Climate Change and Population

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HISTORY    Roderick J. McIntosh

FUTURE    Brian C. O'Neill


Climate change results from alterations (sometimes quite subtle) to the heat and mass exchange between land, ocean, atmosphere, ice sheets, and space. The major driving forces of climate change are those generated by plate tectonics (the distribution of mass around the world) and variation in incoming solar radiation (insolation).

Relatively small changes in plate tectonics can have large and geographically distant consequences. The beginning of the northern hemisphere ice ages, for example, can be linked to uplift between 4 and 5 million years ago (abbreviated Ma) that shut off the Isthmus of Panama and altered flow of the seas around Indonesia and Iceland. Around 5.9 Ma, the shift and crunch of the African land mass moving against Europe produced the Messinian Salinity Crisis. The Mediterranean Sea dried out to a stark salt desert, then refilled with water and redried multiple times. The familiar Mediterranean climate ended, disrupting eastern African forests and, apparently, changing the trajectory of primate evolution–these climate changes yielded the divergence of the evolutionary lines, leading to chimpanzees and humans.

Seemingly small deviations in the amount of incoming solar radiation can have enormous and sometimes abrupt effects on climate. Overlapping solar cycles of different periods produce a complex rhythm of solar radiation reaching the earth. Terrestrial cycles in turn influence how much of that insolation strikes different latitudes. Further complicating matters, heat is transported along the ever-changing land-ocean-atmospheric system, and water vapor and other gasses keep some fraction of heat from reradiating out into space.

There are solar production cycles with periods of 11.2, 22, 66, 80, 150, and 405 years. Total insolation passed to the Earth is also affected by a 2,400-year cycle in the Earth's magnetic field and (perhaps) by a return, about every 100,000 years, of intergalactic dust clouds. However, the climate effects are often unpredictable. For example, the well-attested 11.2 year sun-spot cycle is correlated with an approximately 11 year cycle of oscillations in the global monsoonal system, upon which a majority of the world's populace depends for its rains. While the Indian Ocean and Asian monsoons generally correlate well with the West African monsoons, sometimes the latter can be out of phase with the sun-spot cycle. This happened in 1985, frustrating predictions of an early end to the Sahelian Drought.

Three other driving mechanisms of climate change, all well-researched, are the variations in insolation controlled by the so-called Milankovitch or orbital-beat cycles. These are:

  1. Eccentricity (changes in the shape of Earth's orbit), cycling at 100,000 years, overlain by an important 413,000 year "complementary eccentricity" cycle;,
  2. Obliquity (changes to the tilt of the Earth's axis), cycling at 41,000 years; and
  3. Precession (shifting schedule of the equinoxes), with a paired cyclicity of 23,000 and 19,000 years.

The overlay of these cycles produces a complex rhythm. For example, new dating for the majority of ice ages blanketing the high latitudes over the last several million years reveals a remarkably regular orbit-beat. If continued, this pattern would suggest that the Earth is nearing an end to the current Holocene (interglacial) warm conditions, which have lasted 10,000 years. However, the last time these cycles aligned as they do today (around 420,000 years ago, abbreviated 420 Ka), there was a 30,000-year super-Holocene–more than double the usual duration, very much hotter, and with sea level 15 meters above today's. Whatever the extent of future global warming based on human activities, it is possible that there will be a natural warming trend for another 20,000 years.

Measuring Climate Change

Advances in observation methods, modeling, and research methods, particularly deep-sea drilling and ice-cap or glacial coring, have made the measurement of climate change possible. No less important are advances in absolute dating. Scientists are able to date variability resolvable at the annual and decadal time-scales by dendrochronology, counting the yearly growth of tree rings. Tree-ring growth can also be used to reconstruct annual precipitation–a process called dendroclimatology. Coral, ice cores, and laminated marine drift also allow year-by-year dating in addition to bearing evidence of climate effects. At the century time-scale, climatologists can count the layers in deep-sea cores and begin to explore the record of global temperature change by measuring relative proportions of oxygen isotopes 18O and 16O in the annual strata of ice cores or in shells of marine organisms. For dating at the millennial time-scale, investigators rely upon radiocarbon (14C). At the 100,000 year time-scale, dates can be derived from thermoluminescence, amino acid racemization, and uranium series.

