Category: Oceanography

There’s a report on the BBC website about a new robotic submarine that is currently undergoing final preparations ahead of an attempted dive to the deepest part of the world’s oceans, The Challenger Deep in the Mariana Trench (~11,000 metres down). This depth is deeper than Mount Everest is high (incidentally, did you know that George Everest’s name was actually pronounced Eve-Rest rather than the Ever-Est that we now use to describe the mountain that was named after him?). The robotic submarine has been developed by scientists and engineers at the Woods Hole Oceanographic Institution in the USA and is named Nereus (after the son of Pontus [the sea] and Gaia [the Earth] in Greek mythology). Challenger Deep has previously been visited only twice before, both times by human-operated vehicles, so there is plenty of potential for Nereus to turn up some interesting information. The Challenger Deep is part of a major subduction zone in the western Pacific in which oceanic crust that forms the base of the Pacific Ocean is forced down and underneath the oceanic crust that neighbours the Asian landmass and for this reason it is a major earthquake region. At this kind of depth the pressure experienced due to the weight of water supported is over 1000 times greater than the pressure we experience at sea level (due to the weight of the overlying air in the atmosphere).

It is becoming common knowledge that sea-ice is melting at increasing rates in the Arctic with predictions now suggesting that the region might be ice free in the summer by 2030. The question is, should we really be worried about this, and if so, just how worried should we be? Much of the media attention on the Arctic region is focused on how melting sea ice might alter ocean currents in the north Atlantic, but the real danger lies in what might happen as more and more of the permafrost (permanently frozen soil, water and rock) melts. Locked up in the permafrost are large quantities of carbon (which could be released to the atmosphere as carbon dioxide gas) and, particularly worrying, given its potency as a greenhouse gas, methane. If the permafrost all melts (which apparently could happen within the next 100 years) then the addition of so much carbon dioxide and methane to the atmosphere could lead to an additional increase in global temperatures of 10 degrees Celsius. Put bluntly, that would just about blast humans off the planet. Another key aspect of this issue is that it’s not a tap that can be turned on or off. If temperatures rise enough to melt all of the permafrost then the additional release of greenhouse gases will mean that there’s nothing that can be done to reverse the process. The only hope then is to try to limit the temperature increases that are already in the pipeline to prevent this runaway gas release from occuring. There’s a detailed article on this topic in New Scientist, Issue 2701 [28 March 2009] .

Most people have been aware of the problem of global warming linked to increased levels of Greenhouse Gases in the atmosphere, but there is another problem created by the increased levels of carbon dioxide (CO2) which is only recently coming into the public realm. This is the problem of ocean acidification. The problem is nicely set out in a short film produced by children at a local secondary school (Ridgeway) in conjunction with scientists from the Plymouth Marine Laboratory. There is a piece about the film in the local newspaper from where it is also possible to view the film (or view it directly with this link).

Well done to everyone concerned.

New Scientist, Issue 2699 (14 March 2009) carries a rather confusing news story that reports that recent measurements show that sea level has been rising by 3 millimetres per year since 1993 which is higher than the 2007 forecast of the Intergovernmental Panel on Climate Change (IPCC). It seems that the differences comes from melting from the Greenland and Antarctic ice sheets which was not included in the IPCC forecast because of uncertainty over the models used to predict this. The IPCC forecast was for a sea level rise of 18-59 centimetres by 2100 but apparently if the current trend continues a rise of 1 metre or more by 2100 is likely. However, if you take the 3mm per year figure from the current measurements and extrapolate this out to 2100 you get a rise of about 27 cm (that’s 3 mm/year x 90 years  = 270 mm = 27 cm) which is slap-bang in the lower-middle part of the 2007 IPCC forecast range. So, where’s the story gone…?

A few weeks ago I wrote an entry about generation of electricity from the temperature differences that exist between seawater at the surface and at depth (OTEC) and it is common knowledge that it is feasible to generate power from waves, tidal currents or tidal water level changes. However, it is less well known – by which I mean that I had never ever heard of the idea – that it is possible to generate power from the difference in salt content between freshwater and typical seawater. An article in New Scientist, Issue 2697 (28 February 2009)  introduces this idea, the basis of which is some kind of cell in which freshwater and seawater are separated by a special membrane. There are two ways that this arrangement could then lead to the generation of electricity depending on the set-up and the membrane used. First, the process of osmosis (in which water moves from a weak solution to a strong one across a semi-permeable membrane) can lead to water molecules from the freshwater side crossing the membrane into the seawater and thereby causing an increase in its pressure that can drive the water through a turbine. Alternatively,  a more complex arrangements of membranes can be created that allows the salt ions to move in different directions (e.g. positively-charged sodium ions one way, negatively-charged chloride ions another way) so that the positive and negatively charged ions move towards a cathode and an anode respectively producing a voltage across the cell (basically a big battery). There are plans for a prototype power plant to be up and running soon but it does seem that this technology would only ever be a minor/local player in global power generation (despite figures that suggest it could provide 40% of the world’s electricity demands), especially as any such power plants could only ever be cited in regions where this is ample supply of both seawater and freshwater – namely large estuarine systems that are almost always both environmentally sensitive and quite highly developed already.

New research is suggesting that as sea levels around the world rise the actual rise that occurs in different locations may be very different. Apparently if the West Antarctic ice sheet melts, the meltwater will not spread evenly around the world’s oceans but, instead, will be focussed towards the east coast of North America (with sea levels there being 1-2 metres higher than elsewhere). According to climate models, this effect would occur for a number of reasons. First, as the ice sheet gets smaller it will exert a weaker gravitational pull on the surrounding oceans allowing it to move northwards more easily. Secondly, a lighter ice sheet will allow underlying land to rise displacing water elsewhere. Finally, and perhaps most importantly, the redistribution of this large mass of water would alter the Earth’s spin shifting the locations of existing bulges of water that occur between the equator and the poles. The research is described briefly in New Scientist, Issue 2695 (14 February 2009).

And that’s not all… a completely separate study has calculated that sea levels on the east coast of the USA could rise by around 0.5 metres by 2100 due to a combination of the slow-down of the Atlantic Meridional Overturning Circulation (AMOC) caused by global warming and thermal expansion of ocean waters due to rising temperatures. This research is described briefly in New Scientist, Issue 2700 (21 March 2009) .

All in all, the future doesn’t look bright for low lying parts of the east of the USA.

It has been known for some time that various parts of the world’s climate system can become synchronised, for example the pattern of El Nino events in the Pacific can be linked with the North Atlantic Oscillation (NAO). New research now purports to demonstrate that rather than simply being two climate signals that become coupled, it is the activity in the North Atlantic that is the driving force in the partnership. This finding could have important consequences because the North Atlantic is currently the location of the most rapid climatic changes on the planet so the changes that are occuring there could rapidly lead to other changes around the world. There is a short news item on this research in New Scientist, Issue 2695 (14 February 2009) .

BBC Radio 4 is currently running a four part series “Oceans: what lies beneath” in which the author and broadcaster Gabrielle Walker “looks at how little we understand the waters on which human life depends, and talks to the scientists who are making some extraordinary discoveries deep beneath the ocean waves”. The four parts are titled “Ocean Life”, “The Deep”, “Ocean Resources” and “Oceans and Climate”. Each episode is 30 minutes long and can be listened to online at the BBC Radio 4 Oceans webpage.