Abstract
In this book, Martin Williams reviews past evolution, modern developments and future changes of global deserts and desert margins. Williams classifies deserts as regions ‘where the precipitation is too little and too erratic and the evaporation is too high to allow many plants and animals to survive’ (2014: 3). Deserts may of course include cold-climate ecosystems, such as those found in the Arctic and Antarctic, although Williams only focuses on deserts in warm regions in the book. Such deserts, together with semi-arid and sub-humid desert margins, cover large areas (almost 50% of the global land surface) and are also highly sensitive to future global climate change, and so are of particular concern. In these regions, small climate changes may lead to large hydrological, ecological, economic and social responses.
Williams uses three steps to explain why we should be concerned about climate change in deserts. First, he very carefully examines the various lines of evidence that have been used to reconstruct past climate change in deserts and desert margins. This is followed by an assessment of how deserts and desert margins have responded to past climate change. Finally, Williams considers how they might respond to future climatic changes.
The book consists of 26 chapters and contains abundant well-drawn figures that support the discussion. It is written in a professional and concise way, and draws on the author’s considerable knowledge of chronology, geochemistry, geomorphology, hydrology, biology and archaeology to construct its story. This breadth makes it a good textbook for scientists and students working in the field of desert studies. It is also an effective taster for those unfamiliar with the subject, containing useful sections on, for example, the history of desert studies (Chapter 5).
The book devotes several chapters to exploring the past of deserts, which still remains something of a mystery. Although deserts have been studied for more than 100 years, we still do not fully know why large deserts exist and how they respond to climate change. In principle, four drivers lead to deserts: subtropical high pressure associated with dry subsiding air mass; inland location far from sources of moisture resources; rain-shadow effects of high mountain ranges; and ocean cold currents or upwelling. The first reason is clearly the most important, because most warm deserts are located near 30o north or south of the equator, and are controlled by subtropical high pressures. These drivers are considered in detail in the book, and with a level of critical interpretation that is required for a fuller understanding of desert processes. Indeed, most textbooks teach that the Sahara desert is caused by subtropical high pressure. However, in the early Cenozoic the Sahara desert was a landscape of vegetation, rivers and lakes that favoured occupation by fauna and humans, rather than a desert; though subtropical high pressures remained at 30o north or south (Huber and Goldner, 2012). Williams reminds us that there are other important reasons leading to the formation of deserts. Recent studies have demonstrated that land–sea distribution was also a very important forcing for the onset of Sahara desertification about 7–11 Ma (Zhang et al., 2014). Since then, Sahara aridity has been controlled by earth precession cycles and glacial–interglacial cycles. Understanding the drivers and history of deserts remains a challenge – we still do not fully understand, for example, when and why many of the Asian, Australian and American deserts came into being.
The future of deserts in relation to changing climate also remains a mystery for us, and is a topic the book considers in detail. Williams elaborates on how the evolution of deserts through the past is useful for predicting their future. For example, we know that the Sahara desert is caused by subtropical high pressures, land–sea distribution and relatively lower summer heating. Since these conditions do not change much, we know that the Sahara cannot totally disappear, though it might become smaller due to shifts caused by long-term climate change. Over short time scales, drought events that can last for years to decades have occurred many times in our history. These historical droughts are linked to tropical sea surface temperature (SST) variations. La Niña-like SST anomalies in the tropical Pacific often lead to widespread droughts in North America, and El Niño-like SST warming in the Pacific cause droughts in East Asia. The southward shift of the warmest SSTs in the tropical Atlantic and warming in the Indian Ocean are the main causes of the recent Sahel droughts. It is important to consider whether, if tropical SST variability shifts in the coming future, drought events will occur more frequently in desert areas.
It seems likely that many deserts will become even drier in the coming decades. The development of new climate modelling approaches sheds some light on the possible future of deserts, and the book could have included more detailed discussion of these. For example, a recent model synthesising work based on climate modelling projections under the IPCC AR4 A1B greenhouse emission scenario (Dai, 2011), suggests continued drying over many regions, including those with deserts. These include most of Africa, southern Europe and the Middle East, most of the Americas (except Alaska, northern Canada, Uruguay and northeastern Argentina), Australia and Southeast Asia. People living in these regions may see persistent severe droughts in the coming decades, and our understanding of desert processes as summarised in this book will be crucial in responding to this impending environmental crisis.
Importantly, deserts are not biological wastelands. Two particularly interesting chapters, Chapters 16 and 17 (‘Plant and animal fossils in deserts’, and ‘Prehistoric occupation of deserts’), show that plants, humans and other animals have gradually adapted to climate change in deserts and their semi-arid and seasonally wet margins for several million years. Williams also discusses how the recent rapid, and to some extent extreme, climate change brings new challenges related to land degradation and desertification. Although we still do not fully understand the causes and extent of desertification, natural climate variability or human impacts, accelerated land degradation and desertification do bring more environmental pressures to life in the desert world.
Given a changing climate, how can people in the desert areas meet future challenges? At the end of the book, Williams suggests four valuable principles for the sustainable use of deserts. He advocates: 1) the conservation of soil and water; 2) keeping groundwater use in balance with aquifer recharge; 3) avoiding pollution of water, soil and air; 4) alleviation of poverty, and the safeguarding of health and food. Ancient people clearly adapted to past climate change in deserts. There is reasonable hope that modern people, with more advanced technology, will be able to do better in the future.
Of course, we are still on the way to truly understanding the past, present and future of deserts. This comprehensive and stimulating book is certainly an important milestone along the route.