The analysis challenges assumptions that the buildup of greenhouse gases in the atmosphere has been a significant driver of the increase in temperatures observed over many decades in the ocean and along the coastline from Alaska to California.
Changes in ocean circulation as a result of weaker winds were the main cause of about 1 degree Fahrenheit of warming in the northeast Pacific Ocean and nearby coastal land between 1900 and 2012, according to the analysis of ocean and air temperatures over that time. The study, conducted by researchers from the National Oceanic and Atmospheric Administration and the University of Washington, was published Monday by the Proceedings of the National Academy of Sciences.
Natural, wind-driven climate patterns in the Pacific Ocean, such as El Niño and the Pacific Decadal Oscillation, are already known to exert a powerful influence on sea and land temperatures over years and even decades.
This latest research shows that similar changes in atmospheric and ocean circulation can drive trends that last a century or longer, overshadowing the effects of human-generated increase in greenhouse gases, the study's authors said.
"Changing winds appear to explain a very large fraction of the warming from year to year, decade to decade and the long-term," said study leader James Johnstone, an independent climatologist who did most of the work when he was at the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean.
When coastal wind speeds weaken, they result in less evaporation from the sea surface and unusually low pressure that alters ocean currents and causes temperatures to rise over time.
The study found that weakening winds accounted for more than 80% of the warming trend along the Pacific Northwest coast between Washington and Northern California. In Southern California, weaker winds were responsible for about 60% of the increased warming.
If global warming had been the most powerful influence on land and sea temperatures, those temperatures would have been different, the study's authors said. Most of the warming in the region occurred before 1940, when greenhouse gas concentrations were lower and winds were weaker, the study found. In contrast, winds have strengthened since 1980 and coastal ocean cooled, even as the rise in greenhouse gases has accelerated.
The study focused only on trends at the regional level and did not offer conclusions about the influence of naturally occurring winds on warming throughout the world. If anything, the results reinforce what scientists have known for years: that global climate projections fall short in predicting how temperatures are actually changing at the regional scale.
Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved in the study, said its conclusions about long-term trends were probably overstated because the quality of data from the early 20th century was poor and unreliable. The results may also reflect the fact that the northeast Pacific is an area of the globe where past studies have shown the "signal" of climate change is low relative to the "noise" of natural variability.
"There is no doubt that regionally, the changes in temperature are dominated by changes in the atmospheric circulation that likely have little or nothing to do with climate change," Trenberth said. But, he added, "this does not call into question the concept of global warming."
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