29/9/09

INFORME CIENTÍFICO INTERNACIONAL: “CAMBIOS CLIMÁTICOS ABRUPTOS E IRREVERSIBLES¨


ECO ACTUALIDAD

 








Un informe científico internacional, publicado este jueves, prevé para el mundo un progresivo riesgo de “cambios climáticos abruptos e irreversibles” causados por un calentamiento global más fuerte de lo previsto.
El documento que se extiende por 36 páginas sintetizó más de 1.400 estudios presentados en la conferencia sobre el clima celebrada en marzo en Copenhague; mismo lugar que en diciembre espera recibir a las Naciones Unidas para negociar en dicha reunión un acuerdo que tome el relevo del (próximo a expirar en e 2012), Protocolo de Kyoto.
A seis meses de la conferencia de Copenhague, este informe habla de las previsiones de los expertos en cuanto por ejemplo a: los acontecimientos climáticos extremos; el nivel del mar; el deshielo en el Ártico; la superficie global y la temperatura de los océanos han quedado escasas, y estos se están incrementando significativamente más rápido de lo previsto un par de años atrás, expusieron.
Se plantea entonces en la exposición, que las emisiones de gases con efecto invernadero y otros indicadores climáticos están en o cerca de los límites previstos por el Panel Intergubernamental de la ONU sobre el Cambio Climático (IPCC – informe del 2007),  y que la actividad humana es directa parte responsable en contribuir al calentamiento global.
Ahora no solo estamos ante nuestras acciones directas contra el medio, sino contra las consecuencias mismas de nuestras acciones pasadas; grandiosas cantidades de entre otros, el fuerte gas de efecto invernadero, metano, atrapadas durante milenios en la capa subterránea de hielo del Ártico estarían a punto de ser liberadas a la atmosfera, acelerando significativamente el proceso de calentamiento global.
Tambien se encontraría en peligro la capacidad de los oceanos y bosques de absorber naturalemente el CO2 originado por la quema de combustibles fósiles, según lo indica la investigación..
El nuevo informe fue escrito y revisado por muchos de los científicos que estuvierone n la tarea de recopilación del documento del IPCC; y su proposito es llamar a los politicos a dar pasos necesarios y urgentes:
“Una moderación rápida, sostenida y efectiva… es necesaria para evitar el peligroso cambio climático, sea como sea que está definido”; “Una subida de las temperaturas de más de 2 grados dificultará la vida de las sociedades futuras, y es probable que causen mayores trastornos medioambientales y sociales durante y a partir del próximo siglo”, se advirtieron.
Pero según el IPCC, las naciones industrializadas deberán reducir drástica y significativamente sus emisiones de gas de efecto invernadero; y los números propuestos y necesarios, están entre un 20 y un 45% en comparación con los niveles de 1990. De no ser así será difícil, tal vez imposible, revertir el proceso.
“Objetivos más débiles para 2020, incrementan el riesgo de impactos serios, incluyendo la superación de la barrera a partir de la cual las fuerzas naturales empiezan a empujar las temperaturas hacia arriba incluso más rápidamente”.
Los impactos del cambio climático podrían ser peores de lo previsto, y llegar aun más pronto que tarde; por su parte expertos en clima del Instituto Tecnológico de Massachusetts (MIT), hacen cálculos acerca de la cantidad de grados que la temperatura de la Tierra incrementará para el 2010 y en comparación con lo que se predijo por el 2003: ellos diagnostican que a pesar de los grandes esfuerzos que se han hecho para reducir drásticamente la contaminación, la temperatura de la superficie de la Tierra se incrementará más del doble previsto hacia el 2010, en un total de 5,2 grados.
El primer ministro danés, Lars Loekke Rasmussen, ha insitido a los científicos a dar “indicaciones concretas” a los políticos, cuando el informe sea presentado este jueves durante la Cumbre de la Unión Europea (UE) en Bruselas.
Artículos relacionados:

28/9/09

ARE WE ON THE BRINK OF A 'NEW LITTLE ICE AGE?


By Terrence Joyce, Senior Scientist, Physical Oceanography and
Lloyd Keigwin, Senior Scientist, Geology & Geophysics


When most of us think about Ice Ages, we imagine a slow transition into a colder climate on long time scales. Indeed, studies of the past million years indicate a repeatable cycle of Earth’s climate going from warm periods (“interglacial”, as we are experiencing now) to glacial conditions.

The period of these shifts are related to changes in the tilt of Earth’s rotational axis (41,000 years), changes in the orientation of Earth’s elliptical orbit around the sun, called the “precession of the equinoxes” (23,000 years), and to changes in the shape (more round or less round) of the elliptical orbit (100,000 years). The theory that orbital shifts caused the waxing and waning of ice ages was first pointed out by James Croll in the 19th Century and developed more fully by Milutin Milankovitch in 1938.

undefinedundefined Ice age conditions generally occur when all of the above conspire to create a minimum of summer sunlight on the arctic regions of the earth, although the Ice Age cycle is global in nature and occurs in phase in both hemispheres. It profoundly affects distribution of ice over lands and ocean, atmospheric temperatures and circulation, and ocean temperatures and circulation at the surface and at great depth.

Since the end of the present interglacial and the slow march to the next Ice Age may be several millennia away, why should we care? In fact, won’t the build-up of carbon dioxide (CO²) and other greenhouse gasses possibly ameliorate future changes?

