by Adrian Drago

An international team of scientists led by John Lyman, an oceanographer at NOAA’s Joint Institute for Marine and Atmospheric Research, have recently concluded a study that reveals a steady positive trend of increasing heat content in the upper ocean layers from 1993 to 2008. This is a clear climate change signal.

As the planet warms up due to the trapping of heat energy by higher concentrations of greenhouse gases in the atmosphere, up to 80 to 90 percent of the heat energy ends up in the oceans. The oceans cover 70.8% of the earth’s surface. With an average depth of around 4000m, the ocean’s volume is some 50 times more than that of the earth’s land masses; combined with the large heat capacity of water, this makes the oceans the biggest heat reservoir in the climate system. The researchers claim that the average daily uptake of heat energy by the oceans is estimated to be equivalent to that of five hundred 100-watt light bulbs lit continuously by each of the roughly 6.7 billion people living on the planet. This is equivalent to a warming of the upper 2000m of the oceans by 0.16 oC. A warming ocean is a direct cause of global sea level rise, since seawater expands and takes up more space as it heats up. The scientists say that this expansion accounts for about one-third to one-half of global sea level rise.

Fig. 1: Trends in ocean warming between 1993 and 2008. The black line with error bars shows the annual changes in heat content in the global oceans to 700 m depth, and gives an average warming trend of 0.64 W m?2 (red line). ARGO floats data since 2003 provide an estimate down to 2,000 m depth (blue line). [From the paper ‘Robust Warming of the Global Upper Ocean’ (Lyman John M. et. al,) published in Nature May 2010]

The study relies on measurements collected by autonomous free-floating ocean ARGO floats in combination with earlier devices, called expendable bathythermographs or XBTs, that are small temperature probes dropped from ships to obtain temperature profiles of the upper ocean. The ARGO programme is a global array of currently 3200 free-drifting profiling floats that measures the temperature and salinity of the upper 2000 m of the ocean. The programme is a joint international effort that renders continuous monitoring of the temperature, salinity, and velocity of the upper ocean possible. ARGO floats descend and profile the ocean until they reach a stationary depth of around 2000m, where they continue to drift and sense deep ocean currents for around 10-day period, until they ascend and profile the ocean again; as they surface briefly, data is relayed to a land station via a satellite link, before starting a new cycle of measurements. ???

Fig. 2: Positions of active ARGO floats up to 12th June 2010 [From the ARGO Programme Homepage http://www.argo.ucsd.edu/]

The effects of a warming sea are more evident at the surface. From the recent Malta’s 2nd National Communication to UNFCC, the mean sea surface temperature in the coastal waters of the Maltese Islands is reported to have been steadily increasing at an average rate of close to 0.05 ± 0.015oC per year since the late 70s. The biodiversity, health and functioning of marine ecosystems respond both physically and biologically to such changes. Some evident impacts from rising sea temperatures include the increasing number and abundance of alien species in the Mediterranean, the northward extension of thermophilic species and the increased vulnerability of cold-water species, and the proliferation of jelly-fish swarms that we know from experience have become a more common summer plague even in the Maltese coastal waters.