Page 307 - Earth's Climate Past and Future

CHAPTER 15 • Humans and Preindustrial Climate 283

BOX 15-1 CLIMATE DEBATE

CONTINUED

Black Sea flood Slowly melting ice sheets
"Black Sea" lake caused the Aegean Sea to rise until it flooded
into a freshwater lake that was located in the
region of the modern Black Sea. (Adapted
from W. Ryan and W. Pitman, Noah’s Flood
[New York: Simon & Schuster, 1998].)
Aegean Sea
(low sea level) Sea of Marmara

A Last glaciation

Floodwater
Giant waterfall

Gorge
Rising
sea level
Sea of Marmara

B Deglacial flood (7600 years ago)

Black Sea 7600 years ago would have inundated settle- years ago but that some of the flooding occurred earlier,
ments on the shores of the freshwater lake within a single near 9400 years ago, perhaps because of runoff from the
year, displacing thousands of people. Subsequent investi- meltwater lakes to the north. The ancient flood legends
gations of this region have suggested that some of the may have their origin in the sea level rise at the end of the
flooding of the modern Black Sea region occurred 7600 last deglaciation.

previous dry intervals of nearly comparable intensity had have exhausted the limited supply of nutrient-rich soils
not driven them from the region. Abandonment also in a region of limestone bedrock, making agriculture
coincided with the near disappearance of tree pollen difficult or impossible. Wars with neighboring cultures
from climate records, indicating that the Anasazi had cut may also have been a factor, along with disease. Isolat-
down the juniper and piñon trees previously used as fuel ing climate change as the sole cause of changes in early
for cooking and for winter warmth on the high, cold civilizations is often difficult.
plateau. If so, depletion of this crucial resource may also
have been a major factor in the abandonment. Early Impacts of Humans on Climate
Similarly, the Mayas of Central America may have
altered their own local environment. They may have Eventually, the relationship between humans and their
contributed to regional drought by cutting trees and environment began to change. Instead of being passive
reducing the positive moisture feedback from evapo- players, humans began to actively alter their environ-
transpiration. Alternatively, their farming methods may ment and Earth’s climate.