Geoscience Reference
In-Depth Information
Less
Ice
Sea
Level
(feet
below
modern)
A
s
p
j
p
0
s
n
t
p
h
130
260
More
Ice
390
500 400 300 200 100 0
Age (1000 years before present)
Warmer
B
o
sGn
t
OsntP
Gk
Colder
20000 15000 10000 5000 Now
Years (before present)
f igu r e 16. (A) Sea level changes (in feet below modern sea level) over the past
500,000 years, showing glacial/interglacial cycles. During glaciations, sea level
dropped as seawater was transferred to the poles to build huge ice sheets; during
interglacials, the ice sheets melted to release the water back to the oceans, raising
their level by 390 feet. (B) Temperature over the past 20,000 years measured in
the Greenland ice cores, showing the transition from the Last Glacial Maximum,
20,000 years ago, to the Holocene epoch, which began 11,000 years ago. h e climate
returned to near glacial conditions during a 1,300-year period called the Younger
Dryas. (Drawn by B. Lynn Ingram based on open online sources.)
h e i rst human immigrants made their way to North America from
Asia across a land bridge between Siberia and Alaska that would later be
swallowed up by the rising sea. h ey likely entered at er the planet warmed
sui ciently to melt a narrow corridor between the two enormous ice sheets
over Canada, some 13,500 years ago. Archaeologist Brian Fagan reasons that
a southward migration through this long corridor (approximately 930 miles)
