Page 128 - Earth's Climate Past and Future

104 PART II • Tectonic-Scale Climate Change

FIGURE 6-11 Closing of the
Isthmus of Panama Simulations with
ocean models indicate that gradual
closing of the Central American
isthmus between 10 and 4 Myr ago
More ocean heat redirected warm, salty water
released to northward into the Atlantic Ocean,
atmosphere reduced the extent of high-latitude sea
ice, and handed off additional heat to
the atmosphere. (Adapted from E.
Maier-Reimer et al., “Ocean General
Circulation Model Sensitivity Experiment
with an Open Central American
Stronger
northward flow Isthmus,” Paleoceanography 5 [1990]:
Cut-off of of warm, salty water 349–66.)
warm, salty
water inflow

Cut-off of
cool, low-salinity
water outflow

Climate modelers have used ocean general circu- 6-5 Assessment of Gateway Changes
lation models (O-GCMs) to test this hypothesis by
running pairs of experiments with the Panama region These two case examples invoke very different roles for
configured both as an open gateway passage and as a the ocean in glacial inception. For Antarctica, a reduced
closed-off isthmus. These two configurations roughly poleward flow of warm ocean water was proposed to
correspond to the end points of the tectonic changes have caused a cooling and subsequent glaciation. For
that occurred between about 10 and 4 Myr ago. These the Isthmus of Panama, an increased poleward flow of
simulations confirmed the prediction that warm, salty warm ocean water was invoked as the cause of an
water would have been retained within the Atlantic increase in moisture flux that promoted glaciation.
Ocean and redirected toward northern latitudes (Figure These differing assumptions reflect disagreement
6-11). Closing of the Panama Isthmus also cut off a among climate scientists about how the ocean affects ice
return flow of low-salinity Pacific water into the sheets. Some climate scientists emphasize that a warmer
Atlantic. Blockage of the low-salinity return flow by the ocean will release more latent heat (water vapor) to the
isthmus further increased the salinity of the northward- atmosphere and thereby supply more moisture (snow) to
flowing Atlantic water. aid ice growth. Most climate scientists, however, empha-
In a critical respect, however, the model simulations size the fact that a warmer ocean will release more sensible
contradicted the gateway hypothesis. The stronger heat to the overlying atmosphere and thereby potentially
northward flow of salty water in the Atlantic and the melt more ice (companion Web site, pp. 14–15).
resulting reduction of sea ice caused by closing the In any case, neither set of modeling experiments
Panama Isthmus did not greatly alter precipitation supports the hypothesis that changes in poleward flow
patterns around the high-latitude North Atlantic. As a of warm ocean water tied to gateway changes would
result, the hypothesized increase in moisture needed to have had a large enough effect on climate to initiate the
grow ice sheets did not occur. Instead, the stronger growth of ice sheets. This criticism needs to be tem-
northward flow of water from the tropics and subtropics pered by the realization that O-GCMs are still at a rela-
transferred a large amount of heat to the atmosphere and tively early stage of development and that precipitation
warmed the regions where ice sheets were eventually to patterns are often rather poorly simulated in GCMs. A
form. This warming increased summer melting of snow second criticism of the gateway hypothesis is that each
and opposed the conditions needed for the inception of opening or closing of a gateway occurs over perhaps
glaciation, contrary to the gateway hypothesis. 10 Myr of time, while the cooling has lasted far longer.