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| Die vrees-industrie - feite en verwarring oor global warming [boodskap #19838] |
Tue, 12 January 1999 00:00  |
Bees
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Ferdi se:
> Baie is al feit.
> Soos n stygende gemiddelde temperatuur.
Ek is minder seker hieroor.
Nou vir die mees belangrike feit: akkurate, herhaalbare temp metings het
relatief onlangs begin, verseker in hierdie eeu, en metings wat klein genoeg
inkremente het waarskynlik eers in die laaste paar dekades.
Soos enige ou wat aandeel grafieke bestudeer weet - moenie langtermyn
gevolgtrekkings maak met korttermyn data nie.
As 'n voorbeeld, beskou die volgende paar datapunte: 20, 19, 18, 17. Lyk
soos 'n vermindering met tyd.
Wat as dieselfde datapunte slegs 'n klein deel uitmaak van die volgende
reeks: 16, 17, 19, 21, 20, 19, 18, 17, 18, 20, 22, 23, 25, 28.....Jy kry die
idee.
Kortom, global warming mag aan die gebeur wees, of nie. Ons weet eenvoudig
nie.
Wees maar versigtig met die wat 'n beroep of een of ander aandeel het in die
"vrees-industrie". Nie alle motiewe is so suiwer nie. Ek dink 'n mens moet
luister na alle idees en argumente, en dan self besluit. Ek persoonlik staan
maar bra skepties oor die verwarming-storie. (Daar nie nie veel global
warming waar ek my nou bevind nie :-)
Geniet die warmte
Bees
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| Re: Die vrees-industrie - feite en verwarring oor global warming [boodskap #19866 is 'n antwoord op boodskap #19838] |
Wed, 13 January 1999 00:00  |
ferdinand
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On Tue, 12 Jan 1999 19:03:53 -0700, "Bees" wrote:
inkremente het waarskynlik eers in die laaste paar dekades.>>[/color]
Die punt is, as daar aanduidings, moet ons dit seker met ers bejeen in
plaas van te se: Kom ons wag tot dit finaal bewys is en dan begin ons
panic.
Op Nasa se website is n berig dat die afgelope jaar die warmste is was
wat nog gemeet is.
Ek wou dit gou cut en paste maar die betrokke blad is aflyn vandag.
Dit sal later weer op wees. As jy wil kan jy self gaan kyk by
www.nasa.gov Miskien is jy gelukkiger as ek en kry dit dadelik.
Hier is intussen n stukkkie wat ook vanaf die blad gekry kan word:
Nog n site hiervoor is: http://www.giss.nasa.gov/data/update/csci/
Dan het ek die volgende by New Scientist op die web gekry:
Ice-cold in Paris
Global warming could have a nasty surprise in store for Europe, warns
Stefan Rahmstorf. Instead of bringing year-round warmth, it may herald
an era of freezing winters. By Stefan Rahmstorf
To northern Europeans shivering in the grip of an unusually icy
winter, global warming might suddenly seem an attractive proposition.
How pleasant to bask in a balmy Mediterranean climate without ever
leaving home.
Dream on. Evidence now emerging reveals a risk that global warming
could plunge most of Europe into a big chill lasting hundreds of
years, bringing with it effects that could be felt right around the
world.
What Europeans tend to forget is that the Atlantic Ocean keeps them
relatively warm. By rights, northern Europe should experience the same
chilly climate as the northern US and Canada, since they are all at
more or less the same latitude. But warm surface waters originating in
the tropics are drawn northwards by gigantic undersea 'pumps', which
pull the balmy water towards the European continent. It is this vital
pumping process that could be under threat from global warming.
So how does Europe's cosy heating system work? The warm surface waters
arrive through the Gulf Stream, on the western side of a huge eddy
known to oceanographers as a subtropical gyre. Similar gyres are found
in all oceans and are driven by the winds and the Earth's rotation.
