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Abrupt Climate Change: Inevitable Surprises PLATE 1 Analyses of such ice cores reveal features of the climate when the ice was deposited. In this photo, Geoff Hargreaves, curator, stores a sample of GISP2 deep ice core from central Greenland in the main archive of the National Ice Core Laboratory (a joint effort of the National Science Foundation and the United States Geological Survey with the University of New Hampshire as academic partner, at the Denver Federal Center). PLATE 2 Temperature changes are reflected in changes in the oxygen-isotopic (δ18O) ratios, with low δ18O indicating high temperature. Records of δ18O from ice cores in central Greenland (GISP2) and West Antarctica (Byrd Station) are shown, synchronized to the GISP2 time scale by Blunier and Brook (2001). These records show that both hemispheres experienced an ice age at similar time, that millennial oscillations superimposed on the ice-age cycle were especially large during the slide into and climb out of the ice age, that millennial oscillations were of larger amplitude in the north than in the south, and that at many times (e.g., around 70,000 years ago) antiphase behavior is exhibited between north and south in these millennial oscillations. Note that in this figure, time increases toward the left.
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Abrupt Climate Change: Inevitable Surprises PLATE 3 The top photo is a false-color scanning electron micrograph of assorted pollen grains showing how the size, shape, and surface characteristics differ from one species to another. By counting the number of species and their abundance in sediments and peat deposits (called “palynology”), it is possible to analyze how ecosystems and climate have varied over time. The bottom photo shows pollen from Tilia (Linden or Basswood), which is a temperate deciduous species.
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Abrupt Climate Change: Inevitable Surprises PLATE 4a The great ocean conveyor belt. This is a schematic generally summarizing some important features of the world’s ocean circulation. Warm, low-salinity water, flows north along the surface of the Atlantic, becoming saltier (red arrows). Cooling of this to saltier water in the North Atlantic produces high enough densities for the water to sink and flow southward in the deep ocean and into other ocean basins (blue arrows) (after Broecker, 1995).
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Abrupt Climate Change: Inevitable Surprises PLATE 4b A slightly more complex representation of the global ocean circulation than in Plate 4a, simplified from Ganachaud and Wunsch (2000), as estimated from modern oceanographic data. The figure shows the integrated flow across the latitudes where observations were taken during the World Ocean Circulation Experiment (WOCE) in the 1990s. The red arrows designate near-surface flow (typically warm; technically, water density less than 1027.72 kg m-3), blue and green arrows are deep and bottom flows, respectively. Units are Sverdrups (million cubic meters per second); for comparison, the Gulf Stream transports around 31 Sverdrups northward through the Florida Strait. Notice the vigorous sinking in the North Atlantic and the near-complete absence of sinking in the North Pacific.
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Abrupt Climate Change: Inevitable Surprises PLATE 5 January and July surface air temperature anomalies in degrees Celsius averaged over the years 1970-1980. The “anomaly” is defined as the deviation of air temperature from the average air temperature along a latitude circle passing through the point in question. Removing the mean makes it easier to see East-West variations in temperature without being distracted by the much larger North-South variations.
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Abrupt Climate Change: Inevitable Surprises PLATE 6 Paths along the solution curves of two versions of Stommel’s box model showing the rate of the ocean overturning when the freshwater forcing flux H is increased and then decreased. Only in the case of weak diffusion (orange) does the model respond with an abrupt change, once a threshold in H is crossed. In the case of strong diffusion (green), at any time, there is a unique equilibrium. PLATE 7 Changes in Atlantic circulation in different models forced by prescribed increases in atmospheric CO2. Most models show a reduction of the THC in response to increasing greenhouse gas forcing (Intergovernmental Panel on Climate Change, 2001b).
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Abrupt Climate Change: Inevitable Surprises PLATE 8 During the last ice age, sea levels were approximately 100 m lower than present because more water was contained in ice sheets, and thus not available to the oceans. The dark blue shading around Florida and Southeast Asia demarcates areas that, while 20,000 years ago were dry land, are now under water. Sea levels would rise and flood coastal regions, to the approximate levels shown in the figure (dark green area), if the West Antarctic ice sheet or much of the Greenland ice sheet were to melt. The black line shows the present coast. SOURCE: Burroughs, 1999.
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Abrupt Climate Change: Inevitable Surprises PLATE 9 Composites of fire-scar chronologies for 55 forest stands in Arizona, New Mexico, and northern Sonora, Mexico from the years 1600 to 2000, as reconstructed from tree-ring analyses of fire-scarred trees. The red and yellow vertical tick marks indicate the fire dates recorded by the fire-scarred trees sampled in each stand. The red tick marks represent regional fire years that were recorded by fire scarred trees in 10 or more of the sampled stands. Many of these regional fire years occurred during droughts that were probably associated with La Niña events. Note the very striking decrease in sites recording fires more recently than circa 1890, coinciding with the introduction of large numbers of livestock and organized suppression of fires by government agencies (Swetnam et al., 1999). Major changes in anthropogenic land use affect the frequency of fires associated with droughts.
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