The Atlantic Multidecadal Oscillation is one of two climate patterns that new research suggests are an illusion, caused not by the internal logic of the climate system, but by human influences and natural variability. Photo by Giorgiogp2/Wikimedia Commons
Jan. 3 (UPI) -- There is only one confirmed climate oscillation, the El Niño/Southern Oscillation, or ENSO, according to a new study.
Analysis by a team of meteorologists suggest another pair of atmospheric patterns, the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation, or AMO and PDO, aren't real.
Scientists previously estimated that the AMO and PDO patterns, like ENSO, were defined by shifts in warming and cooling. But the latest research, published Friday in the journal Nature Communications, suggests these shifts in temperature -- mistaken as climate oscillations -- are best explained by human activities and natural variability.
"Our analysis throws cold water (forgive the pun) on the idea, advanced by some climate change contrarians, that certain aspects of climate change -- for example, the increase in recent decades in North Atlantic hurricane activity -- can be dismissed as a part of a natural internal climate cycle," study author Michael Mann, a climatologist and geophysicist at Pennsylvania State University, told UPI in an email.
Unfortunately, the latest findings mean climate modelers can't rely on the AMO or PDO patterns to help them predict climate shifts across smaller scales.
"When it comes to season and longer term climate forecasting, the only predictable signals may be the El Nino/Southern Oscillation phenomenon and human-caused climate change itself," Mann said.
To determine whether the AMO and PDO patterns were real, scientists used the best climate model simulations developed by researchers all over the world. For some models, the researchers supplied the simulations with external factors like volcanoes and human-induced greenhouse gas emissions. For the rest, the control models, scientists removed the external drivers.
"In the case of the 'control' experiments, there are no changes in external drivers -- no greenhouse gas increases, no sulphate aerosols, no volcanoes, no solar fluctuations," Mann said. "That means that any apparent 'signals' that emerge in those simulations must be of internal origin."
While control models successfully produced periodic climate shifts matching the ENSO pattern, they failed to yield anything resembling the AMO and PDO patterns.
Instead of an internal climate pattern, a predictable oscillation, Mann claims the AMO pattern is best explained by the "changing nature of competing human influences."
In the 1950s, '60s and '70s, a buildup of sulfur aerosol pollutants in the atmosphere, caused primarily by coal power plants, had a cooling effect on Earth's atmosphere. In the 1970s, the Clean Air Act and other regulatory measures helped curb sulfur aerosol pollution. As a result, the influence of greenhouse gas emissions reasserted itself, and global warming began to accelerate.
"In the case of the PDO, I think it was simply a misidentification of an apparent 20-year cyclicity in the Pacific climate system based on one or two apparent 'cycles' that simply turned out to be a chance fluctuation of the broad-band 'noise' of the Pacific climate system," Mann said.