Normally, this gyre and the Gulf Stream would have little effect on
the climate of Europe. But the North Atlantic is home to two mammoth
oceanic pumps-one east of Greenland and one in the southern Labrador
Sea-which exert an extra tug on the warmed surface water. Just as
bathwater is sucked down into the plughole, the pumps pull Gulf Stream
water from the surface down into the deep ocean, dragging it far
enough north to heat Europe.
The Gulf Stream, and the North Atlantic pumps form part of the global
ocean circulation system that has been dubbed the 'conveyer belt' by
oceanographers. Warm surface waters are drawn north throughout the
Atlantic at a flow rate more than a hundred times that of the Amazon
River. They then sink to the deeps of the Greenland and Lab-rador
Seas, and return to the Southern Ocean at 2 to 3 kilometres below the
surface as the so-called North Atlantic Deep Water. The waters release
heat into the cold northern atmosphere at a rate of a trillion
kilowatts (10 to the power of 15 W), an amount equivalent to a hundred
times the world's energy consumption. This energy warms the air over
Europe by about 5 degrees C-a free heating service that has operated
reliably over the past 10 000 years or so.
But there is increasing evidence of abrupt and dramatic changes in
Europe's climate throughout the last ice age. Even the warm period
that preceded the ice age may have had major climatic ups and downs.
Average temperatures seem to have swung by 5 degrees C or more within
a few years, leading to icy spells that lasted for centuries.
Climatologists have come to view the past 10 000 years following the
end of the last ice age as an exception in climate history, and some
are saying that human interference with the climate system might
trigger a new period of instability.
Much is already known about the basic workings of the conveyor belt.
Its flow is driven by differences in water density at different points
in the Atlantic. If the North Atlantic Deep Water is to push its way
southwards out of the Atlantic basin it needs to be denser than water
in the south, near South Africa, where it joins the Antarctic
Circumpolar Current, which circles the planet from west to east. Some
of this deep water then rises back to the surface near Antarctica, and
some travels into the other ocean basins at depth, before finally
reaching the North Pacific after a thousand years.
The path by which this water then returns to the Atlantic is still
hotly debated. There are two possible routes: a westward 'warm water
route' passing between the islands of Indonesia and around South
Africa, and an eastward 'cold water route' around the southern tip of
South America via Drake Passage (see Diagram).
The density of the water in the North Atlantic is determined by its
salinity and its temperature, so these factors also determine the
action of the conveyor belt. When the warm surface water comes into
contact with the cold sub-Arctic air, it cools. This increases its
density and encourages it to sink. On the other hand, the
northward-flowing surface waters are diluted by freshwater coming from
rain, rivers and melting snow. This makes the water less dense, and
hence more buoyant. Left to itself, the freshwater would win the
contest. But the conveyor belt brings a perpetual flow of new salty
surface water from regions to the south, which keeps the seawater
dense enough to sink. In subtropical regions, the oceans
characteristically have more evaporation than freshwater input, so
they tend to be more salty.
Detective work This self-maintaining positive feedback system has at
least one glaring hitch: if something interrupts it, and the conveyor
belt grinds to a halt, it remains shut down. This effect was seen in
one of the first climate modelling experiments to include both ocean
and atmosphere. In the late 1980s, Suki Manabe and Ron Stouffer of the
Geophysical Fluid Dynamics Laboratory at Princeton in New Jersey found
that their climate model had two very different, more or less stable
states. One, like our present climate system, had a system of currents
resembling the conveyor belt in the Atlantic and a comfortable
European climate. In the other, however, the conveyor belt had shut
down, and temperatures in northwestern Europe were up to 10 degrees C
colder than today. The existence of these two states has since been
confirmed by many experiments using different models.
But other questions remained. Could the climate switch between these
different states? Had it done so in the past? And what might trigger
such a switch? Luckily, the Earth itself contains several archives of
past climatic conditions which, with a bit of detective work, yield
many clues. Among the most useful of these are the snow layers that
have piled up on the Greenland Ice Sheet and the layers of sediment
that have accumulated at the bottom of the Atlantic.