Indeed, some groups advocate the benefits of global warming, including the Greening Earth Society and the Subtropical Russia Movement. Some in the latter group even advocate active intervention to accelerate the process, seeing this as an opportunity to turn much of cold, austere northern Russia into a subtropical paradise.

Evidence has mounted that global warming began in the last century and that humans may be in part responsible. Both the
Intergovernmental Panel on Climate Change (IPCC) and the US National Academy of Sciences concur. Computer models are being used to predict climate change under different scenarios of greenhouse forcing and the Kyoto Protocol advocates active measures to reduce CO² emissions which contribute to warming.

Thinking is centered around slow changes to our climate and how they will affect humans and the habitability of our planet. Yet this thinking is flawed: It ignores the well-established fact that Earth’s climate has changed rapidly in the past and could change rapidly in the future. The issue centers around the paradox that global warming could instigate a new Little Ice Age in the northern hemisphere.

Evidence for abrupt climate change is readily apparent in ice cores taken from Greenland and Antarctica. One sees clear indications of long-term changes discussed above, with CO² and proxy temperature changes associated with the last ice age and its transition into our present interglacial period of warmth. But, in addition, there is a strong chaotic variation of properties with a quasi-period of around 1500 years. We say chaotic because these millennial shifts look like anything but regular oscillations. Rather, they look like rapid, decade-long transitions between cold and warm climates followed by long interludes in one of the two states.

The best known example of these events is the Younger Dryas cooling of about 12,000 years ago, named for arctic wildflower remains identified in northern European sediments. This event began and ended within a decade and for its 1000 year duration the North Atlantic region was about 5°C colder.

The lack of periodicity and the present failure to isolate a stable forcing mechanism À la Milankovitch, has prompted much scientific debate about the cause of the Younger Dryas and other millennial scale events. Indeed, the Younger Dryas occurred at a time when orbital forcing should have continued to drive climate to the present warm state.

A whole volume that reviews the evidence for abrupt climate change and speculates on its mechanisms was published recently by an expert group commissioned by the National Academy of Sciences in the US. This very readable compilation contains a breadth and depth of discussion that we cannot hope to match here. [ “
Abrupt Climage Change,” National Academy Press, 2002].

Presently, there is only one viable mechanism identified in the report that may play a major role in determining the stable states of our climate and what causes transitions between them: It involves ocean dynamics.

In order to balance the excess heating near the equator and cooling at the poles of the earth, both atmosphere and ocean transport heat from low to high latitudes. Warmer surface water is cooled at high latitudes, releasing heat to the atmosphere, which is then radiated away to space. This heat engine operates to reduce equator-to-pole temperature differences and is a prime moderating mechanism for climate on Earth.

Warmer ocean surface temperatures at low latitudes also release water vapor through an excess of evaporation over precipitation to the atmosphere, and this water vapor is transported poleward in the atmosphere along with a portion of the excess heat. At high latitudes where the atmosphere cools, this water vapor falls out as an excess of precipitation over evaporation. This is part of a second important component of our climate system: the hydrologic cycle. As the ocean waters are cooled in their poleward journey, they become denser. If sufficiently cooled, they can sink to form cold dense flows that spread equatorward at great depths, thus perpetuating the circulation system that transports warm surface flows toward high latitude oceans.

The cycle is completed by oceanic mixing, which slowly converts the cold deep waters to warm surface waters. Thus, surface forcing and internal mixing are two major players in this overturning circulation, called the great ocean conveyor.

The waters moving poleward are relatively salty due to more evaporation at low latitudes, which increases surface salinity. At higher latitudes surface waters become fresher as a consequence of the dominance of precipitation over evaporation at high latitudes.

The freshening tendency makes the surface water more buoyant, thus opposing the cooling tendency. If the freshening is sufficiently large, the surface waters may not be dense enough to sink to great depths in the ocean, thus inhibiting the action of the ocean conveyor and upsetting one important part of the earth’s heating system.

This system of regulation does not operate the same in all oceans. The Asian continent limits the northern extent of the Indian Ocean to the tropics, and deep water does not presently form in the North Pacific, because surface waters are just too fresh. Our present climate promotes cold deep water formation around Antarctica and in the northern North Atlantic Ocean. The conveyor circulation increases the northward transport of warmer waters in the Gulf Stream at mid-latitudes by about 50% over what wind-driven transport alone would do.

Our limited knowledge of ocean climate on long time scales, extracted from the analysis of sediment cores taken around the world ocean, has generally implicated the North Atlantic as the most unstable member of the conveyor: During millennial periods of cold climate, North Atlantic Deep Water (NADW) formation either stopped or was seriously reduced. And this has generally followed periods of large freshwater discharge into the northern N. Atlantic caused by rapid melting of glacial or multi-year ice in the Arctic Basin. It is thought that these fresh waters, which have been transported into the regions of deep water formation, have interrupted the conveyor by overcoming the high latitude cooling effect with excessive freshening.

The ocean conveyor need not stop entirely when the NADW formation is curtailed. It can continue at shallower depths in the N. Atlantic and persist in the Southern Ocean where Antarctic Bottom Water formation continues or is even accelerated. Yet a disruption of the northern limb of the overturning circulation will affect the heat balance of the northern hemisphere and could affect both the oceanic and atmospheric climate. Model calculations indicate the potential for cooling of 3 to 5 degree Celsius in the ocean and atmosphere should a total disruption occur. This is a third to a half the temperature change experienced during major ice ages.

