Decrease in Atlantic circulation?

 

In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?

The first question that is asked is usually how these calculations are done. Due to the predominantly "geostrophic" nature of the ocean circulation (i.e. velocity is generally horizontally perpendicular to pressure gradients because of the Coriolis effect), you can calculate changes in North-South velocities by only considering the East-West changes in temperature and salinity. So given a section across the ocean (say 25N), oceanographers can estimate the transport across that section. The error in these numbers is a little hard to know, but Bryden et al estimate around +/- 6 Sv (1 Sv is 106 m3/s, (or Sv=Sverdrup= 1 million metric tons per second),  the Amazon output is around 0.1 Sv for perspective).

What did Bryden et al find? Their calculations indicate that the Gulf Stream itself has been remarkably stable over the almost 40 years, and this accords with other measurements of the Gulf Stream flow itself. Since what goes north must eventually go south (after taking into account the very small amounts of atmospheric transport and the amount of flow through the Bering Strait), all of the other changes will balance. They show that the amount of deep return flow seems to have gone down about 8 Sv (out of 25 Sv), and the amount of mid-ocean to surface transport has gone up by about the same amount. This corresponds to a roughly 30% apparent weakening in the so-called "Thermohaline Circulation". Since the surface flow is warmer than the deep flow, there is a consequent decrease in the northward heat flux of about 0.2 PW (or about 15%).

Modelling experiments suggest that this kind of decrease should be associated with a decrease in ocean temperatures in the North Atlantic of up to 2C or so, and maybe 0.5 over Europe. Since these changes have not been observed (both the North Atlantic and Europe have warmed significantly over this time period), it might be premature to assert that the circulation definitely has changed. Alternatively, the models may not entirely be capturing the fairly complex oceanic processes involved. Continuous monitoring of this section has already been funded through the UK RAPID program and should provide much better data in the future, and a potential solution to this and other remaining puzzles.

It appears to me from the Bryden paper, that what they are basically saying is less warm water is going to Europe, and more is recirculating back down into the tropics. The circulation is not really getting stronger or weaker, it's changing where it goes. This seems like it's going to keep tropical waters hotter and thus promote more hurricanes --the subtropical gyre, that delivers more warm water to the tropics.

We must distinguish between the wind-driven gyre circulation and the thermohaline-driven meridional overturning circulation.  The Bryden et al findings actually appear to support this--they find significant changes in the meridional overturning circulation, but little if any change in the Gulf Stream (the western boundary current of the subtropical wind-driven gyre that must return any interior southward flow).  Their findings don't support the speculation that changes in the wind-driven subtropical gyre might explain the observed tropical Atlantic Sea Surface Temperature (SST) warming. 

An incorrect premise that is sometimes made is that the THC is a zero-sum game as far as tropical and extratropical North Atlantic upper ocean heat content (and SST) is concerned. Its not. The THC actually transports a positive net amount of heat northward across the equator from the South Atlantic to the North Atlantic. When the THC increases, the entire North Atlantic ocean can warm. High latitude North Atlantic warming is not coming at the expense of tropical North Atlantic cooling!

The survey line in the Bryden paper goes from Africa to the Bahamas; i.e. it catches everything except the gulf current which goes west of the Bahamas. It's really clear that the amount of shallow (less than 1000m) southerly water transport across the survey line is increasing with each survey. That is pretty much true at every depth between 0 and 1000m. Brady et al summarize it as:

...the mid-ocean southward transport above 1000m depth has increased from 13Sv in 1957 to nearly 23 Sv in 2004.

That's the subtropical gyre. It's very clearly increasing from the data. And the water transport across the full survey line must roughly balance since very little goes through the Bering strait (0.8 Sv), the gulf current through the Florida Strait hasn't changed much (33 Sv+-1.1), evaporation is negligible (0.1 Sv) and there's nowhere else for the water to go. So the decrease in southerly flow of cold water below 1000m must be balanced by something - and the data show it's increased southerly flow of shallow water. (The heat flow need not be balanced across the survey line, but the water flow must be (minus any small changes in flow in the Bering Straits, or between Florida and the Bahamas).

It seems like that shallow south flowing water must be warm (it's the portion of the Gulf stream that's turned around and headed back south), and it's headed right where hurricanes form.

The classical (i.e. Stommel) theory of the subtropical gyre requires that the interior equatorward (Sverdrup) transport that takes place everywhere but a narrow strip along the western edge of the basin, precisely balance the poleward transport that takes place in a narrow boundary current along the western edge of the basin. Thus, the subtropical gyre circulation is a horizontal circulation with poleward mass transport along the western boundary, and equatorward transport everywhere east of that, and providing no net northward mass transport integrated across the basin (which is what Bryden et al have done). Bryden et al find that that western boundary current (what is properly called "the Gulf Stream") has not changed in magnitude over several decades. If the classic Stommel model is correct, that means that the southward interior gyre flow also must be changing, since there would have to be an equal and opposite change measured in the Gulf Stream. This change was found.

The decreasing element is the meridional overturning component of the circulation, the thermohaline circulation, which describes a vertical overturning circulation with northward transport in the upper part of the ocean, and equatorward transport at depth.

The increasing element is the returning part of the shallow, warm subtropical gyre.

