Prophets of global warming should restudy the Little Ice Age that abruptly cooled the climate 600 years ago. We may still be in its grip.
Computer simulations that neglect this fact in predicting climate trends are based on a faulty view of the past, according to Paul Mayewski of the Climate Change Research Center at the University of New Hampshire in Durham.
Dr. Mayewski is one of dozens of scientists engaged in several international programs that are deciphering climate records enshrined in ice on Greenland and elsewhere around the Arctic as well as on Antarctica. Their work during the past few years has revolutionized their view of how the climate system functions.
"The simple story that's coming out is that climate has changed more and faster than we ever believed possible," says Richard Alley of the Earth System Science Center of the University of Pennsylvania in University Park.
Gone are easy assumptions of climatic stability. Gone is the notion that climate operated in a series of 100,000-year cycles separated by short warm periods and that the climate cooled slowly.
Summarizing findings that ice-core analysts presented during the recent meeting here of the American Geophysical Union, Dr. Alley noted that significant climate change can happen suddenly within a decade or less.
He explained that "the warm period we're in today is not stable." And he warned: "We modern humans ... have not seen the full range of [natural] climate change," never mind the warming effect of humanly produced greenhouse gases like carbon dioxide.
Ice cores now trace climate back as far as 110,000 years. Among other revelations, they show that, at the height of the last Great Ice Age which ended about 11,600 years ago, central Greenland was 24 degrees C (40 degrees F) cooler than it is today. That's a climate change between then and now of a magnitude that "we did not believe possible until we actually got these records," Alley says.
The Little Ice Age began with significant cooling about AD 1400 to 1420 in what Mayewski calls "the most abrupt onset" of any post-glacial climate event. It has had both colder and milder periods, and its outstanding characteristic is a great deal of variability.
Mayewski explains that the ice-core records show that the natural climate system as measured by atmospheric circulation has been "bouncing up and down ... to its own tune."
"The big punch line," he adds, is that atmospheric circulation patterns may still be dancing to their own tune in spite of the accumulation of humanly produced greenhouse gases. That dance, he says, "is well within the range of Little Ice Age variability."
The ice cores record more than temperatures and ages of climatic events. Dust deposits blown in from distant places reflect hemispheric air circulation patterns. Ratios of different forms of certain elements (called isotopes) provide a unique "fingerprint" that links the dust with its sources. And chemical deposits blown onto the ice sheets from the sea reflect marine biological activity.
One study led by Pierre Biscaye of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., looked at 100 soil samples from 28 locations around the Northern Hemisphere. Using ratios of isotopes of lead, neodymium, and strontium as the "fingerprint," it found that dust frozen into ice at Summit Greenland about 26,000 to 23,000 years ago came from the Gobi and Takla Makan deserts in Mongolia and northern China and from nowhere else. This shows that hemisphere-wide air circulation patterns, not just local winds, were involved.
Ice-core records of atmospheric circulation during the 11,600 years of the present interglacial epoch show significant climate oscillations. The circulation swings between more intense and less intense modes over periods spaced approximately every 2,600 years.
Mayewski explains that these changes seem global, not regional, in nature although their impact will vary from region to region. He cites the collapse of the Mesopotamian Empire about 2200 BC as an example. Ice-core records show the polar-air circulation system was relatively weak then. It wouldn't have pumped as much moisture into the Middle East as it once did, leading to drought.
Mayewski further explains that climate prediction involves more than understanding future temperature trends. It also involves understanding what atmospheric circulation patterns, which affect mid-latitude precipitation, are doing. Mayewski says global-warming predictions should improve when the Little Ice Age variability that characterizes atmospheric circulation today is better understood and included in computer simulations.
Meanwhile, Jeff Severinghaus at the University of Rhode Island Graduate School of Oceanography at Narraganset notes that "one of the really big uncertainties" in global-warming forecasting, is the role of water vapor. When it comes to warming, water vapor is the big gun of the climate system. It is the most abundant and powerful of the greenhouse gases. It plays the main role in keeping Earth warm enough to support life. Significant changes in atmospheric water-vapor content would overwhelm any effect of human pollution.
Dr. Severinghaus and his colleagues can trace water-vapor changes by measuring the methane content of ancient air trapped in ice-core bubbles. Since methane is produced largely in swamps, he explains that "when the world is really wet, you get lots of swamps." This research has shown that the air was fairly dry during the last Ice Age. That means, with less heat-trapping water vapor, the planet would cool down somewhat. Likewise, when the Ice Age ended, both temperature and the air's water-vapor content shot up.
Severinghaus says that, so far, no clear cause-and-effect relationship has shown up. Temperature and water vapor vary together in the ice-core records. One doesn't follow the other. Some external trigger is needed to start the climate change, which then reinforces itself. A warmer planet causes more water vapor to evaporate into the air. The enhanced water-vapor greenhouse effect then helps warm the planet.
Variations in solar intensity may be part of any climate-change trigger. Mayewski says his team finds "a very close correspondence" between about 40 percent of the changes in air circulation and changes in solar intensity as these are reflected in the ice cores.
The bottom line, Mayewski says, is that while scientists are far from understanding the climate system, they now know that "it's not [the] very stable, benign system that we perceived several years ago."