In most parts of the world, precise instrument-measured data on precipitation, temperature, sea surface temperature (SST), and other climate indicators do not extend far back in time. With rare exceptions, such as some Chinese compilations, even the best long-term historical records tend to be anecdotal or refer only to extreme events. However, the combination of these fragmentary records with the accumulating information from ice and coral cores, dendroclimatology, and other seasonal to centennial measures such as oxygen isotope proportions, have revolutionized the study of normal climate variability over the last 10,000 years. These are the foundational data for the global warming debates.

While climatologists cannot directly measure the timing and severity of the hundreds of ice ages that have occurred during the past several million years, they can measure proxies, such as isotopes of oxygen in the ocean waters. Higher levels of 18O oxygen isotopes in the oceans correlate with larger 16O oxygen isotope-enriched ice sheets. The shells or skeletons of phytoplankton or zooplankton that fall to the sea bottom form layers of stratified ooze, identifiable in ocean-bottom cores. Figure 1, based on a 20,000 year long core lifted from the northwest coast of Africa, illustrates the temperature reconstruction of the sea surface as it recovered from its -8.5°C minimum at the Last Glacial Maximum. The same core yields indirect measures of intensified monsoon rains, inferred from decreases in wind-blown dust, and of disintegrating ice, inferred from debris–called lithics–carried long distances on ice floes and eventually dropped to the ocean floor as the floes melt. These jagged variations in rainfall and sea surface temperature contrast with the smooth and gradual changes in the Milankovitch values for solar radiation, underscoring the complexity of the Earth's climate systems.

Climate Change in History

Environmental determinism, popular in the 1920s and 1930s, sought to find climatic and environmental causes for broad historical trends such as the rise or fall of civilizations. Historians and archaeologists now totally reject such efforts. Even at a much more modest level, attempts to correlate climate or habitat variability with societal characteristics (such as ethnic diversity) must be hedged with numerous qualifications. The case for Homo climaticus founders on the complexities of culture. Nevertheless, some observations on how humans respond to climateinduced stress and risk are broadly applicable over time and space. The growing field of historical ecology investigates how communities adapt to normal conditions, even though these conditions may be characterized by large interannual or interdecadal unpredictability.

The most consequential demographic event in human history occurred during the last glaciation, maybe as recently as 30 Ka Cold-adapted Homo neanderthalensis became extinct, perhaps at the hands of his close cousins–Homo sapiens sapiens, or modern humans–recently arrived from Africa. Hominids became a mono-species for the first time in over 6 million years. The demographic consequences of this extinction, in terms of territorial and resource competition, are incalculable.

The monumental changes occurring not long after the Late Glacial Maximum of 18 to 16 Ka are apparent in Figure 1. The abrupt and global climate change episodes, called Heinrich Events, would have had devastating effects on non-adaptive communities. Greenland coring shows a severe warming spike at around 15,000 years before the present (abbreviated b.p.), followed by almost 4,000 years of alternating, rapid-onset warm and cold phases, each lasting at least several hundred years. The coldest such phase was the Younger Dryas, which lasted over 1,000 years, beginning c. 13 Ka. At around 11,650b.p., the Earth warmed five to ten degrees Celsius within perhaps 20 years, an astonishingly sudden increase. A steady rise in sea level–from a low of 121 meters below modern levels around 18 Ka–accompanied this change in global climate at the end of the last glaciation. Archaeology records population dislocations throughout this period, including the movement of Siberian peoples over the Bering Strait land bridge to North America. Archaeology also suggests that c. 15,000 b.p. was a beginning, in the Near East and elsewhere, of radical new dietary and resource habits. Humans showed a new interest in previously ignored plants and animals, matched by migratory ferment as people searched out these new resources–the so-called Broad Spectrum Exploitation. These new habits, the new tools invented for the new foods, and attendant "folk genetic" observations (experience-based knowledge about the effects of purposeful manipulation on future generations of various species) anticipate the first experiments in plant and animal domestication that occur over wide arcs of the Far East, Mesopotamia, Mesoamerica, and savanna Africa at c. 10,000 b.p. With food production came village life, slowly increasing population densities, poor early city sanitation and other public health conditions, and epidemic-scale


