Lull in global warming linked to ocean currents

Nature : Although global warming has appeared to slow since the late 1990s, the slowing is probably only temporary and may be driven by changing currents in the Atlantic and Southern Oceans, according to a new study published in Science. Ocean circulation involves the movement of warm tropical waters toward northern latitudes, where they cool, sink, and then return to the equator. That circulation appeared to slow from the 1970s to the 1990s, which coincided with a period of intense global warming. Then around 1999 ocean circulation sped up, as indicated by the amount of relatively warm water being detected deep in the ocean. The researchers propose that slower ocean currents allow warm water to sit on the surface longer, so more heat and moisture then escape into the atmosphere. The increased heat fuels global warming, whereas the increased evaporation leaves the water saltier and, hence, denser. As the water becomes denser, it sinks faster and speeds up the circulation, and less heat is radiated into the air. Data from the UK suggest that this process has been going on for at least 350 years and cycles approximately every 70 years. In addition, the effect appears more noticeable in the Atlantic Ocean than in the Pacific.