These changes are twice as large as those experienced in the worst winters of the past century in the eastern US, and are likely to persist for decades to centuries after a climate transition occurs. They are of a magnitude comparable to the Little Ice Age, which had profound effects on human settlements in Europe and North America during the 16th through 18th centuries. Their geographic extent is in doubt; it might be limited to regions bounding the N. Atlantic Ocean. High latitude temperature changes in the ocean are much less capable of affecting the global atmosphere than low latitude ones, such as those produced by El Niño.

Whether the pathway for propagation of climate change is atmospheric or oceanic, or whether changes in oceanic and terrestrial sequestration of carbon may globalize effects of climate change, as suspected for glacial/inter-glacial climate changes, are open questions. Yet we begin to approach how the paradox mentioned above can happen: Global warming can induce a colder climate for many of us.

Consider first some observations of oceanic change over the modern instrumental record going back 40 years. During this time interval, we have observed a rise in mean global temperature. Because of its large heat capacity, the ocean has registered small but significant changes in temperature. The largest temperature increases are in the near surface waters, but warming has been measurable to depths as great as 3000 meters in the N. Atlantic. Superimposed on this long-term increase are interannual and decadal changes that often obscure these trends, causing regional variability and cooling in some regions, and warming in others.

In addition, recent evidence shows that the high latitude oceans have freshened while the subtropics and tropics have become saltier. These possible changes in the hydrological cycle have not been limited to the North Atlantic, but have been seen in all major oceans. Yet it is the N. Atlantic where these changes can act to disrupt the overturning circulation and cause a rapid climate transition.

A 3-4 meter, high latitude buildup of fresh water over this time period has decreased water column salinities throughout the subpolar N. Atlantic as deep as 2000m. At the same time, subtropical and northern tropical salinities have increased.

The degree to which the two effects balance out in terms of fresh water is important for climate change. If the net effect is a lowering of salinity, then fresh water must have been added from other sources: river runoff, melting of multi-year arctic ice, or glaciers. A flooding of the northern Atlantic with fresh water from these various sources has the potential to reduce or even disrupt the overturning circulation.

Whether or not the latter will happen is the nexus of the problem, and one that is hard to predict with confidence. At present we do not even have a system in place for monitoring the overturning circulation.

Models of the overturning circulation are very sensitive to how internal mixing is parameterized. Recall that internal mixing of heat and salt is an integral part of overturning circulation. One recent study shows that for a model with constant vertical mixing, which is commonly used in coupled ocean-atmosphere climate runs, there is only one stable climate state: our present one with substantial sinking and dense water formation in the northern N. Atlantic.

With a slightly different formulation, more consistent with some recent measurements of oceanic mixing rates that are small near the surface and become larger over rough bottom topography, a second stable state emerges with little or no deep-water production in the northern N. Atlantic. The existence of a second stable state is crucial to understanding when and if abrupt climate change occurs. When it occurs in model runs and in geological data, it is invariably linked to rapid addition of fresh water at high northern latitudes.

And now perhaps you begin to see the scope of the problem. In addition to incorporating a terrestrial biosphere and polar ice, which both play a large role in the reflectivity of solar radiation, one has to accurately parameterize mixing that occurs on centimeter to tens of centimeter scales in the ocean. And one has to produce long coupled global climate runs of many centuries! This is a daunting task but is necessary before we can confidently rely on models to predict future climate change.

Besides needing believable models that can accurately predict climate change, we also need data that can properly initialize them. Errors in initial data can lead to poor atmospheric predictions in several days. So one sure pathway to better weather predictions is better initial data.

For the ocean, our data coverage is wholly inadequate. We can’t say now what the overturning circulation looks like with any confidence and are faced with the task of predicting what it may be like in 10 years!

Efforts are now underway to remedy this. Global coverage of upper ocean temperature and salinity measurements with autonomous floats is well within our capability within the next decade as are surface measures of wind stress and ocean circulation from satellites.

The measurement of deep flows is more difficult, but knowledge about the locations of critical avenues of dense water flows exists, and efforts are underway to measure them in some key locations with moored arrays.

Our knowledge about past climate change is limited as well. There are only a handful of high-resolution ice core climate records of the past 100,000 years, and even fewer ocean records of comparable resolution. Better definition of past climate states is needed not only in and of itself, but for use by modelers to test their best climate models in reproducing what we know happened in the past before believing model projections about the future. We are not there yet, and progress needs to be made on both better data and improved models before we can begin to answer some critical questions about future climate change.

Researchers always tell you that more research funding is needed, and we are not any different. Our main message is not just that, however. It is that global climate is moving in a direction that makes abrupt climate change more probable, that these dynamics lie beyond the capability of many of the models used in IPCC reports, and the consequences of ignoring this may be large. For those of us living around the edge of the N. Atlantic Ocean, we may be planning for climate scenarios of global warming that are opposite to what might actually occur.


Originally published: February 10, 2003

THE THERMOHALINE OCEAN CIRCULATION

POSTDAM INSTITUTE FOR CLIMATE IMPACT RESEARCH



A Brief Fact Sheet - by Stefan Rahmstorf



 


 
What is the thermohaline circulation (THC)?

As opposed to wind-driven currents and tides (which are due to the gravity of moon and sun), the thermohaline circulation (Fig. 1)  is that part of the ocean circulation which is driven by density differences. Sea water density depends on temperature and salinity, hence the name thermo-haline. The salinity and temperature differences arise from heating/cooling at the sea surface and from the surface freshwater fluxes (evaporation and sea ice formation enhance salinity; precipitation, runoff and ice-melt decrease salinity). Heat sources at the ocean bottom play a minor role.


