Climate models—computer simulations of Earth’s climate system—are crucial tools for scientists, given that it's impossible to run experiments on the entire planet. Access to these digital laboratories also gives people the option to occasionally play “mad scientist” and mess with the Earth a bit. One newly published study falls into that category, asking the question “What would happen if the Earth spun backward?” You can almost hear the maniacal laughter.

Back flip

If you’ve ever learned about the atmosphere, you know that Earth’s rotation makes swirling weather like hurricanes possible through something called the Coriolis Effect. Simply put, fluids heading in a straight line on a spinning globe deflect off to the side—to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. And if the Earth’s rotation reversed, fluids (including ocean currents) would deflect the other way.

It may sound like a trivial bit of pondering, but it’s actually a scientifically interesting question. A group led by Uwe Mikolajewicz of the Max Planck Institute for Meteorology effectively set the planet spinning backward to find out just how many things would change when they let their model run for a few thousand years.

People have long understood the influence of the Coriolis Effect, so some predictions are easy. Prevailing winds in the tropics blow from the east, while they come from the west in the mid-latitudes. Both switch, meaning weather patterns reverse, as well. Most people know that Western Europe is warmed considerably by winds and ocean water coming from the southwest—head straight west across the Atlantic from France and you’ll find Quebec and its considerably cooler climate.

So it’s no surprise that Western Europe gets colder in the backward-Earth simulation. For the same reason, the eastern portions of the Americas and Asia get warmer as warm air and water come their way. But this is not just about temperature; the movement of air masses also controls precipitation patterns and storm tracks. Tropical cyclones, for example, would batter new coastlines.

Precipitation changed in some bigger-than-expected ways, too. The areas of the Americas and Asia that get warmer also get much drier. Northern Africa and the Middle-East, meanwhile, get considerably wetter. That happens, in part, due to a fundamental change in the monsoon rains around the Indian Ocean, forcing them to shift westward.

These weather changes are significant enough to change landscapes. The Sahara Desert turns green, as does the Arabian Peninsula. The title of “World’s Largest Desert” passes to an area covering southern Brazil and Argentina, nestled against a shrunken Amazon rainforest. The American Southeast would similarly turn arid.

In the oceans

The changes in the oceans are just as stark. The world’s oceans circulate in a conveyor-belt-like pattern, with surface water mixing down to the bottom near Greenland and Antarctica, and corresponding surface currents bending counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Bending in the opposite direction brings warm surface water from the equator up along coasts accustomed to cooler currents.

But remarkably, the locations where the conveyor belt drops from the surface to the seafloor change. It shifts a bit near Antarctica, but in the North it dies in the Atlantic and emerges in the Pacific Ocean, instead. Currently, Atlantic currents carry over half the heat energy in the ocean from the equator toward the North Pole. With the Earth spinning the other way, the Pacific’s share rises to almost 80 percent.

One of the biggest causes of year-to-year variability in weather patterns (and even global average temperature) is the El Niño/La Niña seesaw of Pacific Ocean temperatures. That seesaw disappears. Instead, a new temperature oscillation in the Indian Ocean dominates. In short, a lot of the things you used for knowing what weather to expect aren’t applicable to this world.

Apart from being fun to imagine, the surprising parts of this simulation can give researchers new things to think about. The fact that the ocean conveyor belt mixes downward in the North Atlantic rather than the North Pacific, for example, is an interesting puzzle that has been debated. Seeing that pattern flip in this simulation can help test some of the explanations that have been offered. Perhaps we’re fortunate that scientists arguing in a bar can settle things with a model simulation and leave the Earth-reversing death rays to bad action movies.

Earth System Dynamics, 2018. DOI: 10.5194/esd-9-1191-2018 (About DOIs).