From the 15th to the 17th centuries, European sailors rode prevailing winds known as the westerlies to reach lucrative spice markets in Southeast Asia. This powerful atmospheric system, which blows west to east around Earth’s middle latitudes, also brought prosperity to the ancient kingdom of Loulan on China’s Silk Road by way of consistent rain to feed its crops.
In addition to ships and moisture, the westerlies also transport dust, sometimes over astonishing distances. In 2003, scientists traced westerlies-carried particles from China’s Taklamakan Desert to the French Alps. Much of the westerlies’ dust, however, drops into the northern Pacific Ocean, which is why scientists at Columbia University’s Lamont-Doherty Earth Observatory thought that might be the place to look to find concrete evidence to confirm the westerlies are shifting toward the poles as the climate warms.
A number of researchers—using computer modeling and satellite data showing changes to ocean currents—have posited that the westerlies may be shifting. Those investigations, however, relied on modern data sets limited in their retrospective range. “It’s hard to differentiate natural variability from longer-term trends with just several decades’ worth of data,” said Jordan Abell, a doctoral student in Earth and environmental sciences at Lamont.
So Abell, his mentor Gisela Winckler, and their colleagues decided to go back to the Pliocene. “At first, it might sound a little weird, right?” said Winckler, an isotope geochemist at Lamont. “You have to go back 3 million years?” But, she says, that time period mirrors the carbon dioxide levels that exist on Earth now, along with temperature levels similar to those Earth may face in a few decades if it continues to warm—2℃ to 4℃ higher than today’s levels.
Although there are no Pliocene wind records, there are records of millions of years’ worth of dust that have piled up on the ocean floor. Abell, Winckler, and their colleagues analyzed two 8-meter-long sediment cores from two places in the north Pacific at about the 36th and the 45th parallels.
Temperature Extremes Led to Wind Shifts
They also looked at one particular point in time, 2.73 million years ago. The Pliocene was waning, Earth was cooling, and the Pleistocene ice age, with its woolly mammoths and saber-toothed cats, was starting to take hold. During ice ages like the Pleistocene, both the tropics and the poles got colder. The temperature drop at the poles, however, was much greater—around 6℃ to 10℃ compared to 2℃ at the equator, explained Timothy D. Herbert, a coauthor and professor of Earth, environmental, and planetary sciences at Brown University. “So it’s not only a story of the Earth getting colder,” he said, “but that it got much colder at high latitudes than in the tropics.” That temperature difference would have led to differences in air pressure as well as ultimately making the westerlies stronger and shifting them toward the equator.
But how to prove that? Abell and colleagues knew if the westerlies did, indeed, move toward the tropics as Earth got colder, they should find a higher percentage of dust at the more southerly 36th parallel than at the 44th parallel—and they did.
“I think it’s a clever way of using a climate proxy—dust preserved in two marine sediment cores at different latitudinal positions—to try and tease apart how the westerly wind belt shifted during these large-scale climate transitions,” said Sarah Aarons, an isotope geochemist at Scripps Institution of Oceanography who was not involved in the study. “And ultimately, I think it’s important…because scientists will be able to incorporate that information into climate models to more accurately represent what we could expect in the future.”
The team published their results in January in Nature.
New Precipitation Patterns
The upshot of Abell’s paper is that if Earth warms to the level of the Pliocene, the westerlies will no longer blow across the middle latitudes, which could mean much less rain for North America, Europe, and other temperate zones in the north, as well as in parts of Australia and New Zealand in the south. That shift may not happen for centuries, though, and there is still much to learn about how much of a shift will happen.
Abell and his coauthors hope to get more precise data about the degree of shift by studying sediment cores from when Earth was transitioning from the icy Pleistocene to the contemporary Holocene. There are many more sediment cores available for that time period than the earlier Pliocene to Pleistocene, and by comparing a set of cores from north to south, the researchers believe they might be able to say how many degrees the westerlies will shift. “Our work in the Pliocene is really important,” Abell said, “but being able to constrain some of these uncertainties even more is what we hope to do next.”
—Nancy Averett (@nancyaverett), Science Writer