Here's my latest Mind and Matter column in the Wall Street
Journal, with added links and charts. On interglacials.
The entire 10,000-year history of civilization has happened in an
unusually warm interlude in the Earth's recent history. Over the
past million years, it has been as warm as this or warmer for less
than 10% of the time, during 11 brief episodes known as
interglacial periods. One theory holds that agriculture and dense
settlement were impossible in the volatile, generally dry and
carbon-dioxide-starved climates of the ice age, when crop plants
would have grown more slowly and unpredictably even in warmer
regions.
This warm spell is already 11,600 years old, and it must surely,
in the normal course of things, come to an end. In the early 1970s,
after two decades of slight cooling, many scientists were convinced
that the moment was at hand. They were "increasingly apprehensive,
for the weather aberrations they are studying may be the harbinger
of another ice age," said Time in 1974. The "almost unanimous" view
of meteorologists was that the cooling trend would "reduce
agricultural productivity for the rest of the century," and "the
resulting famines could be catastrophic," said Newsweek in
1975.
Since then, of course, warmth has returned, probably driven at
least partly by man-made carbon-dioxide emissions. A new paper,
from universities in Cambridge, London and Florida, drew headlines
last week for arguing that these emissions may avert the return of
the ice age. Less noticed was the fact that the authors, by analogy
with a previous warm spell 780,000 years ago that's a "dead ringer"
for our own, expect the next ice age to start "within about 1,500
years." Hardly the day after tomorrow.
Still, it's striking that most interglacials begin with an abrupt
warming, peak sharply, then begin a gradual descent into cooler
conditions before plunging rather more rapidly toward the freezer.
The last interglacial-which occurred 135,000 to 115,000 years ago
(named the Eemian period after a Dutch river near which the fossils
of warmth-loving shell creatures of that age were found)-saw
temperatures slide erratically downward by about two degrees
Celsius between 127,000 and 120,000 years ago, before a sharper
fall began. See charts here and here.
Cyclical changes in the earth's orbit probably weakened sunlight
in the northern hemisphere summer and thus caused this slow
cooling. Since the northern hemisphere is mostly land, this change
in the sun's strength meant gradually increased snow and ice cover,
which in turn reflected light back into space. This would have
further cooled the air and, gradually, the ocean too.
Carbon-dioxide levels did not begin to fall much until about
112,000 years ago, as the cooling sea absorbed more of the gas.
Our current interglacial shows a similar pattern. Greenland ice
cores and other proxy records show that temperatures peaked around
7,000 years ago, when the Arctic Ocean was several degrees warmer
than today, trees grew farther north in Siberia and the Sahara was
wet enough for hippos (Africa generally gets wetter in warm times).
Data from the southern hemisphere reveal that this "Holocene
Optimum" was global in extent. Here's an example:
source: Willis Eschenbach, wattsupwitthat
An erratic decline in temperature followed, with Minoan, Roman
and Medieval warm periods peaking at successively lower
temperatures, culminating in the exceptionally cool centuries of
the "Little Ice Age" between 1550 and 1850, when glaciers advanced
all over the world. In the Greenland ice cores, these centuries
stand out as the longest and most consistent cold spell of the
current interglacial.
source: murdoconline
In other words, our own interglacial period has followed
previous ones in having an abrupt beginning and a sharp peak,
followed by slow cooling. The question is whether recent warming is
a temporary blip before the expected drift into glacial conditions,
or whether humankind's impact on the atmosphere has now reversed
the cooling trend.
