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A Linkage Between the LIA and Gulf Stream?

Filed under: — mike @ 30 November 2006

Michael Mann & Gavin Schmidt

The precise factors underlying the so-called “Little Ice Age” (LIA) have been intensely debated within the scientific community. One key metric in this debate is the spatial pattern of cooling which may provide a ‘fingerprint’ of the underlying climate change, whether that was externally forced (from solar or volcanic activity) or was part of an intrinsic mode of variability.

Surface temperatures in parts of Europe appear to have have averaged nearly 1°C below the 20th century mean during multidecadal intervals of the late 16th and late 17th century (and with even more extreme coolness for individual years), though most reconstructions indicate less than 0.5°C cooling relative to 20th century mean conditions for the Northern Hemisphere as a whole. There is much less data during these time intervals for the Southern Hemisphere, and that severely limits what conclusions can be drawn there. Just what combination of factors could explain this pattern of observations has remained somewhat enigmatic. A new ingredient in this debate comes with a recent paper in Nature by Lund et al.

Their study uses oxygen isotopes records from foraminifera (small zooplankton) contained within a set of long sediment cores taken off the coast of Florida to infer past changes in currents through the Florida Straits, which represents part of the greater “Gulf Stream” current system. This method is based on the observation that the isotope values are proportional to the water density, and as with the geostrophic calculations that are the basis of ocean transport estimates today, changes in isotope gradients across the strait should tell us something about the strength of the currents. They conclude that the Gulf Stream (at least, the portion of it which travels through the Florida Straits) may have been weaker by about 10% during the broad period (AD 1200-1850) commonly associated with the LIA (see figure, and note the time goes backward). The time resolution of the records doesn’t really allow for a much more precise chronology.

Such changes would have little if any impact on global mean temperatures, and only a modest impact on Northern Hemisphere mean temperatures, though they would have a more substantial impact on North Atlantic Sea Surface Temperatures (SSTs) – possibly up to a degree in some locations. This could help to explain the evidence for greater cooling during that period during the LIA in the North Atlantic and surrounding regions This possibility was touched upon in one news article about the study that appeared in The New Scientist.

How does this fit in with the ideas put forward in other related recent studies? For example, Shindell et al (Science, 2001) showed model results that suggested solar forcing could lead to enhanced winter cooling over certain regions of the Northern Hemisphere such as Europe during the Late Maunder Minimum (the coolest part of the European “Little Ice Age”, the late 17th and early 18th centuries). The mechanism involves the large-scale dynamical response of the atmosphere to the estimated decrease in solar irradiance and subsequent stratospheric ozone change. The response was associated with a shift towards the negative phase of the so-called North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) which projects onto a pattern of regional warmth and coldness that tends to cancel out in an average over the entire Northern Hemisphere. There is supporting evidence for this idea based on widespread proxy evidence for a negative phase of the NAO during late 17th and early 18th century intervals.

In that paper, the Northern Hemisphere temperatures cooled only modestly (at most a few tenths of a degree C) in response to the associated radiative forcing (lowered solar irradiance). A similar argument holds for the impacts of explosive volcanism, as shown in subsequent work by Shindell and colleagues (including us). However, the dynamical response associated with the NAO/AO pattern, which sits on top of the radiative response, leads to greater cooling in some regions at the expense of less cooling (or even warming) in other regions. This mechanism could explain the enhanced cooling in Europe for instance, but it cannot easily explain the additional evidence for an overall cooling of North Atlantic SST at this time. Shindell et al (2001) alluded, however, to the possibility that a sustained negative phase of the NAO would lead to a minor weakening of the North Atlantic meridional overturning circulation,

The GISS model results and empirical reconstructions both suggest that solar-forced regional climate changes during the Maunder Minimum appeared predominantly as a shift toward the low AO/NAO index. Although global average temperature changes were small, modeled regional cooling over the continents during winter was up to five times greater. Changes in ocean circulation were not considered in this model. However, given the sensitivity of the North Atlantic to AO/NAO forcing (37–Delworth and Dixon, J. Climate, 2000), oceanic changes may well have been triggered as a response to the atmospheric changes (38–Broecker, PNAS, 2000). Such oceanic changes would themselves further modify the pattern of SST in the North Atlantic (39–Visbeck et al, 1999) and, to a lesser extent, the downstream air temperature anomalies in Europe.

