A rapidly growing body of research examines whether human conflict can be affected by climatic changes. Drawing from archeology, criminology, economics, geography, history, political science, and psychology, we assemble and analyze the 60 most rigorous quantitative studies and document, for the first time, a remarkable convergence of results. We find strong causal evidence linking climatic events to human conflict across a range of spatial and temporal scales and across all major regions of the world. The magnitude of climate's influence is substantial: for each 1 standard deviation (1σ) change in climate toward warmer temperatures or more extreme rainfall, median estimates indicate that the frequency of interpersonal violence rises 4% and the frequency of intergroup conflict rises 14%. Because locations throughout the inhabited world are expected to warm 2-4σ by 2050, amplified rates of human conflict could represent a large and critical impact of anthropogenic climate change.

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Quantifying the influence of climate on human conflict

A record from Wanxiang Cave, China, characterizes Asian Monsoon (AM) history over the past 1810 years. The summer monsoon correlates with solar variability, Northern Hemisphere and Chinese temperature, Alpine glacial retreat, and Chinese cultural changes. It was generally strong during Europe's Medieval Warm Period and weak during Europe's Little Ice Age, as well as during the final decades of the Tang, Yuan, and Ming Dynasties, all times that were characterized by popular unrest. It was strong during the first several decades of the Northern Song Dynasty, a period of increased rice cultivation and dramatic population increase. The sign of the correlation between the AM and temperature switches around 1960, suggesting that anthropogenic forcing superseded natural forcing as the major driver of AM changes in the late 20th century.

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A test of climate, sun, and culture relationships from an 1810-year Chinese cave record.

Structure and origin of Holocene cold events

For centuries, thinkers have considered whether and how climatic conditions-such as temperature, rainfall, and violent storms-influence the nature of societies and the performance of economies. A multidisciplinary renaissance of quantitative empirical research is illuminating important linkages in the coupled climate-human system. We highlight key methodological innovations and results describing effects of climate on health, economics, conflict, migration, and demographics. Because of persistent "adaptation gaps," current climate conditions continue to play a substantial role in shaping modern society, and future climate changes will likely have additional impact. For example, we compute that temperature depresses current U.S. maize yields by ~48%, warming since 1980 elevated conflict risk in Africa by ~11%, and future warming may slow global economic growth rates by ~0.28 percentage points per year. In general, we estimate that the economic and social burden of current climates tends to be comparable in magnitude to the additional projected impact caused by future anthropogenic climate changes. Overall, findings from this literature point to climate as an important influence on the historical evolution of the global economy, they should inform how we respond to modern climatic conditions, and they can guide how we predict the consequences of future climate changes.

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Social and economic impacts of climate

The processes that control climate in the tropics are poorly understood. We applied compound-specific hydrogen isotopes (δD) and the TEX86 (tetraether index of 86 carbon atoms) temperature proxy to sediment cores from Lake Tanganyika to independently reconstruct precipitation and temperature variations during the past 60,000 years. Tanganyika temperatures follow Northern Hemisphere insolation and indicate that warming in tropical southeast Africa during the last glacial termination began to increase ∼3000 years before atmospheric carbon dioxide concentrations. δD data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial-scale events recorded in Northern Hemisphere monsoonal climate records. This implies that precipitation in tropical southeast Africa is more strongly controlled by changes in Indian Ocean sea surface temperatures and the winter Indian monsoon than by migration of the Intertropical Convergence Zone.

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Northern Hemisphere Controls on Tropical Southeast African Climate During the Past 60,000 Years

A palaeoclimate record spanning the past 16,000 years with nearly annual time resolution has been obtained from Lake Huguang Maar in China. The magnetic properties and titanium content of the lake sediments are thought to reflect changes in the East Asian winter monsoon strength, thus providing a complement to palaeoclimate archives that record the strength of the rain-bearing summer monsoon. The records point to an anti-correlation between winter and summer monsoon strength, best explained by migration of the intertropical convergence zone (ITCZ), a belt of low-pressure air at the Equator. Interestingly, the decline of China's Tang dynasty and the Mayan civilization broadly coincide with drought periods recorded in Lake Huguang Maar and Cariaco basin sediments, respectively. Did major shifts in the position of the ITCZ catalyse simultaneous events in civilizations on opposite sides of the Pacific Ocean? The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits1,2,3, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bolling–Allerod warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons1,2,3. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.

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Influence of the intertropical convergence zone on the East Asian monsoon

Recent studies have drawn attention to differences in the seasonal impact of the 8.2 ka event, with longer cooler summers and shorter cooler/drier winters. However, there are no data available on the simultaneity or the rate of onset of the seasonal changes in Europe. Based on the microfacies and geochemical analyses of seasonally laminated varved sediments from Holzmaar, we present evidence of differences in duration and onset time of changes in summer temperature and winter rainfall during the 8.2 ka event. Since both summer and winter climate signals are co-registered within a single varve, there can be no ambiguity about the phasing and duration of the signals. Our data show that the onset and withdrawal of the 8.2 ka summer cooling occurred within a year, and that summer rains were reduced or absent during the investigated period. The onset of cooler summers preceded the onset of winter dryness by ca. 28 years. In view of the differences in nature and duration of the impact of the 8.2 ka event we suggest that a clearer definition of the 8.2 ka event (summer cooling or winter cooling/dryness) needs to be developed. Based on regional comparison and available modelling studies we also discuss the roles of solar variability, changes in North Atlantic Thermohaline circulation, and North Atlantic Circulation (NAO) during the period under consideration. Wavelet analyses of seasonal laminae indicates that the longer NAO cycles, linked to changes in the N. Atlantic temperatures, were more frequent during the drier periods.

The 8.2 ka event: Evidence for seasonal differences and the rate of climate change in western Europe

Replying to: H. Zhou et al. , doi: 10.1038/nature06408 (2007).
 Zhou 1 raise the possibility that the titanium (Ti) record at Lake Huguang Maar is controlled by local erosion and runoff to the lake, or through hydrological changes in the lake such as level fluctuations, rather than by changes in the inputs of airborne material2. The authors come to this conclusion by considering the Ti record in isolation. They ignore the redox-sensitive parameters of the S-ratio, total organic-matter content, the Mn/Fe ratio, and magnetic susceptibility records. We believe that their case against our interpretation of the Ti record is weak, and that their interpretation can be ruled out if the other measurements are taken into account2.

Yancheva et al. reply

Anti-correlation of summer/winter monsoons? Reply

Paleoclimatic studies based on sedimentary sequences can be supported by the analysis of the sediment’s magnetism. Determination of the magnetization directions, and partly, the intensity can provide useful information for dating when the results can be correlated to dated master curves. So, one main task of a magnetostratigraphic study is to provide an age model for the sediments investigated for paleoclimatic reconstructions. In addition, analysis of the magnetic carrier minerals themselves give additional information about paleoenvironmental processes, since often the input and preservation or alteration of the magnetic particles reflect climatic influences on the sediments bulk composition. Magnetostratigraphy therefore can be one of the most important stratigraphic tools in paleoclimate research.

Gergana Yancheva

Papers

Influence of the intertropical convergence zone on the East Asian monsoon

The 8.2 ka event: Evidence for seasonal differences and the rate of climate change in western Europe

Yancheva et al. reply

Anti-correlation of summer/winter monsoons? Reply

The Contribution of High-resolution Magnetostratigraphic Analyses to Paleoclimatic Reconstructions