 



Figure 1. Schematic representation of the global thermohaline circulation.
Surface currents are shown in red, deep waters in light blue and bottom waters in dark blue. The main deep water formation sites are shown in orange. (After [1], modified by S.R.)






In contrast to the wind-driven currents, the THC is not confined to surface waters but can be regarded as a big overturning of the world ocean, from top to bottom. The thermohaline circulation consists of:
  • Deep water formation: the sinking of water masses, closely associated with (but not to be confused with) convection, which is a vertical mixing process, [2]). Deep water formation takes place in a few localised areas: the Greenland-Norwegian Sea, the Labrador Sea, the Mediteranean Sea, the Wedell Sea, the Ross Sea.
  • Spreading of deep waters (e.g., North Atlantic Deep Water, NADW, and Antarctic Bottom Water, AABW), mainly as deep western boundary currents (DWBC).
  • Upwelling of deep waters: this is not as localised and difficult to observe. It is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind (Ekman divergence).
Near-surface currents: these are required to close the flow. In the Atlantic, the surface currents compensating the outflow of NADW range from the Benguela Current off South Africa via Gulf Stream and North Atlantic Current into the Nordic Seas off Scandinavia (Fig. 2). (Note that the Gulf Stream is primarily a wind-driven current, as part of the subtropical gyre circulation. The thermohaline circulation contributes only roughly 20% to the Gulf Stream flow.)



 


Figure 2. Thermohaline circulation of the Atlantic. 
This highly simplified cartoon of Atlantic currents shows warmer surface currents (red) and cold north Atlantic Deep Water (NADW, blue). The thermohaline circulation heats the North Atlantic and Northern Europe. It extends right up to the Greenland and Norwegian Seas, pushing back the winter sea ice margin.
(From [3].)

Some observational data
The volume transport of the overturning circulation at 24 N has been estimated from hydrographic section data ([4]) as 17 Sv (1 Sv = 106 m3/s), its heat transport as 1.2 PW (1 PW = 1015 W). More recently, an inverse model by [5] yielded 15+-2 Sv NADW overturning in the high latitudes. (Note: when comparing these numbers with models care needs to be taken what exactly is compared - in models, the most common measure of NADW overturning is the maximum of the zonally integrated transport stream function in the North Atlantic, sometimes also the outflow value at 30 S.)



What drives the THC?
The short answer would be: high-latitude cooling. In cold regions the highest surface water densities are reached, this causes convective mixing and sinking of deep water, which drives the circulation.
Reality is more complex. Pressure gradients at depth, resulting from density gradients in the overlying waters, are the driving force in the equations of motion. As the density forcing occurs at the surface (see above), a subtle question is why the density differences and the circulation affect the whole ocean depth and are not confined to a near-surface layer. [6] showed that a deep circulation only arises when heating (buoyancy source) is at depth and cooling at the surface. The reason that there is a deep circulation after all is turbulent mixing, which brings down the heat on a time scale of ~1000 years. It has been shown that in the long-term equilibrium the strength of the thermohaline circulation in models depends on the turbulent mixing coefficient [7], and that the energy required for this turbulent mixing comes to a large extent from the moon via tidal currents ([8]).
This discussion can be labelled: is the THC pushed or pulled ([9])? I.e., pushed by formation of cold deep water, or pulled by downward diffusion of heat through the thermocline? The answer is a question of time scale: ultimately, in the long run, it is pulled. But on shorter time scales, up to centuries, it can be considered pushed in the sense that it is density changes in the deep water formation regions which affect the circulation strength. If this density drops too much so that deep water formation is not possible, the circulation stops. Ultimately, on the long time scale of turbulent mixing, the deep ocean density will drop as well until new deep water formation can start.




Non-linear behaviour of the THC
As mentioned above, highest surface densities in the world ocean are reached where water is very cold, while lower densities are found in the saltier but warmer tropical and subtropical areas. In this sense the THC is thermally driven. Nevertheless, the influence of salinity is important and is what causes the non-linearity of the system. This was first described in a classic paper by [10] with the help of a simple box model. Salinity is involved in a positive feedback: higher salinity in the deep water formation area enhances the circulation, and the circulation in turn transports higher salinity waters into the deep water formation regions (which tend to be regions of net precipitation, i.e., freshwater would accumulate and surface salinity would drop if the circulation stopped). Put simply, in Stommel's model the high-latitude salinity increases linearly with the flow, and the flow increases linearly with high-latitude salinity, which combined gives a quadratic (i.e., non-linear) equation. This leads to two possible equilibrium states, the system is bistable in a certain parameter range. This becomes more than an academic point as complex circulation models behave in the same way, and as the present North Atlantic in many models is in the bistable regime ([11]). The first coupled climate model to show these two equilibria  (discovered quite by accident) is the one by [12].
The situation can be described with a simple stability diagram showing strength of the THC as a function of the freshwater input into the North Atlantic. This shows the bistable regime and a saddle-node bifurcation point where the circulation breaks down. It is discussed in more detail (but for the non-specialist) in [13].
An important point is that the salt transport feedback is not the only feedback rendering the system non-linear. The convective mixing process is itself a highly non-linear, self-sustaining process. In models this can lead to multiple stable convection patterns ([14, 15]), which on one hand can cause artefacts related to the coarse model grid. On the other hand this may be part of a real mechanism for shifts in convection location, as have apparently occured during glacial times.
The bottom line is: salinity leads to non-linearity which causes the existence of multiple equilibria and thresholds in the THC.
A related question is: why is no deep water formed in the North Pacific? Salinity there is too low, but why? A body of literature exists on this topic; it is discussed e.g. in [16]. My opinion is: for geographical reasons so much freshwater enters the North Pacific that it is far in the monostable regime where no deep water formation is possible.








