July 9, 2009 -- A prominent band of rain that circles the equator has moved hundreds of miles north over the last few centuries, probably because of a changing climate.
While much of the shift appears to be from natural swings in the strength of the sun, continued warming from greenhouse gas emissions could push the rain band even further north. That might deprive some tropical regions of freshwater and jeopardize the subsistence lifestyles of more than a billion people.
"So much of the world's population lives a lifestyle that's so incredibly dependent on rainfall," said Julian Sachs, a chemical oceanographer at the University of Washington, Seattle. "If this (rain band) is so sensitive to really small changes in the climate system, it's somewhat disconcerting for the billion-plus people that rely heavily on that rain."
Sachs was digging through mud on Washington Island, a tiny atoll in the central tropical Pacific, when he was struck by an "aha moment." In an attempt to document how Earth's tropical climate had changed before satellite technology existed to document it, he and colleagues were looking at layers of sediment beneath a freshwater lake on the island, which lies about 5 degrees north of the equator and more than 1,000 miles south of Hawaii.
The first three feet of sampling brought up the expected: brown, coffee-colored mud. Then, suddenly, the team hit a layer of strawberry jam-like goo that stretched down for 12 or 15 feet. They knew that the jelly got its color and consistency from cyanobacteria, which only live in super-salty water. That meant that, even though the island gets tons of rain today and researchers sat in an inflatable boat on top of a pure freshwater lake, the island was once much drier.
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"It was such a striking change in the type of material were bringing up from the bottom," Sachs told Discovery News. "We knew right then and there that there had to have been a massive change in the climate regime."
"All we had to do was date the time that the transition occurred from mud to red jelly," he added. "That was a really cool moment."
The researchers determined that the island shifted from dry to wet around the year 1600. Then they went to Palau, which is at 7 degrees north of the equator and very wet today, and the Galapagos Islands, which sit on the equator and are very dry.
In both places, the scientists performed a similar set of experiments: They dug up cores of mud beneath pockets of freshwater (which are rare in the Pacific Islands), brought the samples back to the lab, dated them, and looked at chemical signatures for signs of how wet or dry the region was at that time. They dug down as far as 40 feet and as far back as 1,200 years.
Their results, which they reported in Nature Geoscience, suggested that the band of thick clouds and rain that circles the globe hovered right near the equator during the 1600s.
During that time, Sachs said, sunspots were rare, and the sun was putting out about 1/10th of one percent less solar radiation than today.
Over the centuries, as the Sun heated back up ever so slightly, the band (also called the intertropical convergence zone) moved northward as much as 500 kilometers (300 miles) -- turning dry regions into wet ones and wet regions into dry ones.
The rain band moves towards whichever hemisphere is warmer, said Wallace Broeker, an earth scientist at Columbia University's Lamont Doherty Earth Observatory. As global warming continues, he said, the Northern Hemisphere will warm more quickly because it has more land than does the Southern Hemisphere, where large stretches of ocean buffer some of the heat. That will likely push the intertropical convergence zone even further north.
"I think Julian's finding is just one more indication that rainfall on our planet is extremely responsive to changes in Earth's temperature," Broeker said.
What's more, shifts in the tropical rain band coincide precisely with precipitation and climate changes in other areas around the globe, including China and the United States.
"We're starting now to get a feeling for what's going to happen as the planet warms," Broeker said. "From what we're seeing, the changes are going to be dramatic, and they're going to have large consequences."
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