The challenge to humanity as our planet's destiny unfolds

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.