The Great Ocean Conveyor

The Great Ocean Conveyor: Discovering the Trigger for Abrupt Climate Change

Copyright Date: 2010
Edition: STU - Student edition
Pages: 172
  • Cite this Item
  • Book Info
    The Great Ocean Conveyor
    Book Description:

    Wally Broecker is one of the world's leading authorities on abrupt global climate change. More than two decades ago, he discovered the link between ocean circulation and climate change, in particular how shutdowns of the Great Ocean Conveyor--the vast network of currents that circulate water, heat, and nutrients around the globe--triggered past ice ages. Today, he is among the researchers exploring how our planet's climate system can abruptly "flip-flop" from one state to another, and who are weighing the implications for the future. InThe Great Ocean Conveyor, Broecker introduces readers to the science of abrupt climate change while providing a vivid, firsthand account of the field's history and development.

    Could global warming cause the conveyor to shut down again, prompting another flip-flop in climate? What were the repercussions of past climate shifts? How do we know such shifts occurred? Broecker shows how Earth scientists study ancient ice cores and marine sediments to probe Earth's distant past, and how they blend scientific detective work with the latest technological advances to try to predict the future. He traces how the science has evolved over the years, from the blind alleys and wrong turns to the controversies and breathtaking discoveries. Broecker describes the men and women behind the science, and reveals how his own thinking about abrupt climate change has itself flip-flopped as new evidence has emerged.

    Rich with personal stories and insights,The Great Ocean Conveyoropens a tantalizing window onto how Earth science is practiced.

    eISBN: 978-1-4008-3471-6
    Subjects: Geology, Aquatic Sciences, Physics

Table of Contents

  1. Front Matter
    (pp. i-iv)
  2. Table of Contents
    (pp. v-vi)
  3. Preface
    (pp. vii-xviii)
  4. CHAPTER 1 The Setting
    (pp. 1-18)

    It was not until the mid-1980s that scientists became aware that our planet’s climate system was capable of taking abrupt jumps from one state of operation to another. These jumps are the subject of this book. Before introducing them, however, we need to explore their context, namely, the stately progression of glaciations and interglaciations that are paced by cyclic changes in the configuration of the Earth’s orbit (figure 1-1a).

    Although, early on, physicists identified the precessing (that is, the slow gyration) and wobbling of our planet’s spin axis as the likely drivers of the ice ages, geologists dragged their feet....

  5. CHAPTER 2 A Surprise
    (pp. 19-34)

    Willy Dansgaard, a Danish scientist long interested in the isotopic composition of rain and snow, was the first to demonstrate that the ratio of heavy oxygen (18O) to light oxygen (16O) in precipitation varied systematically with air temperature. For example, winter snow falling on Greenland’s polar plateau contained about 3 percent (30 per mil) less heavy oxygen than did rainfall in the tropics. As has already been discussed, this18O deficiency in polar snow was responsible for the enrichment of18O in glacial ocean. Dansgaard further demonstrated that when the average isotopic composition of annual precipitation at high-latitude sites was...

  6. CHAPTER 3 The Villain
    (pp. 35-50)

    As turning off and turning on the Great Ocean Conveyor is the basis for my idea regarding the cause of the abrupt changes seen in Greenland’s18O to16O record, it is important to place this element of the ocean’s circulation system into context. The ventilation of the ocean’s interior can be subdivided into three parts.¹ Thermocline ventilation refers to the upper ocean. Thermohaline ventilation refers to the deep ocean. Separating these two realms are intermediate waters that penetrate each of the three oceans from the south. The Great Ocean Conveyor describes a branch of the thermohaline circulation involving deep...

  7. CHAPTER 4 Puzzles
    (pp. 51-65)

    A serious challenge to the conveyor concept arose when it became clear that the impacts of both the Younger Dryas and the Dansgaard-Oeschger events were felt throughout the Northern Hemisphere and well into the tropics. If, as I had proposed, the impacts were the result of a shutdown of the heat carried to the northern Atlantic by the conveyor’s upper limb, then only the region surrounding the northern Atlantic should have been affected and, in particular, the region downwind of the northern Atlantic (i.e., northern Europe). Early on, the failure to find a Younger Dryas signature in the pollen records...

  8. CHAPTER 5 Hot Clues
    (pp. 66-76)

    In 1988, Hartmut Heinrich, a young German marine geologist, published an article that turned out to hold the key to solving the puzzle posed in the last chapter. He described six discreet layers of debris in the last glacial section of an eastern North Atlantic sediment core (see figure 5-1). The composition of the coarse fraction grains in these layers differed dramatically from that of the ambient sediment. They were dominated by mineral grains rather than by foraminifera shells. Heinrich noted that the bases of four of these layers were razor sharp, suggesting an abrupt onset for their emplacement. An...

  9. CHAPTER 6 The Solution
    (pp. 77-87)

    The solution to the puzzle posed by the widespread impacts caused by disruptions of the Atlantic’s conveyor operation came in two steps. Step one involved observations made by George Denton on Greenland’s glacial moraines and step two involved model simulations carried out by John Chiang, an atmosphere scientist at the University of California, Berkeley.

    In 2001, the late Gary Comer was able to navigate his yacht,Turmoil, through the Northwest Passage without any interference from ice. In so doing, he experienced first-hand the effects of a subject that would hold his attention for the rest of his life, namely, abrupt...

  10. CHAPTER 7 A Confirmation
    (pp. 88-101)

    The mention of rainfall brings to mind monsoons. Although these intense precipitation events occur everywhere in the tropics, we most often associate them with India. I visited there only once. It was during May, when hot and dry weather prevailed. I learned about monsoons as I sat with my friend and host, Devendra Lal, in a circus tent located in a dry river bed. While we waited for the start of a magic show, he explained that in ten or so days’ time the tent would be gone, for the onset of the monsoons would flood the now dry bed...

  11. CHAPTER 8 The Last Hurrah
    (pp. 102-112)

    As already mentioned, for many years I was convinced that the Younger Dryas was a freak event that resulted from a one-time catastrophic flood of water into the northern Atlantic. I even joked by saying that God had placed it at the close of the last glaciation as a warning of what might happen if we added too much CO₂ to the atmosphere. During the last few years, however, new information has convinced me that this is not the case; rather, the Younger Dryas was very likely an integral part of the sequence of events associated with glacial terminations.


  12. CHAPTER 9 Holocene Wobbles
    (pp. 113-125)

    During the eleven thousand years that have elapsed since the end of the Younger Dryas, only two events that might be classified as conveyor shutdowns have interrupted the relative quietude of the present interglacial period. One, the 8.2-kyr event, has received wide attention, in part because it appears to have been triggered by a flood and in part because it shows up in far-flung records (see figure 9-1). The source of the fresh water is thought to have been a proglacial lake that was situated to the south of what is now the Hudson Bay. As did Lake Agassiz, this...

  13. CHAPTER 10 The Anthropocene
    (pp. 126-138)

    Paul Crutzen, a Nobel laureate, has proposed that the geologic time interval known as the Holocene came to an end at the onset of the Industrial Revolution and that we have entered what he terms the Anthropocene. His logic is that at this point man’s influence on the atmosphere and ocean began to compete with nature’s. Of these impacts, that resulting from the buildup of fossil fuel CO₂ is likely to have the greatest impact. The late Roger Revelle referred to the onging CO₂ buildup as “man’s greatest geophysical experiment.”

    From the conveyor concept’s very inception, the question arose as...

  14. Glossary
    (pp. 139-144)
  15. Figure Credits
    (pp. 145-146)
  16. Supplementary Readings
    (pp. 147-148)
  17. Index
    (pp. 149-154)