Apparent "pause" in surface global warming trend
Over the last decade or so the rate in the rise of global surface temperatures has slowed causing some skeptics to ask how this could be if temperatures are connected to greenhouse gases, the anthropogenic emissions of which have not slowed. As it turns out, the main reason is that the oceans have taken the heat to great depths below the surface. Less significant factors may involve a slight decrease in solar output and an increase in atmospheric aerosols from volcanoes. The concern now is that the oceans will return to more usual circulation patterns that bring the heat back to the surface potentially causing a rapid rise in surface temperatures.
(By Robert Simmon, NASA. Minor modifications by Robert A. Rohde also released to the public domain (NASA Earth Observatory) [Public domain], via Wikimedia Commons)
Ocean circulation is affected by a combination of factors including surface winds, differences in surface water heating, water salinity, and the coriolis effect. Ocean circulation involves the horizontal and vertical movement of water that include surface currents, upwelling of cold water, and downwelling of warm water. Of significant note in this system as it relates to the "pause" is the El Niño/Southern Oscillation (ENSO) phenomenon that occurs in the Pacific Ocean south of the Equator.
So what is ENSO? In a year where conditions are historically average, the Peru current flows north along the west coast of South America. Originating in the Southern Ocean, it is cold and because of the Coriolis effect, it deflects away from the continent drawing cold water from the great depths of the Peru-Chile trench. This cold water upwelling cools the surface waters and surface air temperatures of the tropical Pacific Ocean. During El Niño years, this pattern breaks down and surface temperatures warm.
(CC Image courtesy of NASA Goddard Photo and Video on Flickr)