The melting of arctic ice injects fresh water into the north Atlantic, possibly disrupting the warm ocean currents that make Europe comfortably habitable. Although this is unlikely, the consequences are so unsettling that this subject has drawn undeserved attention. London is at the same latitude as Calgary, Canada; and Rome is in line with Boston, Massachusetts. Troms0, Norway, is 250 miles north of the Arctic Circle; yet its harbor never freezes over. That is why most polar expeditions start there. Technically known as the Atlantic Meridional Overturning Circulation (or MOC), the Gulf Stream picks up heat from the Atlantic Subtropical Gyre and carries it to the Subpolar Gyre. These gyres, or circulating currents thousands of miles across, are driven by winds above the water. Figure 1.15 shows the system of ocean currents over the whole earth. In the north Atlantic, water warmed in the Caribbean flows along the shore of the USA up to Cape Hatteras, and then breaks off eastward toward Iceland and England.8 When it reaches high latitudes, the seawater cools, becomes denser, and sinks to lower depths. The cooled, salty water then flows back to the south underneath the warm water. Fresh water from ice melting from Greenland, how­ever, is lighter than saltwater and stays on top, opposing the northward flow of the Gulf Stream.

Computer models vary greatly on what will happen. The latest results vary from almost 50% slowing of the MOC to no slowing from anthropogenic causes. The

problem is complicated by two other known effects, the Atlantic Multidecadal Oscillation and the North Atlantic Oscillation, which can, respectively, accelerate or delay the MOC slowing by a few decades. Furthermore, it depends on where the temperature rise is greater. Both the injection of fresh water from the north and greenhouse warming of the North slow down the MOC, while warming in the South will enhance it. The 2007 IPCC report concludes that there is a greater than 90% probability that there will be some decrease of the MOC in the next 100 years, but no simulations predict that the MOC will completely stop. There has been no con­clusive evidence of changes so far.