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Author Topic: Global Warming  (Read 14907 times)

Offline JimBob

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Global Warming
« on: 01/06/2006 17:28:36 »
Michael's post about sliding glaciers has again reminded me of the sporatic and dispersed comments in "Enviornmental Issues" about Global Warming. SO ......

Is Global Warming real? Is the data coming in now only reflective of normal changes? WHY do you view the data as you do?



The mind is like a parachute. It works best when open.  -- A. Einstein


 

another_someone

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Re: Global Warming
« Reply #1 on: 02/06/2006 12:38:12 »
Not sure what you mean by 'why do you view the data as you do'?  Are you asking what is the data, or what are the psychological imperatives that cause one person to view the data differently from someone else.

My own view on matters come from the fact that my interest, apart from science and computing, also include history.  Looking at climate change from a longer term perspective, it is very difficult to see anything particularly unusual or out of the ordinary about the climate.  Both the direction of change (which started in the 1660s) and the magnitude of change are all within what might be considered in the longer term as normal.  It may be abnormal if one takes only a very short snapshot of data, but over the longer term, it is perfectly normal.

From a more human perspective, the notion that all environmental change is the fault of humans is itself quite normal.  In the past, when natural disasters happened, it was perfectly normal for people to believe they had somehow angered the God(s), and that the God(s) were somehow punishing them.  If one looks at the challenges that natural climate change brings upon us, we see exactly the same human response, except that the God is now called Gaia, or just called 'the Environment', but it is otherwise exactly the same idea of humans having angered some outside force, and that outside force punishing human kind.

Ofcourse, climate change has, and still can, destroy civilisations.  The point is that the climate change itself is a natural phenomenon, and we cannot stop it.  We can learn to adapt to it, or we simply beat ourselves up for being naughty boys, and feel sorry for all the evil we have done, without having the slightest impact on the process itself.



George
 

Offline Laith

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Re: Global Warming
« Reply #2 on: 02/06/2006 13:26:56 »
When you say 'Longer term' how many years are you taking about? thousands? millions?

Laith
 

Offline JimBob

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Re: Global Warming
« Reply #3 on: 02/06/2006 19:28:26 »
Both George and Laith have asked great querstions. Both are subjective, as short to me as a geologist is a million years, but it is one hell'ov'a long time w.r.t. recorded history.

The "WHY" is whatever reason one holds the position, be it based on data or other.

Each person here will approach the problen from a slightly different point of view.

To set limits limits discussion. I am interested in every opinion, not the one I hold. It is a question posed to get the most diverse responses possible.



The mind is like a parachute. It works best when open.  -- A. Einstein
 

another_someone

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Re: Global Warming
« Reply #4 on: 02/06/2006 20:09:55 »
quote:
Originally posted by Laith
When you say 'Longer term' how many years are you taking about? thousands? millions?



All.

The shortest cycle one can think of is the 11 year cycle of sun spots (technically, it is a 22 year cycle, but the effect upon us is an 11 year cycle).

The more significant historic cycle was at its coldest around 1660, and at its warmest

http://en.wikipedia.org/wiki/Maunder_minimum
quote:

The Maunder Minimum is the name given to the period roughly from 1645 to 1715 A.D., when sunspots became exceedingly rare, as noted by solar observers of the time. It is named after the later solar astronomer E.W. Maunder who discovered the dearth of sunspots during that period by studying records from those years. During one 30-year period within the Maunder Minimum, for example, astronomers observed only about 50 sunspots, as opposed to a more typical 40,000–50,000 spots.


The Maunder Minimum coincided with the middle — and coldest part — of the so-called Little Ice Age, during which Europe and North America, and perhaps much of the rest of the world, were subjected to bitterly cold winters.
Whether there is a causal connection between low sunspot activity and cold winters is the subject of ongoing debate. Some scientists believe that solar variation drives climate change more than carbon dioxide does (see global warming).



http://en.wikipedia.org/wiki/Little_Ice_Age
quote:

Solar activity


During the period 1645–1715, right in the middle of the Little Ice Age, solar activity as seen in sunspots was extremely low, with some years having no sunspots at all. This period of low sunspot activity is known as the Maunder Minimum. The precise link between low sunspot activity and cooling temperatures has not been established, but the coincidence of the Maunder Minimum with the deepest trough of the Little Ice Age is suggestive of such a connection . The Sporer Minimum has also been identified with a significant cooling period during the Little Ice Age. Other indicators of low solar activity during this period are levels of carbon-14 and beryllium-10  The low solar activity is also well documented in astronomical records. Astronomers in both Europe and Asia documented a decrease in the number of visible solar spots during this time period.

