Suppose some cosmic event, say, an asteroid impact, stopped Earth from rotating and then set it spinning the other way. What would the planet’s new climate be like?
Actually, an asteroid impact strong enough to pull that off would do the job a bit too well, says Florian Ziemen of the Max Planck Institute for Meteorology in Hamburg, Germany. “You probably wouldn’t see the Earth anymore.” He used a gentler approach to find the answer.
In a computer model of Earth’s evolving climate, Ziemen and his colleagues reversed a few key physical processes that are the result of Earth’s actual rotational direction, called prograde rotation. It was as simple as that to create a “retrograde rotating Earth,” which they watched develop as the model’s virtual calendar ticked off 7,000 years.
The computer simulation revealed a backward spinning planet with a dramatically different distribution and abundance of desert lands, an otherworldly surge of cyanobacteria, and rerouted ocean currents, among other differences from our real, familiar planet. The findings were presented Monday at the annual General Assembly of the European Geosciences Union (EGU) in Vienna, Austria.
The Expected and the Unexpected
To create their version of a retro-Earth, Ziemen’s team stopped all movement of water and air. They then reversed the sense of the Coriolis force that tugs on all moving matter on a rotating planet, so that any cyclone would henceforth go clockwise on the Northern Hemisphere. They also reversed the daily path of the Sun, with New York 5 hours ahead of London.
The computer model showed a planet with some expected changes. Winds from the east brought a more temperate climate to the Atlantic coast of the United States, for example, but severe winters to western Europe. However, Ziemen was surprised to see the Sahara Desert was gone, and the Middle East had all the precipitation it needed. Instead, the southeastern United States and large parts of Brazil and Argentina had become deserts.
The retrograde rotating Earth also included an unanticipated cyanobacterial surge. In the northern Indian Ocean, the interplay of circulation and high biological production caused low levels of oxygen at greater depths, making it necessary for microorganisms there to consume nitrate instead. The resulting water with low levels of nitrate made it possible for cyanobacteria, which don’t need nitrate, to become the dominant biological producers across a broad expanse of ocean where that nutrient-depleted water reached the surface. Whereas cyanobacterial blooms occur often on the actual Earth, they remain relatively small in scale, Ziemen said. “You can find cyanobacteria, and they may locally be important, but not in a large region.”
These researchers were not the first to model the retrograde Earth; their poster at the EGU conference cites two similar investigations in 2011 and 2008. Robin Smith of the University of Reading in the United Kingdom, who led the 2008 research but was not involved with this latest study, applauds the continued interest in the retrograde approach to climatology. “Our climate and Earth system form an incredibly complex web of processes and feedbacks, and there is only one Earth, so it’s very difficult to run controlled experiments on it to see how it works!” he told Eos. “Experiments like this force us to place our basic understanding of the Earth system on a completely different stage and test that understanding in new ways.”
Replicating the earlier research was one motivation for Ziemen and his colleagues to do the experiment. To go beyond the prior simulations was another. “Our model is run with the full biogeochemistry,” said Ziemen, “so we could study what happens in the ocean and with land vegetation.”
The breadth of affected disciplines was another appeal of the experiment. “The fun thing in this project is that we’re doing it with people of all departments of our institute, so we have this group of 20 people looking into the results,” Ziemen noted.
One process that takes center stage in studies of a backward spinning Earth is the Meridional Overturning Circulation. It consists of cold, deepwater currents starting near the poles and traveling long distances along the bottom of the ocean before surfacing again, with important consequences for the climate.
In Smith’s experiment, deepwater formation in the North Atlantic stopped when Earth’s rotation was reversed. In the 2011 study it didn’t stop, however, raising the possibility that the shape and size of different ocean basins, independent of Earth’s rotation, decided where deep water would form. Now in Ziemen’s simulation, the circulation again collapses in the North Atlantic, and it moves to the northern Pacific Ocean. “That clearly states that it is not the basin shape, it’s the freshwater transport heading into the basins, or at least that’s our understanding of what is happening here,” Ziemen said.
This transformation of oceanic currents between forward and backward spinning Earths resulted from changes to atmospheric water transport, Ziemen explained. “On our Earth, you have freshwater export from the Atlantic into the atmosphere; then the clouds carry the water across Central America and dump it again into the Pacific.” This removal of freshwater makes the surface water of the North Atlantic saltier and thus denser and more likely to sink and initiate the overturning circulation. In the retrograde rotating Earth, it was the Pacific that lost freshwater, he continued. When the direction of rotation was flipped, water evaporated from the Pacific and Indian oceans and ended up in the Atlantic.
A Backward but Kinder Earth?
Looking at the climate of the retrograde Earth, one might conclude that the study falsifies the conclusion of the German philosopher Gottfried Leibniz that for all that may be wrong with Earth, humankind lives in the “best of all possible worlds.” In some respects, the retrograde rotating Earth might seem to be the better place, with, for instance, 11 million square kilometers less desert than what is found on our actual planet.
The simulated retrograde Earth “has more green areas, which is definitely something you can see as a plus,” said Ziemen. “But it depends on where you live. If you’re in western Europe, you’re way better off with [the actual] Earth because on the retrograde Earth it is really cold there.”
—Bas den Hond (email: [email protected]), Freelance Journalist