Jones and Mann later expanded on this point in a review paper published in 2004 in the journal Reviews of Geophysics (see final paragraph of section 5.3 therein). The argument basically goes as follows: a sustained forced negative state of the NAO pattern would lead, based on the experiments done by Delworth and Dixon (2000), to a small, but detectable [1-2 Sverdrups (‘Sv’) – or about 5-10%) decrease in the North Atlantic meridional overturning circulation (MOC), which in turn would lead to an additional pattern of cooling over the North Atlantic during the height of the LIA. This seems consistent with the small change in the strength of the “Gulf Stream” inferred by Lund et al study for the LIA.

An alternative view would suggest that rather than the MOC being a consequence of radiative and subsequent atmospheric change, it was in fact the sole factor and that this observed shift was just part of a multi-centennial oscillation. That implies that the southern Atlantic might be expected to warm slightly instead of cool and that the peak cooling would be in the North Atlantic rather than further afield. Of course, it is also conceivable that some combination of both mechanisms are required to explain the observed changes.

We look forward to further research that might further test these intriguing hypotheses.


57 Responses to “A Linkage Between the LIA and Gulf Stream?”

  1. 51
    Marco Parigi says:

    Re:41 “possible savings [due to] less severe winters” Due to a series of ‘less severe’ winters, just north of here the bark beetles have ravaged millions of hectares of forest. Perhaps this year it will be cold enough to kill many of the bark beetles…

    and 42: Chuckle.

    Western Coal’s PR “co2 is life” advocates will probably claim that a benevolent nature, grateful that the imprisoned fossil coal is being freed and converted into more highly nutritional carbon dioxide, is now removing the Northern boreal forests to prepare them as wheatfields to reward humanity’s efforts:

    Thanks Hank, I always like your comments :). However, I would like to bring up how I believe this fits in to the big picture. We must always mourn the loss of habitat or species, especially when it is felt that it was “preventable”. However, economically speaking, change is both a threat and opportunity in equal measure. This shouldn’t be seen as heartless economic thinking, but as the only way to prioritise action on things humans care about. It should be noted similarly in an economy we should mourn and be sad about businesses going bankrupt and people losing their jobs, but it is unrealistic to “save” them all, and counterproductive to prop up failing businesses or prevent redundancies. If people are willing to put in measurable quantities of money to “save” species or habitats, this could be what is used in economic models to decide the cost to humanity of losing those same things. It is wrong to give them “infinite” value because they can never be again what they were.

  2. 52

    Re #51

    This shouldn’t be seen as heartless economic thinking, but as the only way to prioritise action on things humans care about.

    I agree with Marco Parigi that objective measures are needed in order that rational prioritization occur. I do not agree that the “only way” to do this is known. Measuring things by market value has the virtue of simplicity, but it is not guaranteed as far as I know to yield any sort of long term optimum. Just because we want to think quantitatively (I agree here) doesn’t necessarily mean we must think financially.

    I agree that assigning infinite value makes for a mathematically pathological problem. This is why the “precautionary principle” isn’t a practical guide to policy. Assigning market value to things, on the other hand, is mathematically tractable, but that doesn’t make it appropriate.

    Perhaps it is exactly the mismatch between the time scales of the marketplace and of certain issues relating to the common good that makes those issues environmental issues rather than marketplace issues.

    In any case, waving this issue away on the grounds that the only rational way to inform policy is through economics is circular reasoning. If “economics” wishes to claim that territory it must include matters other than the marketplace in its domain of discourse.

  3. 53
    Marco Parigi says:

    In any case, waving this issue away on the grounds that the only rational way to inform policy is through economics is circular reasoning. If “economics” wishes to claim that territory it must include matters other than the marketplace in its domain of discourse.

    I am not attempting to wave away the issue, and I am not elevating economics to be a basis for *all* policy decisions. I am not sure about what you mean about there being “other” possible numerical value systems useful in making priorities objectively. Call it a “human value quotient”, of which we are trying to maximise for humanity – call the study of it the “science of human values”. The semantics will sound all new-age, life-spiritual and green, but the mathematical calculations would look oddly like those of money and economic science no matter what. Sure – things like future discounting has to be considerably addressed for such long time scales, but there are plenty of economic timescales that deal with such difficulties such as long term demography, the time taken for some kinds of crops/trees to grow, etc.

