The challenge to humanity as our planet's destiny unfolds


EARTH SCIENCE:How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind, Princeton University Press, 718pp, £27.95, By Charles H Langmuir and Wally Broecker

Although awareness of the effects of human activities on the global environment has been growing steadily in recent decades, it has so far evoked only limited willingness to take mitigative action.

In this book, the Harvard geochemist Charles Langmuir and the Columbia Earth scientist Wally Broecker stand well back and approach the environmental challenge in the context of a general introduction to planetary, Earth and climate science.

Their stance is that active planetary stewardship is imperative if the Earth is to remain a habitable abode for humankind.

The book, aimed at nonspecialists, is a revised and greatly expanded version of the highly regarded How to Build a Habitable Planet, by Broecker, published in 1984. Enormous advances have been made in the Earth sciences in the years since the original volume appeared. In addition, climate change has become a much more urgent topic. The revised version aims to bring the science up to date and to give a current environmental perspective.

In this undertaking, Broecker has been joined by Langmuir, who now becomes first author. Their approach of providing each chapter with a clear introduction and summary will help greatly in accommodating the lay reader.

The scene and the style are set with the following early sentences from the book: “The universe as we know it began about 13.7 billion years ago with an explosion that astronomers refer to as the Big Bang. All the matter in the universe still rides forth on the wings of this blast.”

The early chapters sweep through the vast aeons of time that elapsed between this cataclysmic beginning and the time the Earth was formed, about 4.6 billion years ago, from the collapse of the solar nebula in a corner of the Milky Way, one of billions of galaxies in the expanding universe. Succeeding chapters describe how the Earth settled into a habitable abode for life, allowing the emergence, about a billion years after its formation, of the first living organisms and, after further aeons of DNA mutation and Darwinian selection, the rise of homo sapiens some 160,000 years ago.

The question of habitability

A unifying theme in the second half of the book is the question of planetary habitability. A central feature of the Earth’s habitability is that for almost all of its history it has remained at a temperature in which water can exist in its liquid form, an essential prerequisite for life.

This is seen as due mainly to what is called a tectonic thermostat, a long-term climate-stabilising mechanism based on a recycling of the greenhouse gas carbon dioxide (CO2) between the atmosphere and the Earth’s crust. Tectonic movements in the crust cause CO2 to be emitted to the air in volcanic activity.

Balancing this source, chemical reactions involved in the weathering of rocks and soils remove CO2 from the air. The removed carbon is washed to the sea in rivers and streams, enters the shells of marine organisms, eventually falls to the sea bed and, by a commodious vicus of recirculation, is subducted back into the Earth’s crust. The thermostat operates by virtue of the fact that the rate of removal of CO2 from the air is dependent on temperature: the warmer it gets, the faster the weathering occurs, and vice versa. Over geological time, the thermostat acts to keep the atmosphere’s CO2 concentration and temperature close to equilibrium values that are comfortable for life.

Various external influences are described that have perturbed this natural balance throughout the Earth’s history. The external influence with which the latter parts of the book are centrally concerned is human emissions of CO2.

The crux, from the Earth-science perspective, is that the rate at which human emissions are now occurring is more than 150 times the natural background rate due to volcanic sources. Human beings are in the process of certainly doubling, and potentially quadrupling, the CO2 content of the atmosphere. Climate models suggest that a CO2 doubling will increase the global temperature by between three and five degrees centigrade, while a quadrupling will lead to a further warming of a comparable amount.

Coral reefs declining

Independently of the induced warming, the increased atmospheric CO2 content is leading to acidification of the ocean. This is predicted to have a major effect on shell-producing organisms and is already contributing to a global decline in coral reefs. In addition, the loss of soils and biodiversity due to human activities is seen here as an environmental threat comparable to that of climate change. Rainforests are being cleared at an alarming rate, with some 70 per cent of the Indonesian rainforest now gone. The current loss of species due to such invasive human activities is seen as being comparable to the mass extinctions of the geological record.

Although the book presents a pessimistic scenario of a possible future for our planet, the authors are of the view that actions to prevent the worst happening are also feasible. They believe that human beings have the capacity to change their behaviour and manage the planet for the mutual benefit of all species.

A principal factor preventing action is that the economic principles on which our world operates go nowhere near taking environmental costs into account. Globally, people do not pay for the CO2 they emit; agriculture does not take into account the costs of soil depletion; habitat destruction does not take into account the destruction of species. The Earth bank is regarded as being infinite.

The authors hold that a small fraction of the amount spent globally on armaments would permit meaningful action towards a sustainable world.

The greater part of the book, by far, is devoted to the science. We can be grateful to the authors that they had the initiative and energy to undertake a scientific synthesis of such broad scope. In this respect, the book is probably unique. Inevitably, their own scientific specialties predispose the authors to emphasise some areas in which they have in-depth knowledge.

In so far as climate stability is concerned, the book places foremost emphasis on the tectonic thermostat, which acts on geological timescales. Little is said about the climate-stabilising mechanisms acting on the decadal to century timescale, which are important in discussing the scientific background to the 0.8-degree warming of the past century and the predicted changes ahead.

These reservations of another climate specialist aside, the book can be heartily recommended to all who wish to share something of the excitement of seeing the history of the Earth unravelled. Those who already have a background in any scientific area will have their perspective broadened.

All who are concerned with the global environment and who wish to be scientifically well informed in relation to it will find the book a worthwhile and inspirational challenge.

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