Earth's most prominent rainfall feature creeping northward

In contrast, the researchers present evidence that the Galapagos Islands, today an arid place on the equator in the Eastern Pacific, had a wet climate during the little ice age.

They write, "The observations of dry climates on Washington Island and Palau and a wet climate in the Galapagos between about 1420-1560/1640 provide strong evidence for an intertropical convergence zone located perennially south of Washington Island (5 degrees north) during that time and perhaps until the end of the eighteenth century."

If the zone at that time experienced seasonal variations of 7 degrees latitude, as it does today, then during some seasons it would have extended southward to at least the equator, Sachs says. This has been inferred previously from studies of the intertropical convergence zone on or near the continents, but the new data from the Pacific Ocean region is clearer because the feature is so easy to identify there.

The remarkable southward shift in the location of the intertropical convergence zone during the little ice age cannot be explained by changes in the distribution of continents and mountain ranges because they were in the same places in the little ice age as they are now. Instead, the co-authors point out that the Earth received less solar radiation during the little ice age, about 0.1 percent less than today, and speculate that may have caused the zone to hover closer to the equator until solar radiation picked back up.

"If the intertropical convergence zone was 550 kilometers, or 5 degrees, south of its present position as recently as 1630, it must have migrated north at an average rate of 1.4 kilometers – just less than a mile – a year," Sachs says. "Were that rate to continue, the intertropical convergence zone will be 126 kilometers – or more than 75 miles – north of its current position by the latter part of this century."

Other co-authors of the paper that went online June 28 are three of Sachs' former postdoctoral students, Dirk Sachse at the University of Potsdam, Germany; Rienk Smittenberg at the Swiss Federal Institute of Technology Zurich, Switzerland; and Zhaohui Zhang at the Nanjing University, China; as well as Stjepko Golubic of Boston University; and David Battisti, UW professor of atmospheric sciences.

The work was funded by the National Science Foundation, National Oceanic and Atmospheric Administration and the Gary Comer Science and Education Foundation.

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To reach Sachs or Battisti, please contact Sandra Hines, 206-543-2580 or . Sachs is in the Marshall Islands and available through July 8. As of July 9, he will be in the field until August and out of touch. Communication by e-mail is spotty.