  4. 54
    Hank Roberts says:

    Yep. http://www.msnbc.msn.com/id/15924353/site/newsweek/

    “a study called â��Going to the Extremes,â�� coming out in the December issue of the journal Climatic Change, researchers from the National Center for Atmospheric Research (NCAR) and Texas Tech University…

    “… Just how drastic will the extremes be? The report doesnâ��t give a single answer, but plots out three alternate pictures of the future. The most optimistic of the scenarios assumes rapid introduction of clean, efficient technologies to reduce greenhouse-gas emissions. The least optimistic shows a future where we more or less muddle along as we are todayâ��but with a much larger population consuming resources. The effects vary accordingly. The numbers defy simple translation into lay language, as theyâ��re calculated in what statisticians call â��standard deviationsâ��â��two standard deviations being the amount of variation you would expect under normal conditions. When variability gets up to four or five standard deviations, thatâ��s really big. But consider this: under the worst of the three scenarios, by the year 2099, the length of heat waves will increase by a whopping 12 standard deviationsâ��which is statistician-speak for â��holy cow!â�� â��The lengths are so much longer, itâ��s not even on the same scale as today,â�� says Tebaldi.

  5. 55

    Call it a “human value quotient”…

    I do believe something of the sort is necessary. I am not confident it can be sensibly denominated in dollars. (Also, there is a nontrivial moral question of whether other life forms should have some standing in the calculation.)

    The semantics will sound all new-age, life-spiritual and green, but the mathematical calculations would look oddly like those of money and economic science no matter what.

    Maybe. I’m trying to think about this.

    The biggest concern I have is that economics is growth oriented, and the planet is finite. Also, there is a fundamental difference between the time scales on which marketplace analysis is germane and the time scales on which environmental disruption occur, a fact which I believe is finally being widely recognized. Finally I have a concern which as far as I know is original to me, which is that on long time scales and across alternative scenarios, the value of currency is ill-defined. This leads me to suspect that the long range objective function that we need might be incommensurable with money, and that therefore a great deal of mainstream economic thought is much less relevant to environmental issues than is widely believed.

    The growth issue is a key. Optimizing for wealth is equivalent to optimizing for ever-increasing human activity. In a very real physical and biological sense, however, human impact on the planet must asymptote to a finite value on the longest time scales. It also seems likely that we have reached the point in human history where we must begin to grapple with this limitation. This raises the question whether wealth is a useful objective metric on long time scales. If it isn’t, I would think a great deal of economics needs to be reconsidered before it can be applicable to the sorts of long-range planning that are becoming necessary.

    If you would like to follow up, I suggest we take this conversation to http://groups.google.com/group/globalchange
    where it will be on topic.

  6. 56
    Dano says:

    RE 55 (Tobis):

    This leads me to suspect that the long range objective function that we need might be incommensurable with money, and that therefore a great deal of mainstream economic thought is much less relevant to environmental issues than is widely believed.

    This issue of narrow understanding is why Costanza et al’s valuation is not understood and even ridiculed by the econ-leaning types. Fortunately there are folks who are undaunted by this statement on the human condition.

    Best,

    D

  7. 57
    Marco Parigi says:

    The biggest concern I have is that economics is growth oriented, and the planet is finite.

    Start by changing the semantics, because at heart, economic scientists truly believe that money is just an objective representation of human values. If you say that the science of human values is concerned with the attainment of an overall increase in these values, you realise that it matters little whether the planet is finite, and you can have “value growth” even with a shrinking physical base. Human value scientists know this, and that is why the Stern report correctly states that gradual, but steadily more stringent tax and carbon trade measures can be setup to have virtually no “human value nullification” in the long run while considerably curtailing emmissions and resource usage.

    On your other point re: long time scales, “religious” values seem to do well in intergenerational timelines (societies with solid moral belief systems do better in the really long term). I think this is why the Alarmists’ message is so effective at “converting” people to the cause. ” The end of the world is nigh” is standard religious fare, and although I think fear is a blunt instrument for people to do things that “actually” help, it certainly bumps up the average priority several notches for the issue. It also explains why mildly dissenting climate scientists get attacked by the orthodoxy (oops, I meant those signatory to the consensus) :)

    There is a reasonable number of studies that, for example, compare the fortunes of religious societies vs secular ones, the predictable long term effects of demographic changes (eg. baby boomers). The results of those, however are still rarely used in conjunction with other long term predictions (such as climate change)


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