These records show that rapid and severe climatic jumps occurred every
thousand years or so during the last ice age, in sharp contrast to the
stable conditions of the past 10 000 years. The last of these jumps is
the Younger Dryas event which took place as the Earth emerged from the
last ice age. Gradual climatic warming was causing the huge
continental ice sheets to melt and disintegrate, but then, within a
decade, ice-age conditions suddenly returned.
In 1989, a modelling experiment by Ernst Maier-Reimer and Uwe
Mikolajewicz at the Max Planck Institute in Hamburg uncovered a neat
explanation for the Younger Dryas event. It showed that a massive
inflow of meltwater from the Laurentide Ice Sheet could have led to a
sudden collapse of the Atlantic conveyor belt, throwing the Atlantic
region back into the freezer.
Flickering switch Now, researchers are asking whether today's global
warming, the result of accumulated carbon dioxide and other greenhouse
gases in the atmosphere, could have the same effect as the period of
natural warming at the end of the ice age. global warming is, for
instance, expected to warm the surface water in the northern high
latitudes. It should also increase the amount of rain and snowfall
over the ocean, and speed up the melting of high-latitude ice, which
would make the water fresher. Both the warming and the freshening
would make the surface water less dense, which could put brakes on the
pumping mechanism.
In 1993, Manabe and Stouffer studied the effects of CO2 concentrations
on global climate in a model that coupled the ocean, the atmosphere
and sea ice. As the atmospheric CO2 concentration slowly increased to
four times its preindustrial level, the ocean's deep circulation came
to a complete standstill. However, this change required fairly drastic
amounts of CO2 to be released into the atmosphere-the
Intergovernmental Panel on Climate Change does not expect such levels
to be reached before 2100. Also, the deep circulation in the model
ground to a halt slowly-over centuries-unlike the abrupt climate
shifts shown by the Greenland ice core record.
Despite these caveats, there is mounting evidence, both from the past
climate record and from more recent ocean modelling, that the climate
system could be more vulnerable than Manabe and Stouffer's findings
imply. Over the past few years, as researchers have drilled and
analysed more ocean sediment cores, the picture has been getting more
complex. The new evidence shows that during some cold spells, the
conveyor belt may not have switched off but simply shifted south.
Three years ago, marine geologist Michel Sarnthein from the University
of Kiel in Germany, with colleagues from France and the Netherlands,
published reconstructions of deep water flow in the Atlantic at
different times in the past, based on a large number of ocean sediment
cores. They found three circulation modes. The first was a warm
conveyor belt mode that has operated over the past 10 000 years or so.
The second mode was a 'glacial' conveyor belt, which was shallower and
did not extend north into the Greenland Sea, but ended somewhere south
of Iceland. Finally, they found periods where the conveyor belt was
very weak because large amounts of meltwater had entered the Atlantic,
capping off oceanic convection with a surface 'lens' of freshwater.
At that time, I was a researcher at the Institute of Marine Sciences
at the University of Kiel running a series of modelling experiments
investigating how the conveyor belt would respond if a lot of
freshwater suddenly flowed into the Atlantic, and how this would
affect surface temperatures.
Surprisingly, as well as the then familiar climatic modes of operation
with the conveyor belt switched either 'on' or 'off', we too found a
third possibility of a cold conveyor belt extending not nearly as far
north as at present. Although this conveyor belt was almost as
vigorous, it hardly warmed the northern Atlantic region, as its waters
sank and returned south before releasing much heat to the atmosphere.
So a shift in the ocean currents could have thrown the region into a
cold spell without the complete collapse of the conveyor belt. Also in
1994, experiments by Andrew Weaver and Tertia Hughes from the
University of Victoria in Canada showed that with increased
precipitation in the Atlantic the conveyor belt can start to 'flicker'
between its different modes, leading to strong climatic oscillations
over Europe.
