Fwd: ISIS Special Miniseries - Global Warming & then the Big Freeze

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ISIS Special Miniseries - Global Warming & then the Big Freeze

press-release

 

The Institute of Science in Society

Science Society Sustainability

http://www.i-sis.org.uk

 

General Enquiries sam

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ISIS Director m.w.ho

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ISIS Special Miniseries

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Global Warming & then the Big Freeze

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A global circulation of water between the surfaces and the depths of oceans

plays a major role in keeping the earth’s climate congenial to life. But this

circulation is unstable to global warming, with catastrophic consequences. Dr.

Mae-Wan Ho reports.

 

Diagrams and sources for this article are posted on ISIS Members’ website.

www.i-sis.org.uk

 

Thermohaline circulation

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Those of us who live in northern Europe have a special interest in the Gulf

Stream Drift – a surface current of warm water that flows from tropical to

northern Atlantic - which ensures that our mean annual temperature is about 5 to

7 C warmer than would otherwise be the case. The Gulf Stream Drift is linked to

a large thermohaline circulation (THC) in the Atlantic Ocean that redistributes

heat across the globe, and depends both on heating (thermal) and on the salt

content (haline) of ocean water.

 

 

Warm water from the tropics flows to the North Atlantic and gives off heat to

the atmosphere. This cools the water, increasing its density, causing it to

overturn and sink near Greenland and the Norwegian Seas. This down-welling of

cold water is what drives the thermohaline circulations. The cold water make its

way back to the south in a deep current that ultimately involves all the oceans

on earth (see Fig. 1). There is no such actively driven current in the North

Pacific because the water is too fresh and therefore not dense enough to sink

even when cooled.

 

 

 

Figure 1. The thermohaline circulation system.

 

The North Atlantic thermohaline circulation is responsible for much of the total

oceanic heat transport towards the north pole, peaking at about 1.2 + 0.3 Peta

Watts (1015 Watts) at 24oN latitude.

 

 

The effect of the THC is much more extensive than just the Gulf Stream Drift.

The shifts in temperature and salinity of the oceans affect marine ecology.

Colder waters are rich in nutrients and contain a great deal of dissolved carbon

dioxide, whereas warmer waters are oxygenated and stripped of nutrients. The THC

is also a major forcing of hurricanes, which feed off warm surface sea

temperatures. The stronger the thermohaline circulation, the more tropical

cyclones occur, as larger areas have warm sea surface temperatures.

 

 

How fast is the THC? It depends. The idea that it may have many speeds is more

than a century old, but only since the 1960s have quantitative models been

proposed to support the existence of multiple equilibrium states corresponding

to the different speeds of the THC. These models have further shown that

transitions between the different states are often abrupt, and can be brought

about through small perturbations to the water cycle. ‘Abrupt’ in this context

means decades, even years, which is indeed very abrupt in terms of geological

timescales.

 

 

Analyse and models of the abrupt climate change during the last glaciation 11

000 to 12 000 years ago also suggest that the event may have been triggered by

changes THC in response to small perturbations in the hydrological cycle.

 

 

Most, but not all models show a reduction in the strength of the THC with

increasing concentrations of greenhouse gases, and if warming is strong enough

and sustained long enough, a complete collapse cannot be excluded.

 

 

According to the model produced by the research group in the Hadley Centre for

Climate Prediction and Research, Met Office, ashutdown of THC could occur from a

sudden freshening of the North Atlantic, say, due to the influx of water from

melting ice, reducing the mean density of the water and inhibiting the water

column from sinking and feeding the convective circulation. The strength of the

North Atlantic overturning reduces to near zero very quickly, then recovers to

its original strength over a period of about 120 years.

 

 

The atmosphere responds promptly to the cessation of the THC, and a cooling of

the Northern Hemisphere is rapidly established, which is most severe over and

adjacent to the North Atlantic. Over the first three decades after the cessation

of the THC, the southern hemisphere warms due to the loss of northward heat

transport normally carried by the THC. By the third decade, the maximum cooling

over Western Europe will have reduced somewhat.

 

 

But surface temperature is not the only response to the cessation of THC. As the

result of general cooling in the Northern Hemisphere, there is a widespread

reduction in both surface evaporation and precipitation there, with increases

elsewhere. The combined temperature and water-cycle changes would be expected to

have a significant impact on vegetation and agriculture. It is estimated that

primary productivity will be reduced by 12% in the northern hemisphere as a

whole, with larger regional changes: reduction of 16% in Europe, 36% in the

Indian subcontinent and 109% in Central America.

 

Model predicts catastrophic change

A wide range of theoretical models has been proposed for the THC, and a feature

common to many of the models is a non-linear behaviour, involving multiple

equilibrium states, delayed sudden transitions between the states (known as

hysteresis) and threshold effects (sudden change at critical values). In its

simplest form, the model, first proposed by Stommel in 1961, consists of two

boxes representing the high-latitude and low-latitude North Atlantic, with water

exchanged between the two through an overturning circulation. The strength of

the overturning is proportional to the density difference between the two boxes.

The atmosphere is assumed to be able to dampen large-scale sea-surface

temperature fluctuations relatively quickly through radiation, whereas there is

no such feedback regulation on the salinity of the surface water.

 

 

A theoretical model based on Stommel’s gives a starkly simple solution of

overturning T as a function of the rate of freshening F. For F less than a

critical value Fc, three solutions exist (Fig. 2). The middle solution (the

dotted line) is unstable, so in practice there are only two realizable

solutions, the upper branch with strong overturning, and the lower branch with

weak to reversed overturning. For F > Fc, only the lower branch solution is

possible. In this case, the freshening of the surface water is so strong that it

will fail to sink. On the contrary, salt water from the deep will flow towards

the North Atlantic, driving a reverse overturning.

 

 

 

Figure 2. Multiple states and abrupt transitions at critical points

 

 

 

The present THC is assumed to be in the upper, strong overturning branch. But

what would happen when carbon dioxide increases further?

 

 

A robust feature of climate models is a strengthening of the water cycle –

between earth surface and atmosphere - when carbon dioxide increases, due to the

increased amount of water vapour that warmer air can carry and return as

rainfall. This would lead to increase in freshening, F, in the model, and hence

a gradual weakening of the overturning. If F increases to and beyond Fc, the

upper equilibrium becomes untenable and the system jumps to the lower branch.

Such a jump would be quite difficult to reverse, and that’s the characteristic

of hysteresis. Even if F is brought back below the critical value, the system

will stay on the lower branch, unless factors not included in the model can come

into play.

 

 

This simple model is not likely to be the whole story. The prediction of abrupt

change is fairly robust. But whether the THC shuts down, reverses or recovers

after that appears to depend on the precise model.

 

 

As pointed out by Andrey Ganopolski of the Potsdam Institute for Climate Impact

Research in Germany, successful modelling of abrupt climate change in the

earth’s history needs to take into account all the major components of the

earth’s system. Apart from changes in atmospheric composition and the

thermohaline circulation, the dynamics of ice sheets formation and vegetation

cover also played important roles in the evolution of palaeoclimate.

 

 

 

 

 

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General Enquiries sam

Website/Mailing List press-release

ISIS Director m.w.ho

 

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