Thu Nov 2nd, 2006 at 07:57:51 AM EST
Remember this diary by Londonbear from last year? It reported a Nature news article that the Atlantic currents were slowing down.
ET readers were quick to point out that there was nothing definitive within this research. In a comment by talos in the same thread, it was flagged that even the RealClimate scientists were cautious. All the more reason to take heed - but the posts on DailyKos claiming that the Gulf Stream is shutting down have gone into the double digits since...
There is just one snag: There is no indication that the Gulf Stream is shutting down.
This made things a lot clearer for me, thanks Nomad -- afew
First things first, and that's the terminology which repeatedly needs attention. When people write "Gulf Stream shutting down!" they don't really mean the Gulf Stream.
There are several mechanisms to get ocean circulation going. First, there is wind stress. Then there are density differences caused by water temperature and salinity changes which both affect water density. And then there are tidal forces.
The Gulf Stream is the descriptor for the heat transport that is generated predominantly by wind. It is an oceanic surface process. The Gulf Stream is consistently mistaken with the whole oceanic Conveyor Belt - which is a far, far larger system. And this mistake is rampant, even to be found in the academic journals. The scathing letter (pdf) from MIT professor Carl Wunsch in Nature is testimony of that.
Next, the Oceanic Conveyor Belt. In popular press, there is no difference between the Gulf Stream and the Oceanic Conveyor Belt, best known in the literature as the Meridional Overturning Circulation (MOC). Yet, the Gulf Stream is just a small limb of the whole conveyor belt. The currents of the deep oceans are believed to be driven mainly by density differences, affected by temperature loss and salinity increase. (But here too Carl Wunsch is not in agreement, which was somewhat revealing to me.)
Bottom-line in child-speak, the Gulf Stream is a wind-driven limb of the Oceanic Conveyor belt, and the thermo-haline circulation describes the deep oceanic currents.
The report by Bryden et al, published late 2005, dealt with the deep Atlantic currents, and therefore not with the Gulf Stream. The (popular) theory goes that a significant decrease in salinity will prevent the water from getting heavy enough to sink down, hence stop the thermo-haline circulation completely and hell will freeze over Europe. (Likely the Gulf Stream will continue to function but may decrease in its capacity.) So when Bryden and his team announced the results of a thermo-haline slowdown, no wonder this was picked up and reported widely. Perhaps unnecessarily to remind that in news clippings (such as in this one by the BBC) that the decrease was often framed as a 30 percent decrease in the past fifty years, whereas the trend of decrease had began in 1992 and was thus far more recent and shorter spanned - which matters in a subject as oceanography where decadal variations are more rule than exception.
Figure 2 from Bryden et al, 2005, showing a decrease in mass flux since 1992.
We're one year further, and I was wondering how this subject was doing. When I dug through the Geophysical Research Letters last week, I found articles indicating that for the deep water currents before the east coast of the USA no decrease was observed, but surprisingly I found the best update in this very recent post on RealClimate. It reads (emphasis added):
At the meeting this week, Bryden and colleagues gave an update of the work, specifically focusing on the first year of data from the moored array. This is the first time that there has ever been such a continuous set of estimates across the whole Atlantic and so reports of the size and nature of the variability were eagerly anticipated. And they did not disappoint! There were two key observations: first, that the approximations that had been used in the Bryden et al study were actually valid, and secondly, that the variations day by day varied by around 5 Sv (1 Sv is about 10 times the flow of the Amazon). The mean over the year for the MOC was 18 Sv - very close to what was expected and in the middle of recent estimates - and significantly, larger than the value seen in the 2004 snapshot. Given that degree of 'noise', this implies that no conclusions about trends over recent decades can be supported.
Other results presented supported this basic picture: transport estimates at different latitudes were not coherent with the initial results, model variability in the best ocean models was large (suggesting that detectability of a MOC slowdown before 2030-2050 was unlikely), and temperature, salinity and velocity changes in the overflow waters beteen Greenland and Europe showed significant connections to the North Atlantic Oscillation but no obvious trends. A number of records that had seemed to be trending strongly when first looked at, now seem to be simply more variable than first thought. This was something of a theme at the conference - the closer we look at the ocean, the more dynamic it appears.
Which means that the decrease observed in Bryden's work of 2005 falls under a normal variation within the measurements. Meaning: the decrease could simply be part of a natural fluctuation - and not be part of a significant slowdown. As the RealClimate honestly reports, the Guardian reporter missed out on that important bit of nuance. And the myth of an actual slowdown perseveres. Not that a permanent thermo-haline shutdown is inconceivable - it has happened in geologic history - but right now, good old alertness is needed, and not panic stories.