Are We Holding a New Ice Age at Bay?

January 18, 2012

In the early 1970s, after two decades of slight cooling, many scientists were convinced that the end of Earth's 11,600 year old warm spell was at hand.  Since then, of course, warmth has returned, probably driven at least partly by man-made carbon dioxide emissions, says the Wall Street Journal.

A new paper 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 interglacial periods 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 -- saw temperatures slide erratically downward by about two degrees Celsius between 127,000 and 120,000 years ago, before a sharper fall began.
  • 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, further cooling 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.
  • An erratic decline in temperature followed, 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.

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.

Source: Matt Ridley, "Are We Holding a New Ice Age at Bay?" Wall Street Journal, January 14, 2012.

For text:

http://online.wsj.com/article/SB10001424052970204257504577150812451167538.html

 

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