The cold tongue
The disappearance of glaciers goes hand in hand with warming temperatures. But it turns out that the process may be more complicated than rising CO2 levels in the atmosphere. For insight, we look to the past.
The Pliocene epoch was last era in which temperatures were this warm, about 3 to 5 million years ago. CO2 concentrations were 30 percent higher, sea levels 15 to 20 meters higher, and temperatures more than 5 degrees Fahrenheit hotter. And there were no glaciers, except intermittent ice caps on Greenland. What kept the ice sheets at bay has been explored in a recent paper in the journal Paleoceanography by UC Berkeley geographer M. VizcaÃno and colleagues.
The scientists believe that during the Pliocene, a permanent “El NiÃ±o state” may have been taking place. El NiÃ±o is a familiar climate pattern that occurs every three to seven years and is marked by wacky weather patterns. Some countries become dry, others are deluged with floods and tropical storms, all due to a relaxation in trade winds across Pacific, which cut off upwelling of cool, nutrient rich waters and cause sea surface temperatures to rise.
In a permanent El NiÃ±o, sea temperatures remain constant across the Pacific, and the cold water upwelling, known poetically as the “cold tongue” goes limp. Using climate model simulations, the scientists studied what might happen during a permanent El NiÃ±o.
As a result, warmer air invades North America, Greenland, and part of Eurasia around the Black Sea, raising temperatures by as much as 9 degrees Fahrenheit. Conversely, temperatures become the coolest in much of northeast Eurasia. This overlaps nicely with where glaciers would otherwise exist, northeast Eurasia being glacier-free. The researchers write:
The climate reorganization caused by a permanent El NiÃ±o results in temperature anomalies over the northern high latitudes remarkably coincident with known locations of ice sheet growth.
There has been plenty of speculation about what caused the onset of glaciation at the end of the Pliocene. Among the ideas: dropping concentrations of carbon dioxide, greater seasonality in sea surface temperatures, or a closing Panama Isthmus that caused stronger ocean circulation and led to the higher moisture content needed to fuel ice sheet formation.
But since VizcaÃno and colleagues consider temperature the most important factor in ice sheet formation, they turn again to changes in CO2 and El NiÃ±o patterns as the long term determinants. The “cold tongue” could be more important than we thought.