The great climate flip flop

http://www.theatlantic.com/issues/98jan/climate.htm

-- - (x@xxx.cQm), May 04, 2000

Answers

Good article...

"Climate change" is popularly understood to mean greenhouse warming, which, it is predicted, will cause flooding, severe windstorms, and killer heat waves. But warming could lead, paradoxically, to drastic cooling -- a catastrophe that could threaten the survival of civilization

-- (Take@a.look), May 04, 2000.

Aswan dam may trigger ice age, say scientists ----------------------------------------------------------------- Copyright 1997 Nando.net Copyright 1997 Agence France-Presse

LONDON (July 24, 1997 02:21 a.m. EDT) - Egypt's Aswan Dam could cause such oceanic and climatic changes that they trigger a new ice age, according to reports Thursday. Bob Johnson, of the University of Minnesota in the United States, said in the "New Scientist" magazine that his theory, which he described as "pretty far out ... but quite serious", has been backed by researchers at Colorado and Quebec universities. Johnson said that as the dam, built in 1968, stops fresh water entering the Mediterranean, the sea becomes saltier. The more saltier the water, the heavier it gets, and the bigger the flows from the sea through the Straits of Gibraltar into the Atlantic. These extra flows, claimed Johnson, move north, and meet the warm Gulf Stream, pushing more of the Gulf Stream into the Labrador Stream, and causing more snowfalls in the Arctic, and so a huge expansion of ice sheets. Johnson proposed a barrier built across the Straits of Gibraltar to counteract the extra flows.

Copyright 1997 Nando.net -

-- - (x@xxx.com), May 04, 2000.

Johnson said that as the dam, built in 1968, stops fresh water entering the Mediterranean, Really? Can anyone explain just where all that fresh water ends up then?

It is my understanding that water has a natural tendency to flow down hill. This is apparently caused by another natural tendency known as gravity. A dam serves no purpose than to make this downhill flow occur at a single location rather than be spread out along miles of river bed. Some dams can divert water from one catchment area to another, but the water still must flow. In the case of the Aswan dam the water is not diverted, it is merely contained so that the downhill flow is at a single location. From the dam it still flows down the Nile and into the Mediterranean.

On the main issue raised in this thread about a climate flip flop, it has happened in the past and it will happen in the future. The interesting thing (scientifically) is that these past climate flip flops have had a number of causes. There have been the ones mentioned in this article which have changed the gulf stream current, the meteor strikes which may have caused long periods of extreme cold and hence precipitated an ice age, meteor strikes which may have altered sea currents, polar shifts, etc.

The earth is overdue for a massive climate change, and when it happens it will make all the arguments on greenhouse gas emmisions seem minor by comparisom. The main question is WHEN? This year? This century, Next millenium? Or will it still be another 15000 years away?

-- Malcolm Taylor (taylorm@es.co.nz), May 04, 2000.

Malcolm:

In addition, even if NO water flowed from the Nile to the Med, what about all the other rivers that still flow into the Med? The dilution factor in a body of water as big as the Med must be huge, even for a river the size of the Nile.

-- Jim Cooke (JJCooke@yahoo.com), May 05, 2000.

Malcolm,

> Johnson said that as the dam, built in 1968, stops fresh water entering the Mediterranean,
>Really? Can anyone explain just where all that fresh water ends up then?

Mostly, in the air. Though I'm not familiar with the prevailing winds over that region, I suspect that most of this water vapor is not carried over the Mediterranean.

[Just as a guess, I'd say you probably don't live in a desert region, or else you'd have a better feeling for what happens to water that's pooled up in a hot dry climate.]

One of the primary uses of the Aswan Dam is irrigation. As water passses through irrigation systems, unavoidably much of it evaporates before reaching plants. Some of that which does reach plants is taken in by roots, passes through the plants, and transpired through leaves into the air. Another portion of that which reaches plants is not taken up by roots, but is evaporated from soil. The rest travels through the soil in some direction -- some may eventually make its way back to the Nile River, but it would have picked up salts from the soil by that time, rendering it less fresh than it was when it reached Aswan Lake.

>It is my understanding that water has a natural tendency to flow down hill. This is apparently caused by another natural tendency known as gravity.

Another of water's tendencies is to pass from liquid state to gaseous state as its molecules acquire kinetic energy sufficient to exceed intermolecular attractions. This is apparently caused by things known by such terms as "heat", "energy", "thermodynamics", etc.

>A dam serves no purpose than to make this downhill flow occur at a single location rather than be spread out along miles of river bed.

When a careful analysis of the behavior of the dammed water includes the colorless, transparent portion which is water vapor that evaporates from the liquid portion, it finds that some of the water molecules move not downhill, but in directions not inconsistent with the notion of "uphill". Especially in a desert environment, neglect of this gaseous portion could substantially invalidate conclusions drawn without consideration of it.

>In the case of the Aswan dam the water is not diverted, it is merely contained

... though not in _all_ directions. AFAIK, the Aswan reservoir has no roof.

>so that the downhill flow is at a single location. From the dam it still flows down the Nile and into the Mediterranean

... and into the atmosphere and into the soil of irrigated areas.

-- No Spam Please (nos_pam_please@hotmail.com), May 06, 2000.

In the US, there are two desert rivers that, through diversion, never reach the sea. The Colorado river (what's left of it by that time) vanishes at an evaporation pond. The Rio Grande is nearly always dry by the time El Paso is through with it, although other rivers flow into the Rio Grande's dry bed downstream, taking on the name. Some of their water reaches the Gulf of Mexico, but none of the original Rio Grande water from the Rockies.

Irrigation in the desert is essentially an exercise in maximizing evaporation, and highly effective.

-- Flint (flintc@mindspring.com), May 06, 2000.

No Spam Please, and Flint,

You are both correct, and I neglected to consider the amount of water that is lost due to evaporation when used for irrigation in arid areas. I should have picked up on this instantly, as I do live in a semi arid area (400 mm rain per annum) and I am well aware of the fact the little of the water diverted for irrigation ever finds its way back to main river channel.

Data on the River Nile shows that Egypt takes 5.55*10^9 cubic meters of water per annum out of an average flow of 9.97*10^9 cubic meters. Or over half the water is diverted and therefore lost.

-- Malcolm Taylor (taylorm@es.co.nz), May 06, 2000.