Tuesday, March 30, 2010

Ocean Conveyor running AMOC

This post was chosen as an Editor's Selection for ResearchBlogging.orgResearchBlogging.org

If you've ever seen the disaster movie "The Day After Tomorrow", then you've been introduced to the idea that one day the global ocean conveyor might stop.  Its a pity (or perhaps not) that the movie was such a sensational introduction to the concept, because its a pretty serious possibility.  By way of short explanation: one of the things that makes life possible on this rock is that the ocean redistributes heat that arrives on the earth's surface between the tropics, sending it to the higher latitudes by way of warm surface currents.  There, the waters are cooled and made more dense (both colder and saltier) by the polar ice caps; they then sink and begin a slow meander back to the tropics, eventually returning to the surface to complete the cycle.  Without effective redistribution of this sort, the tropics would bake and the polar zones would sink into a deep hard freeze (in both cases much more so than "normal").  The climate in the UK, for example, would be much more like Siberia were it not of the tempering effects of the Gulf Stream continually bringing heat from the Caribbean to the North Sea.  An important point about the conveyor is that it is driven from both ends: by the suns heat near the equator and by the cooling effect of all that ice at the poles.

Why would the conveyor grind to a halt?  The equatorial heat doesn't show any sign of stopping; if anything its getting hotter.  No, the biggest fear is for the other driver: if the polar ice caps melt too much, there will no longer be a big enough reservoir to chill and brine the surface waters and they will cease to sink.  Some data from recent years suggested that this was happening, and happening fast.  Well, it seems as though Armageddon isn't here just yet.  A new paper by CalTech/NASA's Josh Willis in the journal Geophysical Research Letters uses a more complete data set than ever before to conclude that the conveyor, or more specifically a major section of it called the Atlantic Meridional Overturning Circulation (AMOC - hence the corny title of my post) measured at 41°N (near where it says "Atlantic" on the figure above), is not slowing.  In fact, there is some evidence that it may have sped up marginally in recent years, perhaps in response to warming and expansion of Atlantic waters.  The data were consistent across both satellite sources and sensor arrays deployed in the oceans, so it would seem like a pretty robust study (though I am no physical oceanographer).

I am sure I speak for everyone on the bonnie British Isles when I heave a sigh of relief.

But wait?  What light through yonder ice-shelf breaks?  Tis Greenland, and its seeing more of the sun!  In the very same issue of Geophysical Research Letters, a different group of authors report that ice loss is increasing from the Greenland ice sheet.  This is one of the major impacts of recent climate warming and the greatest contributor to increases in sea level globally.  It would also freshen the north polar waters, further reducing the driving force behind the global ocean conveyor.


My response to this news is to marvel at - and grapple with - the complexity and dynamics of the earth and its climate system.  Scientific results with seemingly opposite implications can come out (in this case in the same journal issue), but without threatening the major underlying pattern; I doubt, for example, that Dr. Willis would disagree with the concept of man-made climate change.  Faced with this seeming contradiction, its perhaps no wonder that many folks grapple with the Big Ideas at the heart of global climate change, and even doubt that it exists at all.  I for one have no doubt  that things are changing, and changing fast.  It may just be that some of the really big features of the climate system (including ocean currents) are slower to respond than others.  Its a bit like turning an oil tanker, which may be an unfortunately apt analogy...

Willis, J. (2010). Can in situ floats and satellite altimeters detect long-term changes in Atlantic Ocean overturning? Geophysical Research Letters, 37 (6) DOI: 10.1029/2010GL042372


Khan, S., Wahr, J., Bevis, M., Velicogna, I., & Kendrick, E. (2010). Spread of ice mass loss into northwest Greenland observed by GRACE and GPS Geophysical Research Letters, 37 (6) DOI: 10.1029/2010GL042460

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