One of the world's leading authorities on climate has identified a previously unsuspected force that may be driving abrupt climate changes on earth--water vapor.
Wallace Broecker, a geochemist at Columbia's Lamont-Doherty Earth Observatory, Tuesday reported new evidence that moisture levels in earth's atmosphere in the tropics dropped substantially during the last ice age, along with average temperatures in the region.
Water vapor is a more powerful heat-trapping greenhouse gas than carbon dioxide, he noted, and reductions in earth's atmosphere could cause a reverse greenhouse effect that cools the planet. The new evidence points to changes in the atmosphere's moisture levels in the tropics as a potentially crucial catalyst for dramatic global climate shifts on earth in the past and in the future--in periods as short as a few decades, Broecker said. He unveiled his provocative new theory at the R. A. Daly Lecture at the American Geophysical Union's spring meeting in Baltimore, Md.
"In the past, most thinking has focused on water vapor changes as secondary; that is, as the earth warms or cools, evaporation rates change and the amount of moisture in the air rises and falls," he said. "We opt to turn this thinking around and make water vapor the driver that changes global temperatures." The vast tropical Pacific Ocean is a powerful heat engine for the earth. The equatorial ocean absorbs huge quantities of solar radiation, which moves through the ocean and into the atmosphere in a complex, dynamic system that scientists are just beginning to understand.
Today, the system produces El Niño and La Niña--cyclical shifts in rainfall, winds, and ocean temperatures and currents in the tropical Pacific, which dramatically affect worldwide weather. Similar shifts of the Pacific ocean-atmosphere system may have caused even more intense climate shifts during the ice age from 70,000 to 10,000 years ago, said Broecker, the Newberry Professor of Geological Sciences at Columbia. The special lecture was titled: "Glacial Cooling: Is Water Vapor the Villain?"
"Two independent pieces of evidence send a clear message that the tropical troposphere was drier during glacial time," he said. The first are reconstructions of past snowlines showing that mountain glaciers between latitudes 45 degrees north and 45 degrees south moved more than 3,000 feet down to lower elevations. To account for this, Broecker calculated that air temperatures at high elevations in the tropics were 6 degrees cooler, and cold air holds less moisture.
A second piece of evidence comes from ice cored from an Andean mountain glacier slightly north of the equator. Isotopic analysis showed that snow falling during the ice age contained 8 percent less oxygen-18 than snow that fell afterward. The reduction of this heavier oxygen isotope indicates that air reaching the glacier during the ice age had less than half its current water vapor content, Broecker calculated.
"The greenhouse impact of such a reduction would cool the tropics by several degrees," he said. "And unlike changes in earth's orbital cycles, atmospheric carbon dioxide levels and ice sheet volumes, a change in tropical water vapor could produce large climate shifts on decadal timescales."
Changes in water vapor could be the missing link driving abrupt climate changes that now appear to have occurred synchronously in the northern and southern hemispheres, he said.
"Records from Greenland ice cored in the early 1990's clearly demonstrate that during glacial time the climate of the northern Atlantic region underwent abrupt jumps from one state of operation to another," Broecker said. "Climate appears to have remained locked in one such state for a millennium or so and then during a period as short as a few decades jumped to another."
In the late 1980s, Broecker outlined a theory of the great ocean conveyor--a global system of currents that transfers heat and affects climate around the planet. But the conveyor is vulnerable to disruption and has turned on and off in the past. If it shut down today, winter temperatures in the North Atlantic and its surrounding lands would fall by 5 degrees or more within 10 years. Dublin would acquire the climate of Spitsbergen, 600 miles north of the Arctic Circle.
In recent years, evidence has rapidly mounted that large, abrupt climate changes in the North Atlantic may be linked to similar changes throughout the globe. The evidence has come from past climate records in glaciers in New Zealand's and Chile's alps, pollen in Florida, ocean sediments off coastal California, corals in Barbados, groundwater in Brazil, varnish on rocks in southwestern U.S. deserts, as well as methane records from tropical wetlands.
"I can only see one element of the climate system capable of generating these fast, global changes, that is, changes in the tropical atmosphere leading to changes in the inventory of the earth's most powerful greenhouse gas, water vapor," Broecker said. "The quest must then be to discover the link between global ocean circulation and convective activity in the tropical atmosphere."
Columbia University Record -- May 24, 1996 -- Vol. 21, No. 28