evolutionary epidemiology, arising from the new intimacy of humans and their animal partners.

While it is not possible to say that these early Heinrich Events caused agriculture and pastoralism, the intensified adjustments humans made to climate change clearly included experiments in food production. Globally, the oceanic conveyor system had stabilized in the warm Holocene mode after around 10,000 b.p.; however, there were hiccups in the system at 7,500 b.p. (warm Hypsithermal), at 4,500 to4,000 b.p. (cold sub-Boreal), at 2,760 to 2,510 (sub-Atlantic), and at 950 to 1100 c.e. (Medieval Warm Epoch). Some dramatic regional excursions, such as the European Little Ice Ages of the late 1500s to early 1800s c.e., were not global in reach, however profound their effect upon the economies, political life, and social world of the affected communities. While historians cannot say that the Roman Empire collapsed because of the end of the Mediterranean climatic optimum at around 450 c.e., imperial industrial farming in what is now the North African Sahara was effectively shut down by this global change. That change had its contrasting counterpart south of the Sahara in growing populations and trade, including a thriving urban civilization along the Middle Niger.

Beyond these abrupt shifts of global or sub-global climate, populations throughout history have had to adjust to shorter-duration, but equally abrupt stress conditions. Peter DeMenocol (2001) documents the massive collapse of long-established complex state systems associated with drought or instability at around 2200 b.c.e. (Akkadian, Mesopotamia), 600 c.e. (Mochica, Peruvian coast), and 800 to 1000 c.e. (Classic Maya, Yucatan). Although these appear to be climate-caused collapses, other high-density, centralized states safely pass through analogous stresses. The thirteenth century collapse of the pueblo societies of the American Southwest, with precipitous population declines from warfare and out-migration, was plausibly a consequence not of a single event–the Great Drought of 1276 to 1299c.e.–but of longer episodes of climate unpredictability and environmental degradation at 1130 to 1180 c.e. and 1270 to 1450 These were communities that had endured severe droughts before, but they lacked the economic and political resilience to counter multi-decade periods of unpredictability.

Even more recently at a still shorter time-scale, but reaching far back in prehistory, populations in large regions of the globe have had to deal with another unpredictable system, made familiar through contemporary weather forecasting: El Niños (in full, El Niño Southern Oscillations, or ENSO) and La Niñas. These are just the most notorious of several global barimetric pressure oscillation systems. ENSO have an apparent period of 3.5 to 4.0 years, but regularly skip a beat. Moreover, they appear to fall into clusters of high or low intensity. In spite of great advances in understanding, ENSO are not entirely predictable. The maize farmer in Zimbabwe may have sufficient advance warning after the onset of an ENSO year, but the anchovy fisherman in Peru may not. The fates of Peru's pre-Columbian civilizations have turned on the interacting quasi-periodicity of the ENSO, as have those of millions of South Asians, the victims of monsoonal-driven periods of drought and plenty.

There is, fortunately, a realistic hope that recognition of the rhythms and causes of climate change can be linked to knowledge of natural and human ecology, alleviating a great deal of suffering. Much is already known about regional modes of rainfall variability: The infamous Sahelian Drought is now known to be one of six recurrent African modes. Twenty-first century research is investigating what causes the abrupt shifts from one mode to another, with the aim of finding means of predicting the next mode shift. Such indicators may one day allow governments and international agencies to devise early warning mechanisms that are not only predictive, but preemptive.