The effect on climate

The climatic effect of the THC is still to some extent under discussion, and is due to the heat transport of ~1 PW of this circulation. Back-of-the-envelope calculations suggest that this amount of heat transported into the northern North Atlantic (north of 24 N) should warm this region by ~5K. This is indeed roughly the difference between sea surface temperature (SST) in the North Atlantic as compared to the North Pacific at similar latitudes. A look at sea ice margins suggest that they are pushed back by the warm surface currents in the Atlantic sector as compared to the North Pacific (Fig. 1), this in turn leads to reduced reflection of sunlight and thus warming (albedo feedback). A look at global surface air temperatures is also quite suggestive: over the three main deep water formation regions of the world ocean, air temperatures are warmer by up to ~10K compared to the latitudinal mean.
These observations are, however, no quantitative proof of the climatic effect of the THC, and other explanations can be invoked, such as planetary waves in the atmosphere, locked in place by the geography (Rocky mountains).
One way to estimate the effect of the THC is to switch it off in coupled climate models (by adding a lot of freshwater to the northern Atlantic), and compare the surface climate before and after switching it off. Roughly, this leads to a cooling with a maximum of ~10K over the Nordic Seas (e.g., [12, 17]). The maximum tends to occur near the sea ice margin due to the ice albedo effect. Unfortunately, the details of this cooling are model-dependent: one model shows cooling up to 22K  in annual mean and 33K in winter ([18]). Models also differ in how widespread the cooling is: most tend to affect temperatures over land in  northwestern Europe (Scandinavia, Britain) by several degrees, others show strong cooling further west affecting Canada ([19]).







History of the THC
Sediment data document that the THC has undergone major changes in the history of climate (e.g., [21, 22]). Three major circulation modes were indentified: a warm mode similar to the present-day Atlantic, a cold mode with NADW forming south of Iceland in the Irminger Sea, and a switched-off mode ([23]). The latter appears to have occurred after major input of freshwater, either from surging glacial ice sheets (Heinrich events) or in form of meltwater floods (e.g., Younger Dryas event). The most dramatic climate events recorded in Greenland, the Dansgaard-Oeschger (D/O) events, were probably associated with north-south shifts in convection location, i.e. transitions between warm and cold modes of the Atlantic THC. Recent simulations of such shifts show encouraging agreement with paleoclimatic data ([24]).









The THC in anthropogenic global warming


Global warming can affect the THC in two ways: surface warming and surface freshening, both reducing the density of high-latitude surface waters and thus inhibiting deep water formation. [25] was the first to warn that this could lead to a breakdown of the THC and to abrupt climate change. Subsequently, [26, 27] showed that this could indeed occur for strong global warming (i.e., for a quadrupling, but not for a doubling of CO2). In these scenarios there was no surface cooling, as the high CO2 levels more than compensated for the reduced ocean heat transport. The possibility of a real cooling (both a relative cooling, i.e. a drop back to roughly pre-industrial temperatures after an initial warming phase, and in the longer run an absolute cooling below preindustrial values) as a result of anthropogenic warming was first demonstrated in a sensitivity study by [20]. Significant absolute cooling can arise after CO2 levels decline, but the THC remains switched off after its collapse is triggered in a rapid warming phase.
A THC collapse is now widely discussed as one of a number of "low probability - high impact" risks associated with global warming. More likely than a breakdown of the THC, which only occurs in very pessimistic scenarios, is a weakening of the THC by 20-50%, as simulated by many coupled climate models ([28]).
Key open questions include:

  • What changes in freshwater input to the North Atlantic will result from global warming? (Uncertainty e.g. due to uncertain estimates of Greenland meltwater runoff, ignored so far in most models, and due to possible changes in ENSO ([29]).)
  • What is the risk of exceeding a threshold for THC collapse for a given warming?
  • What other thresholds exist? (E.g., a local shutdown of convection in the Labrador Sea as simulated by [30], rather than a full THC collapse.)
  • What consequences would result for marine ecosystems?
  • How would temperatures over land be affected by a collapse scenario? (Just a reduced warming, or a warming followed by abrupt cooling?)

References

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  3. Rahmstorf, S., Risk of sea-change in the Atlantic. Nature, 1997. 388: p. 825-826.
  4. Roemmich, D.H. and C. Wunsch, Two transatlantic sections: Meridional circulation and heat flux in the subtropical North Atlantic Ocean. Deep Sea Research, 1985. 32: p. 619-664.
  5. Ganachaud, A. and C. Wunsch, Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data. Nature, 2000. 408: p. 453-457.
  6. Sandström, J.W., Dynamische Versuche mit Meerwasser. Annalen der Hydrographie und Maritimen Meteorologie, 1908. 36: p. 6-23.
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  10. Stommel, H., Thermohaline convection with two stable regimes of flow. Tellus, 1961. 13: p. 224-230.
  11. Rahmstorf, S., On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics, 1996. 12: p. 799-811.
  12. Manabe, S. and R.J. Stouffer, Two stable equilibria of a coupled ocean-atmosphere model. Journal of Climate, 1988. 1: p. 841-866.
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  15. Rahmstorf, S., Rapid climate transitions in a coupled ocean-atmosphere model. Nature, 1994. 372: p. 82-85.
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  23. Alley, R.B., et al., Making sense of millennial scale climate change, in Mechanisms of global climate change at millennial time scales, P.U. Clark, R.S. Webb, and L.D. Keigwin, Editors. 1999, AGU: Washington. p. 385-394.
  24. Ganopolski, A. and S. Rahmstorf, Rapid changes of glacial climate simulated in a coupled climate model. Nature, 2001. 409: p. 153-158.
  25. Broecker, W., Unpleasant surprises in the greenhouse? Nature, 1987. 328: p. 123.
  26. Manabe, S. and R.J. Stouffer, Century-scale effects of increased atmospheric CO2 on the ocean-atmosphere system. Nature, 1993. 364: p. 215-218.
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  29. Latif, M., et al., Tropical stabilization of the thermohaline circulation in a greenhouse warming simulation. Journal of Climate, 2000. 13(11): p. 1809-1813.
  30. Wood, R.A., et al., Changing spatial structure of the thermohaline circulation in response to atmospheric CO2 forcing in a climate model. Nature, 1999. 399: p. 572-575.





