become hotter and drier. Maps of other major extreme events are shown in the below image gallery; comparing these to global population density, and to global projected population change, provides a better sense of where future impacts will be most negative or positive, and what combinations of impacts will conspire to affect a given region differently than in the past.
Above: Global maps of the climatological distribution of (clockwise from top left) tropical storms; extreme heat; dust storms; extreme cold (NH winter); thunderstorms; and wildfire.
A detailed analysis of the intersection of population and climate change is well beyond the scope of this blog post. Some simple observational remarks:
-Densely populated East and Southeast Asia are heavily exposed to tropical storms, extreme heat, and thunderstorms (leaving aside geological phenomena like earthquakes and volcanoes)
-Rapidly growing northern India is exposed to extreme heat, dust storms, and thunderstorms. Not only that, but fertility rates are highest in precisely the most climate-vulnerable areas
-While Eastern Europe and the US Great Plains are the loci of strong projected increases in extreme heat, the health effects (though not the agricultural ones) are mitigated by the population decreases in those areas
-Accelerated urban warming will further compound cumulative exposure to extreme heat, as already observed in Chinese cities, among other locations
Further study of specific regions and connections is warranted -- after all, it's the areas where the population is densest, or economic value highest, that are the most critical for supporting and protecting human society. The existence of many climate feedbacks of course strongly argues for the protection of sparsely populated areas like the Arctic and the Amazon basin, but to zeroth order, local problems have to be dealt with before remote ones. Avoiding a devastating heat wave or storm in India should, in my view, take precedence over preventing sea-ice loss on the grounds that it probably bears some connection with mid-latitude extreme events. Another way to frame this argument is that specific problems take precedence over non-specific ones (i.e. those whose exact form, location, and timing are not yet known).
On decadal timescales, population projections like those in the above map are wrapped up with economic ones in the Shared Socioeconomic Pathways dataset -- like the Representative Concentration Pathways projections for greenhouse gases, but for the more-nebulous human aspect of the future. This brings up another reason for periodic revisitation of studying the population-climate interface: it's always changing. The above map doesn't include (as it couldn't foresee) the exodus of people from Syria due to civil war, as reflected in this 2015 map of population change. Capital investment, too, is always moving around in search of profit; the economic impact of flooding in Thailand would have been much less than the actual $6 billion 20 years ago, before a surge in foreign investment and the country's incorporation into global supply chains. Anders Levermann makes the argument that adaptation to climate risk in this arena is much overdue.
Finally, a nice map of the US sums up at a glance current populations and future growth. Comparing it with the types of extreme events experienced in each region is instructive as to where and what kinds of events we should expect more of in the future. For the last decade Texas has added the most people in absolute terms, but is also highly exposed to many of the most damaging weather extremes. Florida is beginning to see sea-level rise affect its valuable coastal properties, in addition to the habitual challenge of tropical storms. Of the six categories of events below, all are expected to remain constant or increase in frequency or severity in the generally more energetic atmosphere of the future. The longstanding trends toward population and economic consolidation on one hand make adaptation easier (as we have to protect smaller areas), but on the other make it harder (as we have more to lose if a concentrated area is heavily affected). Globally, nationally, and locally, policies will have the power to shape climate-relevant decision-making to some extent, but realistically advocating for preparation for the future is the main tool we have as scientists. Climate remains low on people's list of concerns, meaning that its effects -- pure effects as well as those tangled up with societal issues, e.g. 'climate justice' -- mainly flow from decisions made for other reasons. But this doesn't make plotting and understanding climate-related problems any less worthy of scientific study or any less valuable for our collective future.
Billion-dollar weather events for each US state for the 1980-2016 period, by primary categorization. In this methodology, if a state was affected by a given event, it is included in the count, even if its portion of the total damage was much less than $1 billion. Source: https://www.popsci.com/natural-hazard-risk#page-2
What comprises climate? It can be considered loosely but succinctly as the study of air: its properties, movement, and composition. Such major elements of study as temperature, precipitation, clouds, wind, radiation, and air pollution all fall under this definition. Rephrased, climate science consists of the study of anything that occurs in the atmosphere or is directly connected with or affected by the atmosphere, and on timescales long enough for meaningful statistics (typically at least several years). But there are some additional atmospheric phenomena not traditionally studied as such that nonetheless have bearing on subjects that are in the more traditional climate purview. In fact, they are so common as to be absolutely mundane: light and sound.
