Page 127 - Earth's Climate Past and Future

CHAPTER 6 • From Greenhouse to Icehouse: The Last 50 Million Years 103

After the continents separated, a strong, unimpeded
eastward flow developed around Antarctica. Kennett
proposed that the loss of the warm poleward flow of
heat caused the continent to cool and glaciation to
begin. Recent investigations indicate that Australia sep-
arated from Antarctica around 37 to 33 Myr ago, the
same interval as the first glaciation on Antarctica. The
opening of Drake’s Passage between South America and
Drake’s Antarctica occurred near 25 to 20 Myr ago (see Figure
Passage
6-10). This final establishment of unimpeded circula-
South tion around the Antarctic continent falls roughly mid-
Pole way between the initial onset of glaciation and the large
increase in Antarctic ice near 13 Myr ago.
Climate scientists have used sensitivity tests with
O-GCMs to evaluate this hypothesis. Drake’s Passage
was closed in one experiment and left open in another
while all other features of Earth’s geography were
kept the same. The model results suggested that open-
ing Drake’s Passage did not significantly alter ocean
(or atmospheric) temperatures near Antarctica. Instead,
the model simulated a frigid climate over Antarctica
FIGURE 6-10 Opening of Drake’s Passage Opening of an regardless of the kind of ocean flow. The combined heat
ocean gap between South America and Antarctica near 25 to transport by the ocean and the atmosphere remained
20 Myr ago allowed a strong Antarctic circumpolar current about the same in both experiments. Apparently, the
(arrows) to flow uninterrupted around the Antarctic opening of the circum-Antarctic gateway was not a crit-
continent. The passageway between Australia and Antarctica
had opened 10 Myr earlier. (Adapted from E. J. Barron et al., ical factor in the onset and development of Antarctic
glaciation.
“Paleogeography: 180 Million Years Ago to the Present,” Ecologae
Geologicae Helvatiae 74 [1981]: 443–70.) Case Study 2: Central American Seaway During
the last 10 Myr, uplift in Central America gradually
closed a deep ocean passage that had previously sepa-
rated North and South America in the region of
Panama. The last stages of uplift created the Central
6-4 Gateway Hypothesis
American part of the Cordilleran mountain chain. Final
Some climate scientists have called on the opening or closure of this open passage occurred just before 4 Myr
closing of ocean gateways to explain the onset of both ago with the emergence of the Isthmus of Panama, and
southern and later northern glaciation during the last the first large-scale glaciation of North America fol-
50 Myr. These hypotheses focus on narrow passages lowed at 2.75 Myr ago.
that allow or impede exchanges of ocean water between Several climate scientists have speculated that these
ocean basins. They propose that changes in key gate- two episodes are linked. They hypothesized that con-
ways caused glaciations by altering the poleward trans- struction of the Isthmus of Panama blocked the strong
port of heat or salt. westward flow of warm, salty tropical water that had
Case Study 1: Antarctica During the last 50 Myr, previously been driven westward out of the tropical
the last of the Gondwana continents connected to Atlantic Ocean and into the eastern Pacific by trade
Antarctica split off and moved north, leaving Antarctica winds. The newly formed isthmus should have redi-
isolated and surrounded by a circumpolar ocean (Figure rected this flow into the Gulf Stream and toward the
6-10). In the late 1970s, marine geologist James Kennett high latitudes of the Atlantic. They further hypothe-
proposed that this breakup caused the onset of glacia- sized that this strengthened northward flow of warm,
tion on Antarctica. Before the continents separated, salty water would have suppressed the formation of sea
oceanic flow around Antarctica had been impeded by the ice in north polar regions because saltier waters resist
land connections with South America and Australia. freezing better than fresher water (companion Web site,
Kennett hypothesized that these barriers had diverted pp. 41–42). According to this hypothesis, the reduced
warm ocean currents poleward from lower latitudes cover of sea ice would have made more moisture from
and had delivered enough heat to Antarctica to prevent the ocean available to nearby landmasses and triggered
glaciation. the growth of ice sheets.