27/9/09

UN NUEVO TIPO DE «EL NIÑO» SERÁ CINCO VECES MÁS FRECUENTE QUE EL ACTUAL


ABC

A. ACOSTA


Actualizado Viernes, 25-09-09 a las 16:01


El fenómeno «El Niño», que se caracteriza por una entrada de agua más cálida en el Pacífico ecuatorial oriental con implicaciones en el clima de todo el planeta, sobre todo por la aparición de fenómenos extremos, está dejando paso a un nuevo tipo de alteración. Según detallan en «Nature» investigadores de Corea, Francia y Estados Unidos, el nuevo tipo de «El Niño» será cinco veces más frecuente que el convencional a finales de este siglo.
Al nuevo fenómeno lo han llamado «El Niño Modoki», una palabra que en japonés quiere decir algo así como «similar, pero diferente». Pues bien, este nuevo tipo cada vez más común se caracteriza por una zona en el Pacífico central de aguas cálidas con forma de herradura, rodeada por el este y el oeste por corrientes inusualmente frías.
Según los investigadores, la culpa para este cambio habría que buscarla en el calentamiento global del planeta. A medida que la temperatura ha ido aumentando, modificando también la capa bajo la superficie océanica donde se mezclan aguas cálidas y más frías, este nuevo fenómeno se vuelve más común. No son buenas noticias, teniendo en cuenta que «El Niño», que hasta ahora se producía con una frecuencia de entre tres y ocho años, provoca alteraciones climáticas y fenómenos extremos en muchas partes del mundo.

IPCC REPORTS. Working Group I: The Scientific Basis

D.2 
The Coupled Systems As noted in Section D.1, many feedbacks operate within the individual components of the climate system (atmosphere, ocean, cryosphere and land surface). However, many important processes and feedbacks occur through the coupling of the climate system components. Their representation is important to the prediction of large-scale responses.

Modes of natural variability There is an increasing realisation that natural circulation patterns, such as ENSO and NAO, play a fundamental role in global climate and its interannual and longer-term variability. The strongest natural fluctuation of climate on interannual time-scales is the ENSO phenomenon (see Box 4). It is an inherently coupled atmosphere-ocean mode with its core activity in the tropical Pacific, but with important regional climate impacts throughout the world. Global climate models are only now beginning to exhibit variability in the tropical Pacific that resembles ENSO, mainly through increased meridional resolution at the equator. Patterns of sea surface temperature and atmospheric circulation similar to those occurring during ENSO on interannual time-scales also occur on decadal and longer time-scales.


The North Atlantic Oscillation (NAO) is the dominant pattern of northern wintertime atmospheric circulation variability and is increasingly being simulated realistically. The NAO is closely related to the Arctic Oscillation (AO), which has an additional annular component around the Arctic. There is strong evidence that the NAO arises mainly from internal atmospheric processes involving the entire troposphere-stratosphere system. Fluctuations in Atlantic Sea Surface Temperatures (SSTs) are related to the strength of the NAO, and a modest two-way interaction between the NAO and the Atlantic Ocean, leading to decadal variability, is emerging as important in projecting climate change.

Climate change may manifest itself both as shifting means, as well as changing preference of specific climate regimes, as evidenced by the observed trend toward positive values for the last 30 years in the NAO index and the climate �shift� in the tropical Pacific about 1976. While coupled models simulate features of observed natural climate variability, such as the NAO and ENSO, which suggests that many of the relevant processes are included in the models, further progress is needed to depict these natural modes accurately. Moreover, because ENSO and NAO are key determinants of regional climate change and can possibly result in abrupt and counter intuitive changes, there has been an increase in uncertainty in those aspects of climate change that critically depend on regional changes.

The thermohaline circulation (THC) The thermohaline circulation (THC) is responsible for the major part of the meridional heat transport in the Atlantic Ocean. The THC is a global-scale overturning in the ocean driven by density differences arising from temperature and salinity effects. In the Atlantic, heat is transported by warm surface waters flowing northward and cold saline waters from the North Atlantic returning at depth. Reorganisations in the Atlantic THC can be triggered by perturbations in the surface buoyancy, which is influenced by precipitation, evaporation, continental runoff, sea-ice formation, and the exchange of heat, processes that could all change with consequences for regional and global climate. Interactions between the atmosphere and the ocean are also likely to be of considerable importance on decadal and longer time-scales, where the THC is involved. The interplay between the large-scale atmospheric forcing, with warming and evaporation in low latitudes and cooling and increased precipitation at high latitudes, forms the basis of a potential instability of the present Atlantic THC. ENSO may also influence the Atlantic THC by altering the fresh water balance of the tropical Atlantic, therefore providing a coupling between low and high latitudes. Uncertainties in the representation of small-scale flows over sills and through narrow straits and of ocean convection limit the ability of models to simulate situations involving substantial changes in the THC. The less saline North Pacific means that a deep THC does not occur in the Pacific.