Sound is the main casualty of simplification in the primary set of equations used in atmospheric science and particularly in modeling, for the reason that otherwise models would have to simulate energy moving very rapidly between gridcells and this would require very short timesteps, making the whole modeling enterprise considerably more computationally intensive. As they bear little energy compared with other wavelengths, they are routinely neglected. But that's not to say that they are inconsequential: sound is a critical medium for many animals as well as humans, affecting their ability to do everything from attracting mates to finding food. Animals that use sonar, of course, are particularly vulnerable. For these reasons and others, the European Environment Agency has set a goal of reducing sound pollution 'significantly', while acknowledging attainment of this goal in the near future is highly unlikely. That is mostly because the major sources of anthropogenic sound pollution, vehicle and air traffic, are increasing or remaining constant. It is also worth noting that natural environments can be quite noisy as well -- tropical forests, for instance -- but that the key is that organisms there have adapted to the noise, whereas those in other biomes generally have not.
Likewise, while visible light is of course an essential component of radiation, and its effects on atmospheric temperature are well-characterized by observations and models, its effects on ecosystems (and thus indirect effects on climate) are very rarely considered. A study from 2014 makes the point that tropical bats avoid areas with even small amounts of light pollution, and that this avoidance spells trouble for plants in fragmented forests that rely on bats' seed-dispersal proclivities. The resultant potential change in forest cover further implies an impact on traditional climate metrics like temperature and precipitation, given the strong positive-feedback relationship between deforestation and future drying. A direct effect of light pollution on plants has been observed (at least in terms of correlations) regarding springtime budding in the UK. It's not hard to imagine other similarly complex but very real linkages connecting changes in animal or plant behavior due to artificial-light exposure with aspects of local or regional climate.
On the non-traditional impacts side, many of the studies that have been produced concern temperature. There are several reasons for this focus: it's easily measured, constantly affects us (unlike, say, precipitation), varies widely across the globe, and not infrequently reaches values that we find uncomfortable, if not outright dangerous. Indoors, high nighttime temperatures are closely linked to sleep disruption in the U.S., despite the widespread availability of electricity and air conditioning. The effects are particularly strong among the impoverished and the elderly, and cannot help but be read in the context of the extensive medical literature implicating poor sleep in a host of maladies. Indoors, warm wintertime indoor temperatures are perhaps too comfortable, suggests a study pointing to lower caloric loss through metabolic heating than in decades and centuries past. Pleasant temperatures and sunshine, on the other hand, have been shown to improve cognitive performance.
While traditional ways of measuring climate and its impacts are deservedly predominant, it's often instructive to at least consider the other ways in which the atmosphere and life on the surface of the Earth interact. These are often more complex than they would seem at first glance, which is part of the joy of this field of study. And just as health and environmental impacts are maximized in urban areas as measured by traditional metrics, so too are they maximized there by non-traditional ones, making consideration of the factors behind them worthy of at least occasional consideration in an era when our ability to understand and consciously work to modify the interconnectedness of climate, economics, health, and the environment is increasing year by year.
Resettlement, like migration, is a product of fragility. Unlike migration, it's typically coordinated and forced, rather than sporadic and voluntary. The fragility that spurs it can be sociopolitical (as with harried minority groups), economic (as with smallholder farms during the Great Depression), climatic (as along the Louisiana coast), or any combination of these. Where climatic fragility leads to economic fragility, making climatic and economic incentives consonant, self-initiated movement is most likely, even in the absence of a disaster per se. But typically, it takes a disaster, and even then few people want to leave their homes and neighborhoods if at all possible. This post examines resettlement in the 20th century United States and its implications for the future we all face, which includes the challenges of extreme heat, sea-level rise, and stronger storms (among others).
