The global mean surface temperature has been increasing very slowly, if at all, for the past 10-15 years, even though levels of CO2 emissions continue to rise, sea levels continue to rise, Arctic ice continues to thin, and ocean acidification continues to increase.
Since global mean surface temperatures aren’t rising as expected, where has the heat gone?
We have known for a while that about 90% of the extra heat energy absorbed by the Earth goes into warming the seas, which then contribute in complex ways to increasing the global mean surface temperatures. So the missing heat has got to be somewhere in the oceans.
The slow global circulation of ocean water, known as the Global Conveyor but also as Thermohaline Circulation and more recently as the Meridional Overturning Circulation, plays a huge role in keeping the planet climate relatively stable and in general equilibrium.
What drives this global circulation? Sea water gets denser as it gets colder, reaching its maximum density not at 4 degrees C like freshwater, but at its coldest unfrozen state at -1.8 degrees C. Only in the very highest latitudes, in the North Atlantic and in the Southern Ocean near the West Antarctic Peninsula, does it remain saline enough and get cold and dense enough to sink into abyssal depths in the deep ocean basins 4km or more below the surface.
The sinking of this very cold and saline water drives the global circulation. The deep cold water flows slowly toward lower latitudes, rises or upwells for a variety of reasons and becomes the warmer surface waters, driven into familiar currents by winds and tides.
A great deal of data has now been gathered about the temperatures and salinity levels of ocean waters from all depths around the planet, including from the Southern Ocean that rings the Antarctic. They tell us that the deep ocean water mass around Antarctica, particularly near the West Antarctic Peninsula, is getting warmer and it is freshening. This warming and freshening of the deep cold water has now also been tracked north in all directions to around 30 degrees South latitude.
So that is what has changed. Much of the missing heat, along with West Antarctic glacial melt water, has gone into the deepest, densest, coldest basins of the oceans, rather that into its surface waters.
It will not stay there for long. Further data indicate the mass of the cold dense bottom water is shrinking, and its rate of flow is slowing. These are not reassuring changes.
Still, we have learned that the increasing heat on planet Earth is not only absorbed by surface waters, resulting in a relatively rapid warming of global mean surface temperatures, but that it can also be absorbed first in the high latitudes by very cold water and transported into deeper layers of the ocean. Surface temperatures may then not rise as soon or quickly, but the planet continues to warm anyway, and the long-term impact will be profound.
Ships and satellites are collecting ever more data. If nothing else, the development of this dramatic change in global climate will be incredibly well documented. Perhaps at some point the weight of evidence will be sufficient to push us into effective response.
Meanwhile, last month 140 boats sailed together from San Francisco to the tip of Baja and back. One of the boats first sailed over from Sweden, taking the Northwest Passage to get to the Pacific.