Non-linear events and rapid climate change
 
The possibility for rapid and irreversible changes in the climate system exists, but there is a large degree of uncertainty about the mechanisms involved and hence also about the likelihood or time-scales of such transitions. The climate system involves many processes and feedbacks that interact in complex non-linear ways. This interaction can give rise to thresholds in the climate system that can be crossed if the system is perturbed sufficiently. There is evidence from polar ice cores suggesting that atmospheric regimes can change within a few years and that large-scale hemispheric changes can evolve as fast as a few decades. For example, the possibility of a threshold for a rapid transition of the Atlantic THC to a collapsed state has been demonstrated with a hierarchy of models. It is not yet clear what this threshold is and how likely it is that human activity would lead it to being exceeded (see Section F.6). Atmospheric circulation can be characterised by different preferred patterns; e.g., arising from ENSO and the NAO/AO, and changes in their phase can occur rapidly. Basic theory and models suggest that climate change may be first expressed in changes in the frequency of occurrence of these patterns. Changes in vegetation, through either direct anthropogenic deforestation or those caused by global warming, could occur rapidly and could induce further climate change. It is supposed that the rapid creation of the Sahara about 5,500 years ago represents an example of such a non-linear change in land cover.


D.3 
Regionalisation Techniques Regional climate information was only addressed to a limited degree in the SAR. Techniques used to enhance regional detail have been substantially improved since the SAR and have become more widely applied. They fall into three categories: high and variable resolution AOGCMs; regional (or nested limited area) climate models (RCMs); and empirical/statistical and statistical/dynamical methods. The techniques exhibit different strengths and weaknesses and their use at the continental scale strongly depends on the needs of specific applications.

Coarse resolution AOGCMs simulate atmospheric general circulation features well in general. At the regional scale, the models display area-average biases that are highly variable from region to region and among models, with sub-continental area averaged seasonal temperature biases typically ±4ºC and precipitation biases between -40 and +80%. These represent an important improvement compared to AOGCMs evaluated in the SAR.

The development of high resolution/variable resolution Atmospheric General Circulation Models (AGCMs) since the SAR generally shows that the dynamics and large-scale flow in the models improves as resolution increases. In some cases, however, systematic errors are worsened compared to coarser resolution models, although only very few results have been documented.

High resolution RCMs have matured considerably since the SAR. Regional models consistently improve the spatial detail of simulated climate compared to AGCMs. RCMs driven by observed boundary conditions evidence area-averaged temperature biases (regional scales of 105 to 106 km2) generally below 2ºC, while precipitation biases are below 50%. Regionalisation work indicates at finer scales that the changes can be substantially different in magnitude or sign from the large area-average results. A relatively large spread exists among models, although attribution of the cause of these differences is unclear.

26/9/09

THE YALE FORUM.- CLIMATE CHANGE & THE MEDIA


Common Climate Misconceptions
 
By Zeke Hausfather | October 11, 2007
 
Some in the news media may be overplaying the extent of the risk that Northern Europe might soon plunge into a new Ice Age. They risk going beyond where the best science can now take them.
“Britain could be heading for a climate like Alaska,” the BBC reported back in 2003. It painted a stark picture of a life in which “our ports could be frozen over. Ice storms could ravage the country, and London could see snow lying for weeks on end.”