To start, it's important to put resettlement in context. In a previous post, I explored the issue of climate-induced migration and resettlement in comparison with the adaptation measures that are more conventionally favored by planners. 'Adaptation' encompasses everything from human behavior to upgraded electrical grids and taller seawalls, and this will be sufficient for most of the population. The risks and costs of climate change are of course unevenly distributed geographically, but such is the raison d'etre of government -- to smooth out expenses and revenues across the full population, so that projects can be undertaken that are too complex, expensive, time-consuming, or unmarketable to fall within the province of private-sector activity. But of course government redistribution has its discontents, and particularly pertinent in the climate context is the disconnect between behaviors generated by current incentives and optimal behaviors from a utilitarian point of view. The basis of the objection is a simple economic axiom: the lower the cost of a good, the more of it is demanded, a prototypical example being taxpayer subsidy of the National Flood Insurance Program incentivizing the development of flood-prone areas. For the foreseeable future, much of this shared risk will probably remain politically and financially tolerable. But consider the most extreme cases: the sinking island, the isolated canyon community... these are the margins where serious public conversations need to be had about whether and how governments should provide an avenue (and perhaps additional incentives) for residents to relocate to less-fragile locations. The government thumb is already on the scales affecting where people live, in ways as varied as the infamous mortgage-interest deduction, the aforementioned NFIP, and the commitment to footing the entire bill for wildfire suppression without passing on any of the costs to homeowners. In the best case, climate-related challenges force a clear-eyed look at the purpose and consequences of these kinds of subsidies. If the major externalities are not internalized, market distortions and the resulting inefficiencies will continue indefinitely.
Although inexact, the closest parallel to the present-day issues posed by ongoing climate trends is probably the 1930s Dust Bowl and Great Depression. There was in fact a short-lived "Resettlement Administration", under the umbrella of the New Deal, whose aim was to resettle farmers as a response to rural poverty brought on by a combination of antiquated agricultural practices and adverse environmental conditions (heat and drought). As the former program administrator described it, the driving force of the program was explicitly economic, with the aim of addressing the suboptimal productivity of labor and land that the small struggling Great Plains farms represented. Buyouts were extensive, but the new suburban communities for the displaced that were envisioned were never realized on any consequential scale. Despite support from many small farmers, these new developments faced ultimately crippling opposition on both financial and sociocultural fronts. This provides a valuable lesson for the present, in that it highlights the importance of considering where people will go and how they will be assimilated into a new community, beyond simply getting them out of harm's way. An alternative is to move a group of people wholesale from one place to another, as happened with both Native Americans and African Americans. That neither example was terribly successful points to the critical role of economics, in addition to community cohesion.
An idealized 1920s vision of the progressive optimization of land use in New York State, with substantial implied movement of people from the less-productive to the more-productive areas. The report argued for government-managed resettlement to speed along this process and improve the economic fortunes of the poor in the less-productive areas, even without the motive force of climate-related disasters. Source: Jacobs 1989, quoting Mumford 1925.
At the same time, state initiatives were begun to alleviate the primarily economic hardship being suffered by smallholder farmers in the Northeast. With advances in technology and transportation stemming from the Industrial Revolution, their farms were no longer economically viable due to inherent climate and topographic factors, but many were trapped by debt. A decades-long political debate ensued between the 'social Darwinists' (advocating resettlement of the affected) and the 'repopulationists' (advocating investments allowing them to adapt to the changing times). The former group prevailed and buyouts began in earnest in the early 1930s in New York State under the governorship of FDR, who saw them as efficient ways to manage growing rural malaise. This is one of the few examples where a potential humanitarian disaster was foreseen and successfully avoided. However, they lasted just a few years before funding was cut back to skeleton levels.
A humbling lesson from these past efforts is that even in an era of substantial climatic stress and economic upheaval, these programs' achievements were ultimately narrower than their original visions. Key elements that seem necessary for long-term success are the preservation of some kind of social fabric, a plan for economic integration into the new community, and having community buy-in from the outset. A reliable and continuing revenue stream is also necessary from a programmatic point of view.
Looking to the future, the social-fabric issue seems particularly pertinent given rising inequality, especially in urban areas. Exacerbating it is the fact that, even without climate stressors, the wealthy and highly educated are much more mobile and have more transferable skills, so that the economic and social shock of resettlement is smaller for them. This means that the poor are often forced to make larger changes in response to climatic threats, despite not having the resources to comfortably do so. Subsistence farmers are arguably the worst off, being highly vulnerable to climatic whims, having the fewest resources, and having a livelihood which depends on the scarce resource of land (as opposed to practicing a trade or service-based skill). Fortunately, some efforts are underway in the international-policy sphere to smooth out the expected necessary transitions -- in fact, there's even a non-profit organization expressly dedicated to facilitating climate-related displacements. Climate and social-science researchers are brainstorming solutions, and localized efforts have been implemented and largely proven successful, though in all likelihood large-scale initiatives will only begin when the true scale of the issue comes into focus. While not something that's likely to affect most of us, the exigencies of resettlement will have a large-enough impact on those who do experience them that we all should be interested in finding efficacious and cost-effective solutions. While climate change exists on a bigger scale than anything seen before in human history, no disaster has ever been purely natural (including the Dust Bowl), which should give us heart that the answers are not out of the realm of experience either.
