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A line that runs through Africa, Europe, Alaska and both poles divides Earth into two halves that reflect the same amount of light – and this newly discovered symmetry may play a critical role in the planet’s climate.

It was previously known that the northern and southern hemispheres have almost equal reflectivity, or albedo, but at the National Oceanic and Atmospheric Administration in the US and his colleagues have now uncovered a second line of symmetry along the 27° east and 153° west meridians.

The hemispheres separated by this line are nearly equal in three respects: their albedo in clear skies, the reflectivity of clouds and the fractions covered by ice-free oceans. This symmetry has persisted throughout 25 years of satellite observations analysed by Zhang and his colleagues.

At first, Zhang thought it must be a coincidence. “What convinced me that the east-west symmetry is not trivial are three features: its uniqueness, its persistence and what we call the triple symmetry feature,” he says. “Finding one division with equal total reflection might be expected. But finding a persistent, unique east-west division that also balances land-ocean distribution, clear-sky reflection and cloudy-sky reflection is much less trivial – especially given how variable and dynamic clouds are.”

While the east-west symmetry is centred near 27° east when averaged over the 25-year satellite record, in any individual year, the exact line of symmetry shifts slightly. The team found that these small year-to-year shifts are strongly related to the phase of the El Niño-Southern Oscillation (ENSO), a global climate phenomenon related to fluctuations in sea-surface temperatures in the Pacific Ocean.

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“In other words, the symmetry may not simply be a geometric accident,” says Zhang. “It may be connected to one of the most important modes of climate variability on Earth. ENSO may act as a large-scale adjustment mechanism that helps keep the long-term east-west symmetry centred near 27° east.”

at the Centre for International Climate Research in Oslo, Norway, who wasn’t involved in the study, says he doubted the discovery at first.

“I was a bit sceptical of an east-west symmetry separated at around 27 degrees east, which intuitively seems much less obvious than a separation at the equator, and I wondered if it could be a coincidence.”

However, Hodnebrog says he now agrees it is likely a “robust feature, and potentially another fascinating property of the Earth”.

The connection to ENSO may also be significant, says Hodnebrog. Unlike the north-south symmetry, which appears to be weakening due to the effects of climate change on sea ice and clouds, the east-west symmetry is currently stable, though models suggest it could weaken in future. “A potential future asymmetry could be an indication of changes in the atmospheric circulation,” he says.

at the University of New South Wales, Sydney, says there is a high potential that the east-west symmetry is a coincidence.

“Earth’s weather and climate communicate easily across longitudes,” says Jucker. “This is due to Earth’s rotation, which creates bands of circum-global easterly and westerly winds, and atmospheric perturbations preferentially propagating in the east-west direction as well.”

If there are mechanisms that maintain the east-west symmetry, it could have implications for geoengineering schemes, Zhang says. For example, attempts to increase the albedo of one hemisphere might be negated by a global-scale feedback loop.

“Before making confident claims about the effects of geoengineering, we need a stronger understanding of how clouds, circulation, precipitation and planetary reflectivity respond across the coupled Earth system,” says Zhang.