How far can we push our luck?

Gaia has illuminated our understanding of our planet, says Andrew Goudie, but it isn't clear whether the earth goddess can survive whatever we throw at her

The 20th century was a time of extraordinary global environmental change. The human population increased fourfold from 1.5 billion to 6 billion, the world's economy increased fifteenfold, its freshwater use increased ninefold, the irrigated area by fivefold and energy use by thirteen to fourteenfold. Indeed, humankind probably used more energy in the 20th century than in all preceding human history. The Earth's biota is experiencing the sixth great extinction event in its history - the first such event to be caused by one species, Homo sapiens .

Furthermore, we are living in a world where the human population is likely to be greater by 2 billion to 4 billion people by 2050, where earth systems will be massively altered by global climatic change and other types of human impact, and where environments that until now have been left relatively pristine will become increasingly exploited. Sir Crispin Tickell has gone so far as to ask whether humans have become suicidally successful.

Given the speed of transformation, it is scarcely surprising that since the early 1980s, global change has become a central concern for many disciplines and for society as a whole. In 1986, the International Council of Scientific Unions developed the International Geosphere-Biosphere Programme "to describe and understand the interactive physical, chemical and biological processes that regulate the total earth system, the unique environment that it provides for life, the changes that are occurring in this system, and the manner in which they are influenced by human activities". Both books reviewed here relate to this concern, though one is the outcome of an idea that was partially responsible for the concern, whereas the other is a direct product of it.

The Gaia concept, named after the Greek goddess of the earth, arose some four decades ago when a British scientist, James Lovelock, was employed by US space agency Nasa to help design means of detecting life on Mars. He recognised that a planet with abundant life will have a very different atmosphere from one without it. The hypothesis that he developed with microbiologist Lynn Margulis postulated that the climate and chemical composition of the earth's surface are kept in homeostasis at an optimum by and for the biosphere. Life actively participates in shaping the environment in a way that optimises the conditions of life.

The concept evolved at a time when the developing space programmes of the Russians and the Americans were leading to an increasing appreciation of Planet or Spaceship Earth. Gaia and the space programme promoted thinking at a global scale, which is something we now take for granted. Gaia also stimulated an interdisciplinary approach that required the attention of biologists, chemists, earth scientists, climatologists and oceanographers.

This was different from the reductionist approach that was and is so characteristic of much science. The Gaian perspective also underlies another widespread concern of the modern era - the principle of sustainability. As Tim O'Riordan, professor of environmental sciences at the University of East Anglia, put it: "A healthy planet will support, and will be supported by, healthy people."

Lovelock's first book, Gaia: A New Look at Life on Earth , appeared in 1979. This vision, as initially formulated, attracted much attention and a cult following. However, it also attracted some strong criticism. The sharpest came from Darwinian theorists such as Richard Dawkins and W. Ford Doolittle, who questioned how the idea of natural selection in a world where nature was red in tooth and claw tied in with Gaia. Other scientists questioned whether organisms stabilise the global environment and make it more suitable for life, and pointed out that some biological feedbacks destabilise, rather than stabilise, the global environment. Others condemned the idea of a planet-sized organism as being part of pseudo-scientific myth-making of the type that produced the surge in astrology, fringe medicine, faith healing and religious mysticism. Some believed that, paradoxically, Gaia was not helpful to those concerned about human impacts on the environment. It was unsettling to some environmentalists that Gaia implied that the earth system can survive human affronts, and that Gaia will clean up the mess we make by a series of reinstating feedback mechanisms. There was also hostility to the name itself, implying as it did that the Earth was alive, and to its perceived teleological implications.

In the late 1980s, the Gaia hypothesis was modified and re-presented as the Gaia theory. The final verdict on Gaia is not yet in. Its history is no simple tale of a maverick scientist winning through against academic conservatism or of the vindication of a scorned innovator. At the very least, however, Lovelock's intuition that there was something special about the Earth's environment that could best be accounted for by the evolution of its biosphere has helped to stimulate research. Gaia's proponents have also done a great service by championing the need to consider the Earth as a coupled system of the biotic and the abiotic.

