CO2 emissions not the cause of U.S. West Coast warming?

The rise in temperatures along the U.S. West Coast during the past century is almost entirely the result of natural forces — not human emissions of greenhouse gases, according to a major new study released today in the Proceedings of the National Academy of Sciences.

Northeast Pacific coastal warming since 1900 is often ascribed to anthropogenic greenhouse forcing, whereas multidecadal temperature changes are widely interpreted in the framework of the Pacific Decadal Oscillation (PDO), which responds to regional atmospheric dynamics. This study uses several independent data sources to demonstrate that century-long warming around the northeast Pacific margins, like multidecadal variability, can be primarily attributed to changes in atmospheric circulation. It presents a significant reinterpretation of the region’s recent climate change origins, showing that atmospheric conditions have changed substantially over the last century, that these changes are not likely related to historical anthropogenic and natural radiative forcing, and that dynamical mechanisms of interannual and multidecadal temperature variability can also apply to observed century-long trends.

From a Seattle Times newspaper story: (h/t Dale Hartz)
The vast majority of coastal temperature increases since 1900 are the result of changes in winds over the eastern Pacific Ocean, the authors found. But they could find no evidence that those weather patterns were themselves being influenced by the human burning of fossil fuels.
…
Since the ocean is the biggest driver of temperature changes along the coast, the authors tracked land and sea surface temperatures there going back 113 years. They found that virtually all of the roughly 1 degree Celsius average temperature increase could be explained by changes in air circulation.

“It’s a simple story, but the results are very surprising: We do not see a human hand in the warming of the West Coast,” said co-author Nate Mantua, with NOAA Fisheries Southwest Fisheries Science Center. “That is taking people by surprise, and may generate some blowback.”

Source: http://seattletimes.com/html/localnews/2024601865_climateweatherstudyxml.html
The paper:

Atmospheric controls on northeast Pacific temperature variability and change, 1900–2012

James A. Johnstone and Nathan J. Mantua

Abstract

Over the last century, northeast Pacific coastal sea surface temperatures (SSTs) and land-based surface air temperatures (SATs) display multidecadal variations associated with the Pacific Decadal Oscillation, in addition to a warming trend of ∼0.5–1 °C. Using independent records of sea-level pressure (SLP), SST, and SAT, this study investigates northeast (NE) Pacific coupled atmosphere–ocean variability from 1900 to 2012, with emphasis on the coastal areas around North America. We use a linear stochastic time series model to show that the SST evolution around the NE Pacific coast can be explained by a combination of regional atmospheric forcing and ocean persistence, accounting for 63% of nonseasonal monthly SST variance (r = 0.79) and 73% of variance in annual means (r = 0.86). We show that SLP reductions and related atmospheric forcing led to century-long warming around the NE Pacific margins, with the strongest trends observed from 1910–1920 to 1940. NE Pacific circulation changes are estimated to account for more than 80% of the 1900–2012 linear warming in coastal NE Pacific SST and US Pacific northwest (Washington, Oregon, and northern California) SAT. An ensemble of climate model simulations run under the same historical radiative forcings fails to reproduce the observed regional circulation trends. These results suggest that natural internally generated changes in atmospheric circulation were the primary cause of coastal NE Pacific warming from 1900 to 2012 and demonstrate more generally that regional mechanisms of interannual and multidecadal temperature variability can also extend to century time scales.

http://www.pnas.org/content/early/2014/09/16/1318371111.abstract