New Scientist in 2005 cautioned us that stuttering ocean warm currents may “plunge the continent into a mini ice age.”
National Geographic that same year reported that “Chilling new evidence from the Atlantic Ocean is raising fears that western Europe could soon be gripped by a mini ice age.”
The potential shutdown of the Thermohaline Circulation (THC), commonly misidentified (.pdf ) as the Gulf Stream, often makes the list of the most dangerous potential impacts of climate change. However, the current state of the science suggests 1) that the THC, rather than abruptly shutting down, is likely to slow over the course of centuries; and 2) any cooling in Northern Europe would be more than offset by the larger human-driven global warming trend.
The THC is a global ocean circulation driven primarily by changes in density and salinity of ocean waters. In the North Atlantic, low temperatures, combined with high evaporation rates driven by strong winds moving over water, increase the density of the surface water. As a result, surface waters sink, drawing in warmer surface waters from the south. These processes drive a global ocean current often referred to as the “ocean conveyor belt.” The THC is one of the reasons (.pdf) that the United Kingdom and other Northern European countries enjoy such a mild climate, despite sharing the same latitude as Siberia.
Paleoclimate records show that the THC has shut down in the past, causing temperatures in Northern Europe to fall substantially. These shutdowns are hypothesized to be caused by massive freshwater releases into the North Atlantic from enormous glacial lakes that develop as the world moved out of ice ages.
Similarly, many scientists are concerned that increasing freshwater icemelt in Greenland and Northern Europe as a result of anthropogenic climate change could potentially slow or even shut down the THC.
The question, like many involving climate change, is really one of timescales. Climate models show the THC slowing down over the next century by anywhere from zero to 50 percent, but almost none show it actually stopping. In even the most pessimistic cases, the rate of icemelt occurring would produce an order of magnitude less freshwater than what caused past THC shutdowns.
In any of these modeled cases a slowdown of the THC would still have a cooling effect on Northern Europe, but it would likely be more than offset by the larger global warming trend.
In 2005, Harry Bryden, an oceanographer, and his research team published an article in Nature that shocked many in the climate field and generated a considerable amount of media attention. Bryden found that, compared to data from 1957, 1981, 1992 and 1998, the volume of the THC appeared to have decreased by about 30 percent.
Many others in the field have since criticized Bryden’s study. They argue that measurements are not yet able to effectively distinguish a trend from natural variability in the current. An actual decrease of 30 percent in the current, these scientists maintain, would have caused measurably cooler temperatures in Europe.
M.I.T. oceanographer Carl Wunsch compares Bryden’s method to “measuring temperatures in Hamburg on five random days and then concluding that the climate is getting warmer or colder.” As scientists writing in RealClimate explain, “now that data has been properly published, it confirms what we thought all along. The sampling variability in the kind of snapshot surveys that Bryden et al had used was too large for the apparent trends that they saw to be significant.”
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany is strongly skeptical of the idea that a thermohaline shutdown in today’s climate would lead to a situation where large parts of Europe would be frozen. And Gavin Schmidt, from NASA Goddard, maintains that, “while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of Europe is a ways off yet.”
Wallace Broecker of Columbia University, who first posited that THC shutdowns could explain climate shifts in the distant past, puts it even more strongly, pulling no punches: “The notion that [a modern THC shutdown] may trigger a mini ice age is a myth.”
Scientific debate unquestionably will continue over this issue. The recent installation of a new system for measuring THC flows should improve data on any changes that are occurring.
In the mean time, journalists need to be careful in painting an unjustified picture of dramatic cooling in Northern Europe, one not supported by the current state of the science.
Author
Zeke Hausfather is a regular contributor to the Yale Forum (E-mail: zeke@yaleclimatemediaforum.org).

QUESTIONS AND ANSWERS ON ABRUPT CLIMATE CHANGE

MINISTRY FOR DE ENVIRONMENT -NEW ZELAND

Man-made climate warming could lead to some impacts that are abrupt or irreversible

The IPCC assessment is that climate change is likely to lead to some irreversible impacts. Abrupt changes cannot be excluded, but they are not currently considered likely. Such changes will depend on both the rate and the magnitude of climate change.

What is abrupt climate change?

When scientists talk about climate change, they are usually referring to "gradual climate change." In other words, if the planet warms steadily, the climate changes steadily. But there is evidence that some parts of the climate system work more like a switch than a dial; if a certain temperature level is reached, there may be abrupt climate change.
Some scientists have pointed out the risk of catastrophic climate change caused by gradual increases in greenhouse gas concentrations - like the breakup of the West Antarctic ice sheet, or the collapse of the thermohaline circulation (THC).

Could abrupt climate change really happen?

Scientists have only recently begun to study the possibility of abrupt climate change in more detail. Our knowledge is still too limited to know at what stage abrupt climate change could be triggered by increasing greenhouse gas concentrations, but it is generally considered very unlikely to happen within the next several decades. However, the probability of an abrupt change in some part of  the climate system is expected to increase if the rate, or the magnitude, or the duration of climate change from greenhouse gas emissions increase.
Thermohaline circulation (THC) is often called the "Ocean Conveyor". This system consists of water sinking at high latitudes in the North Atlantic, then flowing south at depth through the North and South Atlantic Oceans, around Antarctica in the circumpolar current and then flowing north into the Indian and Pacific Oceans. The core of deep water is continuously eroded by mixing with waters above it, and finally peters out in the North Pacific. The conveyor is completed by warm water flowing back towards the North Atlantic, via the Indonesian Throughflow from the Pacific Ocean, around the Cape of Good Hope from the Indian Ocean and then northwards through the South and North Atlantic Ocean. This return flow of warm water reinforces the Gulf Stream in the North Atlantic and warms the North Atlantic region by ~10°C. The sinking in the North Atlantic is critically dependent on temperature and on the salinity of the water.

Could climate change shut down the thermohaline circulation?

Climate change is expected to increase ocean temperatures particularly in polar regions and to increase the flow of freshwater into the ocean through rain, run-off, and melting of glaciers. The increased surface ocean temperatures and reduced salinity (saltiness of the water) are very likely to weaken the thermohaline circulation during the 21st century. However current thinking is that it is very unlikely there will be an abrupt change this century

What would happen to New Zealand and the Pacific if the thermohaline circulation shuts down?

Current studies suggest a shut down of the THC would lead to an extra 1-2 degree celsius increase in temperature in the New Zealand region, on top of that caused by other global warming processes. This would reinforce the effects of global warming. We do not know what effect this could have on our day-to-day weather as scientists are only beginning to study this phenomenon. The additional warming effect of a shut-down of the THC in the Pacific region is the opposite to the expected effects in the northern hemisphere, where a reduction in the THC is expected to lead to a cooling, or at least a reduced rate of warming in northern regions of Europe and eastern Canada and the United States.

Should we be really worried?

The scientific consensus is that we are not on the brink of a shutdown of the THC. Many scientists believe that it cannot happen or is unlikely to happen for at least another century. However, the THC system is pivotal to climate stability, and if it shuts down this would have an enormous impact likely to last for several centuries.
To find out what you can do to help reduce the impacts of climate change, please visit www.sustainability.govt.nz.