Gary Comer was a wealthy retired American who found on a private voyage in 2001 that he was able to easily navigate the normally ice-bound Northwest Passage in northern Canada. It perturbed him that he could do so and resulted in his substantial funding of scientific research into the global extent of abrupt climate change. Glaciologist Richard Alley, oceanographer Wallace Broecker and geologist George Denton were leading scientists in the research he funded, and are co-authors with editor Philip Conkling of a newly published book which draws on the past decade of their and others’ work. The Fate of Greenland: Lessons from Abrupt Climate Change is a handsomely produced volume, lavishly illustrated with stunning photographs, many of them taken by Comer himself. It’s written for a general audience, albeit at times requiring close attention from readers when the complexities of some of the scientific detective work are explained.
Past climate is the main focus of the book and the work of the scientists concerned, but their study of the past is aimed at better understanding what will happen in the future as the atmosphere continues to absorb a heightened level of greenhouse gases. The book begins with the theory of ice ages, each glacial and interglacial period initiated by changes in the earth’s orbital features and amplified by consequent changes in carbon dioxide levels. The understandings gained in the study of ice ages of how Earth’s orbital changes affect the atmosphere, oceans and vegetation systems and how all these parts of the complex biosphere interact with each other have enabled climate models which replay the past accurately and give some confidence that they will provide useful results when run forward to ask what happens as conditions change.
A chapter on ice cores has Richard Alley present at a major central Greenland drilling site in the early 1990s, describing the collection of ice cores covering more than 100,000 years of ice formation and explaining how they reveal such an extraordinary amount of information about the climate at specific times. (I covered some of the detail in a review of Alley’s earlier book The Two-Mile Time Machine.) It’s the surprises in the ice core that point to abrupt climate change. There are a couple of dozen big jumps that draw the eye. A sudden warming occurred over Greenland, often 10 degrees or more, in roughly a decade or less. Then gradual cooling was followed by fast cooling, and centuries of really cold conditions before another warming jump. The book explains various factors which make it clear that the sudden climate change was not confined to Greenland. Big, widespread, abrupt climate changes have happened repeatedly before there were a large number of modern humans on the planet.
The Younger Dryas event, a period of cold reversal superimposed on the last deglaciation, from 12,900 years ago to 11,700 years ago, 1200 years in all, is seen as the latest in the series of climate reversals that punctuated the last glacial cycle. Geologist George Denton enters the scene at this stage of the book. An early theory had it that a rapid draining of North America’s largest proglacial lake into the North Atlantic had triggered the Younger Dryas by flooding the North Atlantic with fresh water and curtailing the circulation of the ocean current that brings warm water from the tropics. But Denton and others were unable to find evidence for any channel that could have carried that water. The book explains a new theory, developed from his observations in Scoresby Sound in Eastern Greenland, that it was markedly different summer and winter seasonal temperatures, meaning winter sea ice in the North Atlantic, that resulted in the Younger Dryas.
The Great Ocean Conveyor Belt and the work of Wallace Broecker receive close attention at this point in the book, particularly the current that carries warm water to the northern reaches of the Atlantic before cooling and sinking to flow south. It’s a fascinatingly complex picture, which includes the evidence from North Atlantic deep-sea sediment of periodic armadas of melting icebergs in the North Atlantic likely to have originated in the collapse of a Canadian ice sheet. Such an ice armada may have been the trigger for the Younger Dryas. The jolts of surface fresh water in the North Atlantic are thought to have caused stops in the ocean conveyor with subsequent spontaneous restarts provided by the heat stored up in the deep sea. Other mechanisms are also proposed to account for stoppages, though the book acknowledges that much remains to be learned about the underlying physics of the stops and starts of the ocean conveyor which must have led to the various abrupt climate change events. The chapter ends somewhat reassuringly with Broecker’s revised consideration that a conveyor shutdown as a result of the current ongoing buildup of greenhouse gases is not a likelihood, even if it can’t be eliminated from the list of consequences.
What the book calls wobbles in the North Atlantic conveyor are thought to have been associated with smaller climate changes such as the so-called Little Ice Age, from approximately 1550 to 1850, when the extent of ice and snow may have reached the maximum glacier pulse of the ongoing interglacial. It was preceded by similar but less extensive pulses. The book recognises that these changes may simply be a response to forcings from the sun or from volcanic eruptions, but the authors are interested in the possibility that the strength of the conveyor circulation in the North Atlantic may lie at the root of these relatively minor climate fluctuations. The conveyor controls the distribution of sea ice around Greenland and Iceland and provides a comprehensive mechanism for understanding abrupt climate changes.
The book leaves the intricacies of paleoclimate investigation in its final two chapters and sets out the generally agreed case that global warming is caused by the carbon dioxide from our burning of fossil fuels, with a temperature rise of between 2 and 4.5 degrees expected from doubled levels. The authors are determinedly phlegmatic, one suspects as a matter of temperament rather than a result of the science they investigate. They declare that the best science indicates that global warming will stress out society, especially poor people living in hot places and conclude that the wisest response is to take action. No need for panic, they say, but business-as-usual is not recommended either since it is likely to bring high costs for this generation and especially future generations.
However, in spite of their low-key approach they don’t minimise the risks ahead. Sea level rise will inevitably follow the loss of ice from Greenland, up to 7.3 metres in all, though they don’t believe the disintegration would occur faster than over a few centuries. Although they find no single type of abrupt change likely in the future they acknowledge that Greenland’s evidence of abrupt changes in the past may indicate they have greatly underestimated the coming damage of climate change. Readers of this book will hardly be able to claim that the authors are alarmist. But it would be unwise to overlook the depth of the threat that underlies their calm statements about the future possibilities.