Increases of population and expansion of rice agriculture in Asia, and anthropogenic methane emissions since 5000 BP

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Abstract

CO2 and CH4 composition of the atmosphere increased rapidly following the industrial revolution. Recently Ruddiman has suggested that increases in the anthropogenic contribution to atmospheric greenhouse gases had actually begun thousands of years earlier. Research on climates indicates that a cooling and drying trend developed from about 5000 BP across the Asian monsoonal region. Archaeological and biological data reveal that from about the same time there was an important transition point corresponding to the rapid growth of population and expansion of cultivated rice areas. Extensive deforestation also occurred from this time. The expansion of rice agriculture and extended wetland areas provided more sources of methane emissions, and thus contributed to greenhouse gas budgets. The climate impacts of increased anthropogenic methane emissions were possibly counterbalanced in part by any natural decrease from orbital forcing. The methane contribution from rice paddy areas is estimated to be smaller than 250 ppb for the middle-late Neolithic. The 1000-ppb methane rise after industrial era coincides closely with the rapid growth of global human populations, and anthropogenic driven sources.

Introduction

The Anthropocene is now regarded as a geological episode beginning from the time that human impacts greatly affected the natural behaviour of Earth Systems (Crutzen and Stoermer, 2000, Crutzen, 2002, Ruddiman, 2003, Liu, 2006). Generally, the Anthropocene is thought to have started in the late eighteenth century, when the industrial revolution began producing greenhouse gases which significantly altered the composition of the atmosphere (Crutzen, 2002). Ruddiman and Thomson, 2001, Ruddiman, 2003 and Ruddiman et al. (2008) have recently hypothesized that anthropogenic emissions of greenhouse gases first altered atmospheric concentrations many millennia ago as a result of the discovery and spread of agriculture and subsequent technological innovations in the practice of farming.

As one of the principal greenhouse gases, methane plays a central role in atmospheric oxidation chemistry and effects stratospheric ozone and water vapour levels, yet its sources and sinks are less well understood than those for carbon dioxide (Houghton, 1999, Keppler et al., 2006). Studies of atmospheric methane and its sources and sinks have been carried out intensively over the two past decades (e.g. Gon et al., 2000, Crutzen et al., 2006, Verburg et al., 2006). Total current methane emissions are estimated to be 600 million tn yr−1 and are in fact largely of biogenic origin (Verburg et al., 2006). It is generally inferred that the three-fold increase in methane concentration since industrial times is the result of anthropogenic emissions (Crutzen, 2002).

A methane record recovered from central Greenland (GISP2 ice-core), and covering the past 110,000 years, reveals that the magnitude of rapid methane concentration shifts are consistent with variations in Northern Hemisphere summer insolation (Brook et al., 1996). However, the ice-core record shows that the variations of greenhouse gas concentration differ from the natural trends predicted by earth-orbital changes during the Holocene (Blunier et al., 1995, Ruddiman, 2003). The CH4 trends in the middle and late Holocene suggest a different mechanism is needed to explain them. Blunier et al. (1995) argued that the increase of CH4 concentration may be controlled by the hydrological cycle and to have had an increasing contribution from northern wetlands. Ruddiman noted that rice farmers may have been causing elevated global atmospheric methane levels from as early as 5000 BP (Ruddiman and Thomson, 2001, Ruddiman, 2003). There may also be other explanations.

This paper reviews climate variability data on the Asian monsoonal region, the increase of population and expansion of rice agriculture in Asia, and investigates the timing of these in relation to the anomalous CH4 concentrations during the Holocene.

Section snippets

The climate variation in Asian monsoon region during the Holocene

The monsoon system, which covers the region from the western Arabian Sea through East Asia and northern Australia, is a dynamic component of the modern climate system and was a controlling influence during the Holocene (Fig. 1). The Asian monsoon has significantly influenced the nature of environments, and societal and economic activity of roughly half the world's population (Kutzbach, 1981, An et al., 2000, Yancheva et al., 2007). The results from a low-resolution climate model agree with

The rapid growth of human population in China from 5000 BP

From about the start of the Holocene, virtually the entire human population had become predominantly farmers or herders within 5000 years (Legge and Rowley-Conway, 1987). The transition of human lifestyles from hunter–gatherer to sedentary agriculture (Bellwood, 2005) made possible rapid human population growth, a rise of crop areas and the acceleration of technological progress. But the transition occurred at different times in different regions of the world and could have been a result of

Expansion of rice agriculture in Asia

Rice is the world's most important food and is unique among cereals in being an aquatic crop which can be a perennial or annual according to water and temperature conditions. Rice fields are also regarded as a major source of methane emission, and contribute 9–30% to the global methane budget (Gon et al., 2000, Verburg et al., 2006). While rice is grown in 112 countries and 124 million hectares are devoted to it (Bertin et al., 1971), Asian countries contain the main regions of cultivated rice

Discussion and conclusion

Crutzen and Stoermer (2000) advanced the concept of “Anthropocene” and included the notion that human activities have altered greenhouse gas concentration in the atmosphere and thus modified Earth's climate systems from about 300 years ago. Ruddiman and Thomson (2001) and Ruddiman (2003) suggested that an increase in the anthropogenic contribution to atmospheric greenhouse gases had actually begun to increase thousands of years earlier.

The difference in methane concentration in Greenland versus

Acknowledgements

This study was supported by the Grants NSFC (40572108, 40599423), Innovation Program of CAS (KZCX3-SW-146), National Key Basic Research Fund (2004CB720202) and Australian Research Council Discovery Grant.

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