Page 306 - Earth's Climate Past and Future

282 PART V • Historical and Future Climate Change

BOX 15-1 CLIMATE DEBATE
Sea Level Rise and Flood Legends

any early cultures have a flood legend, a story about today, and the level of the Aegean, linked to the global
Ma great flood that swept away earlier people. Many of ocean as part of the Mediterranean Sea, lay well below the
these legends share several features: a deluge sent by a threshold needed to connect the two bodies of water. A
higher authority as punishment for the sins of the people, freshwater lake rimmed by reedy swamps covered a much
a warning that a flood was coming, and preflood advice to smaller part of the basin than the modern Black Sea.
gather up every kind of animal on a large vessel to preserve Later, as the very last portions of the great northern
all forms of life in a postflood world. The story of Noah in hemisphere ice sheets melted, the meltwater they
the Hebrew Old Testament is the most widely known returned to the ocean pushed the rising Aegean Sea into
flood legend, but much the same story is found in the the Sea of Marmara, which then spilled into the freshwater
older Babylonian tale of Gilgamesh and in legends of lake (modern Black Sea). Radiocarbon dates in sediment
other early cultures in the Old World. Storytellers passed cores from the Black Sea suggested that an abrupt transi-
these legends down over many generations. tion occurred near 7600 calendar years ago. Freshwater
In the eighteenth and nineteenth centuries, scientists mollusks that lived in a lake were replaced by mollusks and
attempted to reconcile the biblical story of the flood with plankton that lived in salty ocean water. Within a short
their scientific knowledge by advocating the diluvial interval, it seemed, incoming seawater transformed this
hypothesis. This hypothesis called on a great worldwide freshwater lake into a salty inland sea.
flood to explain the widespread deposits of unsorted Ryan and Pitman also found a huge gorge lying buried
debris (everything from clay to boulders) strewn across the underneath a thin layer of sediments at the bottom of the
northern continents. Today we recognize these deposits as strait known as the Bosporus, between the Sea of Marmara
moraines left by the retreating ice sheets. Scientists who and the Black Sea. Cut into bedrock, this gorge is clear evi-
now interpret the biblical flood legend less literally have dence that an enormous flow of water poured into the
continued to search for evidence of a major regional-scale Black Sea in the past. The coarse sediments along the floor
flooding event, and many of them focus on the Near East of the gorge (sand, pebbles, cobbles, and even boulders)
because of its early civilizations. are arrayed in great dunelike shapes tilted toward the
In 1998 the geophysicists Bill Ryan and Walter Pitman north, consistent with a flow from the Aegean Sea into the
pulled together evidence from the Black Sea region and Black Sea. At the place where the Bosporus meets the Black
posed a dramatic new explanation called the Black Sea Sea, a deep pool is cut into the underlying rock, apparently
flood hypothesis. During the last glacial maximum, the carved by an immense waterfall created by a torrent of
present connection between the Black Sea and the incoming water.
Aegean Sea through the Sea of Marmara in Turkey did not Ryan and Pitman concluded that the rise of floodwa-
exist. Global sea level stood 110–125 m lower than it does ters caused by the torrent of seawater entering the

tions and from marine sediments in the Persian Gulf tered. Some of the people moved northward to coastal
shows that an abrupt abandonment of major northern regions, perhaps because shallow groundwater was still
cities in that region coincided with a period of intense accessible in such regions. Even these coastal popula-
aridity and increased movement of windblown dust that tions dwindled.
lasted a few hundred years. Climate change is thought to Near 1300, the Anasazi people, who had created and
have been responsible. occupied beautiful cave dwellings cut into the sides of
Another example is the sudden collapse of the cliffs in the southern Colorado Plateau, abruptly aban-
Mayan civilization on the Yucatán Peninsula in Mexico doned the entire region. Evidence from tree ring stud-
in A.D. 860. Evidence from lake sediments indicates ies indicates that their sudden departure occurred dur-
that this dislocation occurred during an interval of ing an interval of drought.
severe drought (Figure 15–11). A civilization that had Depletion of resources is also a plausible compet-
built cities and massive stone temples, carved monu- ing explanation for some of these cultural changes.
mental statues out of stone, and developed a system of For example, although the Anasazi did abandon their cliff
writing suddenly abandoned its inland cities and scat- dwellings in the American Southwest during a drought,