Radiative Forcing by Coastal Anthropogenic Emissions Explains Observed 20th Century Southeast Pacific Cooling

The coastal Southeast Pacific cooled by 0.25K from 1979–2006, a period when the nearby Andes warmed by 0.25K. While correlated with atmospheric teleconnections, the magnitude and causes of observed cooling and changes in regional cloud cover have not been explained quantitatively. Here we show that inadvertent cloud modification by anthropogenic emissions from Chile and Peru has increased cloud brightness and lifetime throughout the region's extensive stratocumulus, resulting in net shortwave surface climate forcing and boundary layer cooling consistent with the vertical profile of observed cooling. The extensive stratocumulus observed in the region throughout the satellite era are more widespread and brighter than pre-industrial conditions. Results underscore the need to consider potentially large local and global climatic impacts when setting air quality and greenhouse gas policies in regions with extensive warm boundary layer cloud regimes.