The possibility that an influx of freshwater into the Atlantic could
have such an effect is worrying. Although the kind of sudden climatic
swing shown in these models works through positive feedback, it is
primarily associated not with salt transport in the conveyor belt, but
with the downward mixing of water in the two pumps. If
something-perhaps the effect of global warming-interrupted the
downward mixing, or convection, process at one of these sites, the
incoming freshwater would start to accumulate at the surface. This
would make the surface waters more and more buoyant and it would
become harder and harder to restart the pump. The models suggest that
in this way the pumping at one of the sites could shut down and the
conveyor belt could then change its route within a few years. This
breakdown could be triggered by a relatively small change in the
amount of freshwater entering the ocean, because the two pump sites
are very localised, each being just a few hundred kilometres across.
Not surprisingly, these convective pumps have been dubbed the Achilles
heel of the conveyer belt.
But we do not know whether this will really happen. Existing models
are simply incapable of quantifying how much warming is needed to
switch off convection at one of the pump sites. Though some of the
fastest computers in the world are used for these simulations, lack of
computer power is still forcing models to use a very coarse mesh in
their calculations. This means that they cannot represent the kind of
regional details that could turn out to be crucial.
The models are also unrealistic in that they overstate the role of
salt and heat diffusion. Reducing the amount of diffusion seems to
make the ocean currents less stable, according to comparisons that
Stouffer and I have made over the past six months. What's more, most
models still work with ad hoc fixes, known as flux adjustments, at the
interface between ocean and atmosphere. Without these fixes, the
simulated climate and ocean circulation drifts to a less realistic
state.
So we can take no comfort from the current global warming scenarios,
which tend to show a smooth gradual warming over the next century. We
simply don't know why our present climate is much more stable than the
climate of the past, and whether this stability will continue in the
face of global warming. Though the models suggest the effect of global
warming might be less drastic and rapid than past changes, it could
simply be that present models don't yet capture the physics of abrupt
climate change.
Researchers are attacking these gaps in our knowledge on three fronts.
First, they are looking to see if past swings in ocean circulation
took place only during the last ice age, or whether the ocean was also
unstable in the Eemian Interglacial Stage, a warm interlude between
113 000 and 125 000 years ago. If the first of these is true, we could
be safe. It implies that swings in the conveyor belt arose only
because of meltwater or iceberg flotillas, from the warming of the
large amounts of land ice that had formed in the preceding glacial
period.
On the other hand, climatic swings in the Eemian suggest that large
amounts of land ice are not necessary to cause the changes. That could
presage a rough ride for us in the future, when global warming has set
in. Two recent Greenland ice cores, the European GRIP core taken in
1992 and the American GISP2 core taken in 1993, provided conflicting
views of the Eemian climate.
The former showed a record of frequent fluctuation but was probably
disturbed by motions of the ice, while the latter revealed a period of
stability. Drilling has just started on a new ice core, known as North
GRIP, which should help to settle the issue (This Week, 6 July 1996, p
7).
Research is also opening up on the oceanographic front with
expeditions by European, American and Canadian teams starting this
winter. The oceanographers will be studying the convection processes
and their link to climatic conditions in the northern Atlantic. These
measurements will in turn provide important data needed to validate
and improve model simulations-the third line of attack. More
measurements will also help to decide whether convection in the
Greenland Sea has already weakened since the early 1980s due to global
warming, as some oceanographers have suggested (see This Week, 19
March 1994, p 4), or if wether is a natural climatic fluctuation.
So long as the jury stays out on just how vulnerable the conveyor belt
is, there is still a very real possibility that we will unwittingly
disrupt it and trigger a calamitous cooling throughout Europe. The
consequences for ecosystems, agriculture and society could be severe.
And it's not just a European problem. The effects of past cooling
episodes, such as the Younger Dryas, have been seen in the climate
record from the US, Chile and even New Zealand. Geochemist Wally
Broecker of Columbia University in New York has a blunt way of putting
it: 'We are playing Russian roulette with climate and no one knows
what lies in the chamber of the gun.' Stefan Rahmstorf is an ocean
modeller at the Potsdam Institute for Climate Impact Research in
Germany.
New Scientist, 08 Febuary 1997, Volume 153. Issue 2068.
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