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The 27\u00b0 east meridian, running through Europe and Africa, divides Earth into two equally reflective halves<\/p>\n
PLANETARY VISIONS LTD\/SCIENCE PHOTO LIBRARY<\/p>\n<\/p>\n
A line that runs through Africa, Europe, Alaska and both poles divides Earth into two halves that reflect the same amount of light \u2013 and this newly discovered symmetry may play a critical role in the planet\u2019s climate.<\/p>\n
It was previously known that the northern and southern hemispheres have almost equal reflectivity, or albedo, but Jianhao Zhang<\/a> at the National Oceanic and Atmospheric Administration in the US and his colleagues have now uncovered a second line of symmetry along the 27\u00b0 east and 153\u00b0 west meridians.<\/p>\n 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.<\/p>\n At first, Zhang thought it must be a coincidence. \u201cWhat 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,\u201d he says. \u201cFinding 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 \u2013 especially given how variable and dynamic clouds are.\u201d<\/p>\n While the east-west symmetry is centred near 27\u00b0 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 Ni\u00f1o-Southern Oscillation (ENSO), a global climate phenomenon related to fluctuations in sea-surface temperatures in the Pacific Ocean.<\/p>\n \u201cIn other words, the symmetry may not simply be a geometric accident,\u201d says Zhang. \u201cIt 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\u00b0 east.\u201d<\/p>\n