Abstract
Archaeologists are well versed in landscapes being constructed through the building of monuments, modifications such as agricultural terracing or irrigation canals, impacts like soil erosion and wood-cutting, but most would probably not see this as affecting much more than the land-surface, soils and vegetation, and the cultural world. Ruddiman’s book synthesizes the evidence of the cumulative impact of human activities since the beginnings of agriculture. Its central tenet is that human land-use activities over the past 5000–7000 years raised methane and carbon dioxide levels in the atmosphere. This means that our industrial era represents the intensification of anthropogenic processes underway since the Neolithic.
Quaternary geoscientists and archaeologists often consider the impacts of climate change on past cultures, but Ruddiman argues that past cultures had a profound and increasing impact on global climate. For Quaternary scientists familiar with the Ruddiman hypothesis, this book represents a distillation of all the strands of evidence and argument, as well as a history of their development. For those unfamiliar with it, this is now the essential introduction. The book is divided into 21 chapters, grouped into 6 parts. Each chapter is bite-size, between 10 and 20 pages, and amply illustrated with essential colour charts as well as many maps and photos. It is written in easy-to-follow prose, largely jargon-free, suitable as an introductory textbook, for the layperson, or perhaps more importantly to easily communicate key issues across disciplines.
Part 1 of the book summarizes the cyclical climate system, from the orbital forcing of long-term cycles to earth–sea–atmosphere carbon cycles. Then, the author moves to discussing anomalies in carbon gases over the Holocene in contrast to previous interglacials; both carbon dioxide and methane increase over the last several thousand years against the expected trend. The methane rise is unlikely to have originated from natural sources.
Part 2 considers the origins and spread of agriculture, drawing primarily on archaeological evidence. First, Ruddiman discusses early agriculture in the Near East, with plant domestication starting 11,000 years ago, followed by animals from 10,000 years ago. This package of food production spread across Europe from 8400 to 5500 years ago. The Neolithic in Europe brought with it forest clearance and animal tillage. Britain is considered as a case study in how longer term developments lead to growth of colluvium and efforts to reduce soil erosion in early field systems. Elsewhere in Europe, silt from erosion accumulated in alluvial plains and deltas. The story of European agriculture and reforestation is taken through the medieval and early industrial eras. The origins of agriculture in China and South Asia are treated at some length, as it is only in this region where attempts have been made to quantify the impacts of agriculture, especially methane emissions from rice paddies. The spread of livestock in these regions is important, as sheep and cattle only arrived in the past 5000 years, but are highly productive in these regions – another important but unqualified source of methane. Historical records, especially from China, indicate continued population growth and agricultural intensification. Two chapters treat early farming in the Americas, Africa, and Oceania.
Part 3 considers the reality of the anomalies in terms of how the Holocene is aligned with previous interglacials to support Ruddiman’s hypothesis stated in previous chapters. Alignment to glacial onset, inferred from ice volume decreases, makes the anomalies start slightly later, around 6000 BP instead of 7000 BP for CO2, but the anomalies are still there. Ruddiman also considers possible non-anthropogenic sources of carbon dioxide and methane, such as the dissolution of calcium carbonate by acidifying oceans, which would have begun when post-glacial reforestation finished and stopped drawdown of CO2 into new forests. Coral reef formation also would have increased by about 7000 years ago as rising sea levels stabilized, thus releasing CO2. Growing river deltas have been suggested to produce methane. There is not enough evidence that these processes would be sufficient to explain the anomaly, or to explain why the same processes would not have happened in previous interglacials. For the anthropogenic alternative explanation, one must consider per capita land use over time. Some critics have taken estimates of per capita land use and deforestation, and assumed a linear relationship for backwards extrapolation. Such an approach inevitably led to the conclusion that the very low populations in prehistory are unlikely to have had much of an impact on atmospheric gases. However, this is demonstrably false in historical data for China and also Britain, where the Domesday Book indicates deforestation levels similar to the 20th century but with only ~2.5% of the current population.
Part 4 steps back from the details of evidence and asks ‘how science moves forward’ to consider falsification of hypotheses and the academic process of paradigm shifts. Is this a paradigm shift? Certainly, it is now clear that ‘[the idea that] human impacts on this planet have occurred mainly in the industrial era (the last 200 years or less) is no longer tenable’. This section is somewhat more autobiographical, reflecting reactions, in person, in print, in the blogosphere, of Ruddiman’s hypothesis.
Part 5 looks at climates of the recent past and future. One of the entailments of the early anthropogenic model is that we should have already been in a world that was cooling: northern ice sheets should be growing not melting. Instead, anthropogenic carbon emissions have caused the glacial cycle to miss a pulse. Thus, it would seem that ‘we humans have now ended the 2.75-million-year history of northern hemisphere ice-age cycles for a time into the future that is beyond imagining’. This raises the question as to what climate ‘we’ want and whether climate engineering is our way forward.
Finally, Part 6 illustrates how a paradigm of anthropogenic climate controls links to recent history. The late Roman Era had temperature drops, as did the centuries grouped as the ‘Little Ice Age’ from 1400 to 1900 AD. Ruddiman concludes that these changes were also anthropogenic, representing massive human population crashes, with the abandonment of agricultural land and reforestation. Two of these were when the Bubonic Plague reduced population levels across Europe in the 6th and 14th centuries. The Mongol invasion of China in the 13th-century left farmers dead, irrigation systems damaged and rice land abandoned, correlating with a big dip in methane. Another event was in the 16th century: once European guns and germs had decimated American populations, large-scale reforestation of the Amazon took place.
This is a small book that packs big punches. While it may aim at outlining a new paradigm on the anthropogenic climate, it also lays the groundwork for a new focus of interdisciplinary data integration between palaeoclimate scientists, archaeologists and historians. But it calls for a new large-scale synthesis which runs counter to the regionalist traditions of archaeology or history. The Neolithic Revolution was about more than just settling down and farming: it set the world on the path to a new regime of climatic controls with human unwittingly holding a couple of levers.