The Stommel model described of two circulations with no mass exchange, one wind driven at shallow depth, and one thermohaline at greater depth. However, the model that is clearly inconsistent with Bryden's data. You can clearly see the increase in the southward shallow depth transport (above 1000m) with each later survey. The paper itself says in the abstract:

In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.

COST/BENEFIT OF CLIMATE CHANGE

I think the essential thing to clarify when discussing cost/benefits of climate change is the timeframe. Long term effects of a warmer climate may well indeed be beneficial but I think that there is ample evidence as well as logical reasoning supporting the expectation of negative consequences overwhelming positive ones in the short term. What is short term? Well, for global climate I think short term is measured in one to a few centuries. For the biosphere, short term is millenia.

Unfortunately, by most human measures, these are very long term timeframes. So while, tropical forest inside the arctic circle is not necessarily a negative on its own merits, the ecological turmoil that the planet will go through before we are nice and relatively settled again, at whatever climate, will prove very hard to endure. Cities built long ago will find that they have no viable water supply anymore. Agricultural industries refined and well established in a particular location will find that their fields can no longer grow crops. Populations living on the coastlines will have to move. These are the negatives associated with a changing climate if not necessarily with a new one.

The other point I wanted to make with respect to a cost/benefit analysis of reduction and eventual near elimination of fossil fuel as an energy source is that this is already an unavoidable future. Fossil fuels are non-renewable, we will eventually have to find other sources of energy. So any cost that you wish to attribute to emissions reduction goals must be something that we will not incur in the future anyway. On top one that, I would insist on some kind of credit in the benefit column for the quantity of oil saved from burning and therefore available for the myriad other exceptional uses it has. Burning this resource when alternatives could be developed is a tragic waste already, aside from pollution concerns.

What we can see from stabilisation scenarios and the possibilities of positive feedbacks in the carbon cycle is that we don't have two decades to wait to get detailed information to enable us to make a near-perfect cost-benefit analysis.

Instead we have a situation of risk management. There is a non-negligible risk that we could disrupt the world's climate sufficiently that the world's economy and society would be disrupted extremely seriously. It's a sensible insurance investment to reduce our perturbation to the world's climate.

The point is that no-one knows what will happen. The contrarians can't be certain that it will be easier/cheaper to adapt to changes because we aren't capable of making that sort of prediction

Let me concede the point that sometimes the press gravitates to the scary scenarios. In fact that will keep us on-topic, as the current article will surely be blown out of proportion into a suggestion of glaciers returning to London.

BALANCE IN COMMUNICATION

On the whole, though, the he-said she-said of the media tends to put equal weight on the comments of a committed contrarian and a mainstream scientist. This is clever framing of the debate by the contrarians, leaving the public with the sense of two equally valid positions. Reasonable people often shrug and conclude that "the truth lies somewhere in between" the center and the don't-worry fringe. This amounts to a success of the economic interests behind the contrarian position, as the public is left with a far more equivocal view of the situation than the science actually indicates.

I would prefer that the press, in their pursuit of balance, interview three people, one at the mainstream, one alarmist and one denialist. A genuinely symmetric report would present the scientific mainstream in the middle, and the political fringes as fringes.

A genuinely rational analysis would go beyond symmetry and would risk-weight these positions. There is a possibility that we have grossly overestimated the net future impacts due to anthropogenic perturbation of the climate system, and will therefore erroneously indulge in the sort of policy that denialists invariably call "draconian". There is also a possibility that we have grossly underestimated those impacts, and will therefore undercorrect our behavior and suffer cataclysmic consequences. The latter risk is more important. So given equal scientific weight to their arguments, the argument of the alarmist is rationally of greater policy consequence and should be given more attention, without it being cast as part of the mainstream.

I would have to say that, in North America at least, that is nothing like what emerges from the press.

For instance, we now know that the Greenland ice sheet is melting, and that as it melts the height of the surface of the top of the ice sheet will reduce increasing the temperature there.

The Greenland Ice Sheet has an area of 1,736,095 km2 according to the USGS, the density of an ice sheet is about 900 kg/cubic meter. The mean thickness reported varies from 1500 to 1700 m thick. this yields a volume of 2.5 million cubic kilometers. It must be pointed out that the ice has been thinning more appreciatively in west Greenland of late and that ice sheet melting can only contribute a moderate amount of freshwater volume each year. Freshwater spikes requires extensive sustained calving which the ice sheet is not capable of. A loss of ice from Greenland of 372(37) km3 yr-1 in 1998-2003 was reported by: Hanna, E., Huybrechts, P., Janssens, I., Cappelen, J., Steffen, K., Stephens, A. [2005]

This means that if the climatic conditions remain unaltered, then the melt of the Greenland ice will accelerate and sea level will rise by 5 metres. See http://www.radix.net/~bobg/faqs/sea.level.faq.html This means that not only will most of Florida disappear beneath the waves, so will much of southern England including London. Since the melting is being caused by carbon dioxide, the only way to stop it would be to reduce carbon dioxide levels below those of today. Is that politically or physically possible? Let's face it; it is already time to say "Bye-bye London!"

How have we got into this state? Some blame the press and their "False Objectivity of Balance."  I blame the scientists with their "False Balance of Objectivity."

Article edited heavily by ArizonaEnergy.org

Original webpage by Gavin Schmidt and Michael Mann at http://www.realclimate.org/index.php?p=225