See also: Paleodemography; Peopling of the Continents; Prehistoric Populations; World Population Growth.


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Roderick J. McIntosh


The threat of human-induced climate change, popularly known as global warming, presents a difficult challenge to society. The production of so-called greenhouse gases. (GHG), as a result of human activity, mainly due to the burning of fossil fuels such as coal, oil, and natural gas, is expected to lead to a generalized warming of the Earth's surface, rising sea levels, and changes in precipitation patterns. The potential effects of these changes are many and varied–more frequent and intense heat waves, changes in the frequency of droughts and floods, increased coastal flooding, and more damaging storm surges–all with attendant consequences for human health, agriculture, economic activity, biodiversity, and ecosystem functioning. Some of these consequences could be positive–for example, increased agricultural productivity in some areas–but most are expected to be negative. Responding to this challenge is complicated by the considerable uncertainty that remains in projections of how much climate will change, how severe, on balance, the effects will be, how they will be distributed geographically, and how costly it would be to reduce greenhouse gas emissions. In addition, the long-term nature of the effects of climate change means that if emissions are reduced now, the costs will be borne in the near term while the (uncertain) benefits will be realized largely in the long term–decades and even centuries into the future. Moreover, because sources of emissions are widely dispersed among nations, no single country can significantly reduce future global climate change just by reducing its own emissions. Any solution to the problem must eventually be global. Demographic factors are important to all of the key aspects of the climate change issue: they play important roles as drivers of greenhouse gas emissions, as determinants of the effects of climate change on society and ecosystems, and in considerations of climate change policy.

Population and Greenhouse Gas Emissions

Most studies of the influence of population on energy use and greenhouse gas emissions focus on population size and fall into one of two categories: decomposition analyses and sensitivity analyses. A smaller number consider additional compositional variables such as age structure and household type. Limited attention has been given to the potential role of urbanization.

Decompositions of emissions rates into components attributable to each of several driving forces have been performed on national and regional data on historical emissions, on scenarios of future emissions, and on cross-sectional data. All such decompositions begin with a multiplicative identity, a variation of the well-known I-PAT equation as applied to greenhouse gas emissions. I-PAT describes the environmental impact (I) of human activities as the product of three factors: population size (P), affluence (A), and technology (T). The goal in such exercises is to quantify the importance of the P, A, and T variables in producing environmental impacts, usually in order to prioritize policy recommendations for reducing them. However, such exercises suffer from a long list of ambiguities inherent in decomposing index numbers (such as the I in I-PAT) that make results difficult, if not impossible, to compare.

There are a number of ways to perform the decomposition, and each method leads to a different result. In addition, the choice of variables to include in the decomposition, differences in the level of disaggregation, the need to consider interactions between the variables on the right-hand side of the equation, and the inertia built into trends in individual variables all affect the results and complicate interpretation. These ambiguities have been the basis of attacks on methods of quantitative analysis and have generated heated scientific debates about the relative importance of various factors without, however, resulting in any clear resolutions.

An alternative approach to analyzing the role of population in energy use and carbon dioxide emissions has been sensitivity analysis–that is, comparing scenarios from an energy-emissions model in which various assumptions about driving forces are tested in a systematic way. Models used in such studies have ranged from simple I-PAT-type formulations to more complex energy-economy models. Most work to date has focused on the influence of population size: On balance, the results indicate that although population momentum limits the plausible range of population sizes over the next several decades, in the longer term alternative patterns of population growth could exert a substantial influence on projected emissions. Incorporating relationships between population growth and income growth can substantially change the emissions expected from particular demographic and economic scenarios, but does not significantly change the sensitivity of results to alternative population growth assumptions.