I recently attended the Rutgers Climate Symposium, where this year's theme was 'cities and climate' — especially appropriate for an event situated in the USA's finest megalopolis. A key takeaway from the day was the degree to which -- due to their high concentration of people, money, and physical assets -- cities are at the front lines of climate- and environment-related issues, more so than rural areas, provinces, or countries. One speaker, Bill Solecki, listed a number of unique challenges cities have faced in the last few centuries, which have been encountered and dealt with largely in serial fashion: fire, sanitation, air pollution, open space, and urban decay. From this perspective, sea-level rise or extreme heat is just another challenge that will mainly be felt first in cities, and that will strain the resources of some, but likely fail to change the larger arc of history that bends toward urbanization. The speaker Julie Pullen noted that this trend represents an opportunity across much of the globe to effect major change in urban form and lifestyles as the century progresses.
In fact, this positioning of cities on the front lines of global issues has been the case for about as long as cities themselves have existed, if for no reason other than that local governments are typically and of necessity concerned with more-prosaic matters than national leaders, who have more of a tendency to helicopter in, make a pronouncement, and helicopter out (hence Fiorello LaGuardia's quip that 'There's no Democratic or Republican way to pick up the garbage'). The greater complexity of cities than suburban or rural areas inspires and motivates a greater complexity of governance, and this fact is largely implicated in the strikingly complete sorting of the US (and many other places) into liberal urban and conservative rural halves.
Whatever their political leanings or economic status, another emergent theme was the need for municipalities to work together to tackle problems too big for any one of them to face alone. Not every city can afford an office of environment or climate, nor is there always the political will for it. Leveraging existing knowledge, financial, and political networks brings down the cost associated with addressing climate-related problems through economies of scale, and consequently reduces the potency of the argument that a given solution is too expensive or cumbersome to implement. For example, the various cities in a metropolitan area could all use information from the same study to assess their vulnerability to sea-level rise; could all finance seawall improvements through the same pooling they use to fund transportation infrastructure; could all coordinate these actions through the same conference of mayors they all belong to; etc. Organizations like the Global Covenant of Mayors and the Urban Climate Change Research Network also serve as conduits for resources and information while still allowing cities to take their own individualistic problem-solving approaches.
As a result, one of the main lessons of the symposium and of the most-recent Urban Climate Change Research Network report was that the most effective climate-related consortia all consider their programs as elements of larger programs or strategies, rather than being viewed ipso facto. Another aspect concerns disadvantaged populations: in environment-related issues as in political and financial ones, and in the developed world as in the developing, these communities are where there is the most room for improvement in terms of communication and implementation of cost-effective measures to address problems (whether done proactively or reactionarily). Thus, these populations must be explicitly focused on for substantial equity strides to be made.
Lastly, integrated and participatory decision-making around urban climatology is crucial for making decisions that are good (as these require the input of various leaders and organizations) and also decisions that are actually implemented (as this only happens when the aforementioned people consider the decisions legitimate and accurate). As speaker Cynthia Rosenzweig emphasized, this is exemplified by, essentially, keeping an open mind rather than letting pure science dictate the decisions. Risk reduction, reaction, and long-term planning should be considered hand-in-hand. It's doubly important to keep this integrated framework in mind in the frenzied weeks and months after an extreme event, which is when much of the political energy and financial muscle are available to enact changes. More broadly, Bill Solecki stressed that the intersection of extreme events, climate change, and urban areas is where there's the motivation and wherewithal for society-wide adaptation measures, and where these measures would have the biggest impact. And yet, they also illustrate one of the most fundamental challenges in applying the results of urban-climatological studies: that while we have lots of data on roughly what's expected in terms of climate in the next 100 years, we don't know what to do about it -- that is to say, we don't know how to react viscerally, and certainly not how to plan in an organized and effective manner. Which is exactly why these kinds of mutual brainstorming sessions between urban climatologists and everyone else with a stake in the future of cities in the 21st century are so important.