Scientists Debate Gaia , which emerged out of the Second Chapman Conference on the Gaia Hypothesis held in Valencia in 2000, examines and reviews Gaia (the hypothesis and the theory), addressing how it has changed and how it has been criticised. Fifty-three contributors (including Lovelock and Margulis) consider the principles and processes behind Gaia; earth history and cycles; the philosophy, history and human dimensions of Gaia; the quantification of Gaia (including modelling it); and life forms and Gaia.

Some of the chapters (particularly those dealing with modelling) are quite demanding, others are quite empirical, and others more philosophical. The tone is not uncritically adulatory towards the Gaia concept, but much of it is forward looking, pointing to where further research is required to refute or substantiate aspects of Gaia. Especially useful are those chapters that address the relationship between the evolutionary mechanism of natural selection and Gaian processes, and those that explore evidence for such processes in the geological past. The book is well presented with a welcome consistency of format and approach. It would have benefited, however, from a more penetrating evaluation of where we stand. A concluding chapter to round up the uncertainties, debates and future research directions would have been immensely helpful.

From its onset, Gaia has referred, implicitly and explicitly, to a system. It has contributed to the development of what is known as earth system science - the subject of Global Change and the Earth System . This seeks to address a number of questions. How did the earth system operate in the absence of significant human influence? How can human-driven effects be discerned from those due to natural variability? What are the implications of global change for human wellbeing? How robust is the earth system in the face of these internal forces of change? Can human activities trigger abrupt and potentially irreversible changes to which adaptation would be impossible? How serious is this inadvertent human experiment with its own life-support system?

This book, part of a series published for the International Geosphere-Biosphere Programme, is well illustrated with colour diagrams (some of which are repetitive or too small for the detail they present). It contains boxes that provide detailed information about specific issues, written by contributors. One of these boxes is a defence of the Gaia concept (by Tim Lenton), another is a critique (by Paul Crutzen). The volume is extensively referenced. For a book assembled by a committee of editors, it is remarkably coherent.

The first substantial chapter considers how the earth system operated before significant human influence. It looks at scales of climatic variability, particularly as revealed by ice-core studies, at the interconnections among physical, chemical and biological systems and processes, and at the evidence for thresholds, non-linearities and abrupt change in the pre-human earth system.

The next chapter looks at human impact in the "Anthropocene Era". It is a largely predictable canter across predictable territory, but is notable for saying rather little about one of the most important of all human impacts - accelerated soil erosion.

The third substantial chapter argues that as human activities are so pervasive, one needs to consider how the earth system responds. It is proposed that human-induced changes typically cascade through the system to cause more multiple collateral and interacting changes, which in turn produce myriad further responses. Some systems may be resilient, but others may be pushed across thresholds.

The book next considers the consequences of changes in the earth system for human wellbeing as determined by food availability, water resources, air quality and pests and diseases. It draws attention to so-called catastrophic failures of the earth system - the ozone hole and the oceanic conveyor belt, which is a potential threat. The authors draw conclusions about the contemporary knowledge base, the state of earth system science and the need for a toolkit with which to pursue the goal of global sustainability in a world that is being changed in a way "unprecedented in human history and probably in the history of the planet".

Humans have been thinking about their impact on the environment for long enough. Clarence Glacken pointed to the impact of Count Buffon's ideas in the 18th century, and George Perkins Marsh's Man and Nature (1864) was a major treatment of how we were transforming our environment. There was also the symposium on "Man's Role in Changing the Face of the Earth" that took place in the US in 1956. Since then, we have had major studies of phenomena such as deforestation. Geographers have also always thought in terms of connections, as have ecologists. Global Change and the Earth System also looks at these two themes of the human impact and of connections. Where it is original, however, is that it concentrates on examining these at the global rather than the local or regional scale.

We have here two important volumes that, in differing ways, build on ideas that have developed over the past few decades. They act as a benchmark for evaluating ideas on environmental change. It will be interesting to see how the ideas expressed in them stand up to scrutiny over the coming years.

Andrew Goudie is master of St Cross College, Oxford, and author of The Human Impact (in its sixth edition).