Work focused on both direct energy use by households and indirect use (energy used in the production and transport of other goods consumed by the household) has identified household characteristics as key determinants of residential energy requirements. Household size appears to have an important effect (independently from income), most likely due to the existence of substantial economies of scale in energy use at the household level. Age is also important: Other things equal, households headed by the middle-aged tend to have higher consumption and energy requirements than those headed by the young or the old. These patterns, when combined with projected changes in the composition of populations by age and living arrangements, imply that compositional change may have an important effect on aggregate energy use and emissions above and beyond the scale effect of population size.

There have been few studies of the potential for urbanization (and spatial patterns of settlement in general) to affect future greenhouse gas emissions. Generally, this factor is considered only implicitly in emissions scenarios by assuming it to be essentially an income effect. However, analysis of cross-national variation in energy use and emissions suggests that urbanization leads to greater emissions above and beyond the influence of per capita income.

Population and the Effects of Climate Change

Demographic factors will strongly influence the effects that climate change may have on society, as well as influencing the ways that societies respond to those effects. Perhaps most directly, the expected increase in the population of low-lying coastal areas as urbanization (and urban deconcentration) proceeds is likely to exacerbate the effects of future sea level rise associated with global warming, including increased damage from extreme weather events. In addition, there are potential impacts–some of which might be positive–on agricultural production, one of the most intensively studied areas of climate change consequences; at the same time, population growth will raise the demand for food and fiber. The potential for climate change to expand the numbers of environmental refugees has also attracted wide interest. While global climate change may not presage a century of massive refugee movement, stresses associated with global change may intensify the pressures that already drive internal, regional, and intercontinental migration.

Future levels of fertility, population growth, and age structure will each play a role in societal responses to the effects of climate change. For the remaining high-fertility countries, a case can be made that lower fertility at the household level and slower population growth at the regional and national levels would ease the challenges faced by countries in the areas of health, migration, and food production. A qualification specific to health is that lower fertility accentuates population aging and thus puts pressure on health resources. Another, general, qualification is that policies affecting fertility are unlikely to be key strategies, since more direct means of improving social resilience under conditions of stress are available. Among these are better management of agricultural resource systems, more vigorous development and equitable distribution of health resources, and elimination of institutional rigidities that trap impoverished populations in environmentally unstable environments.

Population and Climate Change Policy

Many population-related policies–such as voluntary family planning and reproductive health programs, and investments in education and primary health care–improve individual welfare among the least well-off members of the current population. They also tend to lower fertility and slow population growth, reducing GHG emissions in the long run and improving the resilience of populations vulnerable to climate change. Therefore, they qualify as winwin policies of the sort identified for priority action in analyses of the potential effects of climate change. The existence of a climate-related external cost to fertility decisions lends support to such programs, not only because they assist couples in having the number of children they want, but also because they tend to lower desired fertility. Several studies have estimated the magnitude of these external factors to be on the order of hundreds to thousands of dollars per birth. These estimates depend on a number of factors, including geographical location (on average, births in developing countries where consumption is lower have a smaller external effect than births in industrialized countries), the magnitude of assumed future greenhouse gas emissions reductions, the costs of emissions reductions, and the discount rate. Nonetheless, the conclusion that the external costs are substantial appears to be robust, partly because meeting long-term climate change limitation goals will eventually require steep emissions reductions, and a smaller population inevitably reduces the need for the most expensive emissions reductions at the margin.

These conclusions do not imply that population policies are the most effective or equitable policies for addressing potential problems of climate change. More direct means of reducing GHG emissions and enhancing the functioning of institutions are available. Arguably, however, policies related to population should be part of a broad range of policies to reduce greenhouse gas emissions and to improve social resilience to the expected effects of climate change, and of global environmental change in general. Population-related policies have not yet entered explicitly into serious discussions of climate change policy. Little consideration has been given even to differential population growth among industrialized countries when negotiating country-specific emissions reduction targets. This is likely due to the sensitivity of the issue, given the long-running debate over the relative importance of population size and growth, as compared to high levels of per capita consumption, in affecting the environment in a deleterious fashion.

See also: Ecological Perspectives on Population; Energy and Population; Natural Resources and Population.


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Brian C. O'Neill

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