Ocean depth and salinity can counter warming

Ocean depth and salinity can counter warming
September 1, 2000

Gee, predicting the climate 100 years from now might be trickier than we thought.

That would be an honest assessment of 25 years of trying to mathematically model how increasing greenhouse gases will affect our planet by the year 2100. Your local paper may not report it, but that fact becomes obvious if you read between the lines of articles in climatology journals.

Here’s a recent example. The term “climate surprise” has grown popular over the past year—as in, if we keep “polluting” the atmosphere with carbon dioxide (a nonpollutant, by the way), a climate surprise may wreak worldwide atmospheric havoc. (Alien abduction specialists Art Bell and Whitley Streiber promoted this notion in their 2000 book, The Global Superstorm.)

Perhaps the most sensational of the theorized “surprises” is the sudden shutdown of the thermohaline ocean circulation, also known as the conveyor belt.

The thermohaline circulation is one mechanism by which excess heat from the tropics is transported poleward. Warm Gulf Stream waters make their way across the Atlantic, keeping Great Britain unusually mild for a location that far north. But as the Gulf Stream rapidly loses heat to the cool polar air masses so common in that region, the ocean waters cool and become more salty because of evaporation, which makes the surface water denser, causing it to sink. That North Atlantic deep water then circulates across the world’s ocean basins, potentially affecting climate around the globe.

Several general circulation model (GCM) runs have indicated this ocean conveyor belt may weaken or even shut down completely because of greenhouse warming. Some evidence links abrupt changes in the intensity of the formation of North Atlantic deep water to major paleoclimate events. During the last major glaciation (about 11,000 years ago), for instance, a slug of freshwater came down the St. Lawrence River into the North Atlantic, reducing its salinity, shutting off the sinking motion and deep water formation, and plunging northern Europe into a cold and dry millennium.

In a recent issue of the Journal of Climate, Latif and three coauthors from Germany’s Max-Planck Institut of Meteorology used their own climate model to arrive at a much different conclusion. Unlike other modeling efforts on this topic, their model had much better spatial resolution in the tropical oceans, allowing them to better reproduce recurring El Niño/La Niña fluctuations.

When they compared conditions in the North Atlantic for increasing greenhouse gases with the control run (sans increases), they found no changes to the North Atlantic oceans. Apparently, when there is warming in the eastern tropical Pacific (in the “El Niño region”), there is related drying over Brazil, which reduces stream flow from the Amazon and increases evaporation over the tropical Atlantic. The reduced amount of freshwater in the tropical Atlantic Ocean raises the salinity. According to this model, the increased saltiness of the North Atlantic would compensate for the projected warming there, effectively producing no change in the thermohaline circulation from present conditions.

Speaking of complex interactions between the oceans and the atmosphere, two U.K. oceanographers put a new spin on potential sea-level change in the Mediterranean. Tsimplis and Baker examined data from a series of Mediterranean basin tide gauges, looking at trends in sea temperature and salinity. They found sea level has been decreasing at a rate of about 1.3 mm per year since 1960, though before that time it was generally rising.

They suggest part of that decline is related to changes in atmospheric circulation over the North Atlantic, while the rest can be explained by temperature and salinity changes in deep waters in the Mediterranean. Even though measurements at about 2 to 3km depth at various locations in the Mediterranean generally show both temperature and salinity increasing (two effects which tend to counteract each other with respect to sea-level rise), the declines in sea level since 1960 are significant. (The authors dismiss land subsidence as a factor.)

Any analysis of this kind is complicated because, to get a complete picture of ocean conditions, you would need a lot of measurements over time at many different depths (for surface, intermediate, and deep water). But given that sea-level changes respond to variations over the entire depth of the water column, the observed declines led the authors to conclude

the reduction of sea level indicates that global sea level rise is not a threat for most of the Mediterranean, contrary to earlier beliefs. Only in areas where coastal subsidence dominates the sea level changes will sea level remain a problem in the foreseeable future.


References

Latif, M. et al., 2000. Tropical stabilization of the thermohaline circulation in a greenhouse warming simulation. Journal of Climate, 13, 1809-1813.

Tsimplis, M.N. and T.F. Baker, 2000. Sea level drop in the Mediterranean Sea: An indicator of deep water salinity and temperature change? Geophysical Research Letters, 27, 1731-1734.