Volcanic activity


Throughout the Little Ice Age, the world also experienced heightened volcanic activity. When a volcano erupts, its ash reaches high into the atmosphere and can spread to cover the whole earth. This ash cloud blocks out some of the incoming solar radiation, leading to worldwide cooling that can last up to two years after an eruption. Also emitted by eruptions is sulfur in the form of SO2 gas. When this gas reaches the stratosphere, it turns into sulfuric acid particles, which reflect the sun's rays, further reducing the amount of radiation reaching the earth's surface. The 1815 eruption of Tambora in Indonesia blanketed the atmosphere with ash; the following year, 1816, came to be known as the Year Without A Summer, when frost and snow were reported in June and July in both New England and Northern Europe.



http://en.wikipedia.org/wiki/Medieval_Warm_Period
quote:

The Medieval Warm Period (MWP) or Medieval Climate Optimum was an unusually warm period during the European Medieval period, lasting from about the 10th century to about the 14th century. It has been argued a better name would be the Medieval Climatic Anomaly. The MWP is often involved in contentious discussions of global warming and the greenhouse effect.
Initial research on the MWP and the following Little Ice Age (LIA) was largely done in Europe, where the phenomenon was most obvious and clearly documented.
It was initially believed that the temperature changes were global. However, this view has been questioned; the 2001 IPCC report summarises this research, saying: "…current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of 'Little Ice Age' and 'Medieval Warm Period' appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries".
During this time wine grapes were grown in Europe as far north as southern Britain although less extensively than they are today (however, factors other than climate strongly influence the commercial success of vineyards; and the time of greatest extent of medieval vineyards falls outside the MWP). The Vikings took advantage of ice-free seas to colonize Greenland and other outlying lands of the far north. The period was followed by the Little Ice Age, a period of cooling that lasted until the 19th century when the current period of global warming began.


The Medieval Warm Period partially coincides in time with the peak in solar activity named the Medieval Maximum (AD 1100–1250).
In Chesapeake Bay, Maryland, researchers found large temperature excursions during the Little Ice Age (~AD 1400–1850) and the Medieval Warm Period (~AD 800–1300) possibly related to changes in the strength of North Atlantic thermohaline circulation. Sediments in Piermont Marsh of the lower Hudson Valley show a dry Medieval Warm period from AD 800–1300.
Prolonged droughts affected many parts of the western United States and especially eastern California and the western Great Basin.[6] Alaska experienced three time intervals of comparable warmth: 1–300, 850–1200, and post-1800 AD.
A radiocarbon-dated box core in the Sargasso Sea shows that sea surface temperature was approximately 1°C cooler than today approximately 400 years ago (the Little Ice Age) and 1700 years ago, and approximately 1°C warmer than today 1000 years ago (the Medieval Warm Period). However, all the reconstructions, as shown above, appear to indicate that it was not.



http://en.wikipedia.org/wiki/Holocene_climatic_optimum
quote:

The Holocene Climate Optimum was a warm period during roughly the interval 9,000 to 5,000 years B.P.. This event has also been known by many other names, including: Hypisthermal, Altithermal, Climatic Optimum, Holocene Optimum, Holocene Thermal Maximum, and Holocene Megathermal.
This warm period was followed by a gradual decline until about 2,000 years ago.

