One Hypothesis out there comes from William M. Gray Professor Emeritus Department of Atmospheric Science Colorado State University: “A slowing down of the global ocean’s Meridional Overturning Circulation (global conveyor belt) is the likely cause of most of the global warming that has been observed since the latter part of the 19th century. Shorter multi-decadal changes in the MOC are hypothesized to be responsible for the more recent global warming periods between 1910-1940 and 1975-1998 and the weak multi-decadal cooling periods between 1945-1975 and 2000 to the present. This current weak cooling is projected to go on for the next couple of decades”.
“When the MOC (or THC) is stronger than average there is more upwelling of cold water in the South Pacific and Indian Oceans, and an increase in global rainfall of a few percent occurs. This causes the global surface temperatures to cool. The opposite occurs when the MOC (or THC) is weaker than normal. There is less upwelling of cold water, global rainfall is reduced a few percent and the global surface temperature warms“.