  • For other temperature fluctuations see: Temperature record

  • For other past climate fluctuation see: Paleoclimatology

  • For the pollen zone and Blytt-Sernander period associated with the climate optimum, see: Atlantic (period)




The Holocene Climate Optimum warm event consisted of increases of up to 4 °C near the North Pole (in one study, winter warming of 3-9°C and summer of 2-6°C in northern central Siberia)[1]. Northwestern Europe experienced warming, while there was cooling in the south.[2] The average temperature change appears to have declined rapidly with latitude so that essentially no change in mean temperature is reported at low and mid latitudes. Tropical reefs tend to show temperature increases of less than 1 °C. In terms of the global average, the typical shift was probably between 0.5 and 2 °C warmer than the mid-20th century (depending on estimates of latitude dependence and seasonality in response patterns).



What seems interesting to me is that the  Holocene Climate Optimum seems to be around the time humans started to develop argiculture.

On the longer term yet:

http://en.wikipedia.org/wiki/Milankovitch_cycles
quote:

Milankovitch cycles are the collective effect of changes in the Earth's movements upon its climate, named after Serbian geophysicist Milutin Milankovi#263;. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of the Quaternary glaciation over the last few million years. The Earth's axis completes one full cycle of precession approximately every 26,000 years. At the same time, the elliptical orbit rotates, more slowly, leading to a 22,000 year cycle in the equinoxes. In addition, the Earth's tilt relative to the Sun changes between 21.5 degrees to 24.5 degrees and back again on a 41,000 year cycle. The Earth's axis today is tilted 23.44 degrees relative to the normal to the plane of the ecliptic.
The Milankovitch theory of climate change is not perfectly worked out; in particular, the largest response is at the 100,000 year timescale, but the forcing is apparently small at this scale – see Ice age for more discussion. Various feedbacks (from CO2, or from ice sheet dynamics) are invoked to explain this discrepancy.
Milankovitch-like theories were advanced by Joseph Adhemar, James Croll, Milutin Milankovi#263; and others, but verification was difficult due to the absence of reliably dated evidence and doubts as to exactly which periods were important. Not until the advent of deep-ocean cores and the seminal paper by Hayes, Imbrie and Shackleton "Variations in the earths orbit: pacemaker of the ice ages" in Science, 1976, did the theory attain its present state.





George
« Last Edit: 02/06/2006 20:11:57 by another_someone »
 

Offline ukmicky

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Re: Global Warming
« Reply #5 on: 02/06/2006 20:59:53 »
Wouldnt it be funny if in 200 or so years time when they have a much better understanding of how climate change works and dont rely so much on guess work, they work out that all the C02 that we're pumping into the atmosphere actually saved us from a devastating ice age.




Michael
« Last Edit: 02/06/2006 21:17:14 by ukmicky »
 

Offline JimBob

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Re: Global Warming
« Reply #6 on: 02/06/2006 21:25:55 »
Yes, That may be a possibility.

But what about the possibility of the warmenr temps. causing all the Methane hydrate in the oceans to melt, all the methane exters the atmosphere and PRESTO, grenhouse? Is this going to happen?



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another_someone

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Re: Global Warming
« Reply #7 on: 02/06/2006 23:25:50 »
quote:
Originally posted by JimBob
But what about the possibility of the warmenr temps. causing all the Methane hydrate in the oceans to melt, all the methane exters the atmosphere and PRESTO, grenhouse? Is this going to happen?



Firstly, the Earth is not at the warmest it has ever been, so if this is to happen, then it is likely to have happened at all.  It may indeed have happened in the past, but if it has happened in the past, then it will be unquestionably inevitable that it will happen some time in the future.

The methane hydrates are a building problem no matter what happens.  One cannot expect the hydrates to continually accumulate without limit.

I understand the Japanese are looking at mining methane hydrates as a fuel.  This sounds eminently sensible, but will probably not have a major impact upon the total amount of methane hydrates under the sea for some considerable time.

One compensating factor regarding the hydrates is that as the sea levels rise one would expect the pressure under which the hydrates are places increases, and so will do more to keep them trapped, until the sea levels start to fall again.

Another factor that is not mentioned regarding increased global temperatures is that it will cause increased rainfall, which could well increase the amount of earthquakes that one would expect (as the increased moisture will help lubricate the tectonic plates).

But the underlying question is, while it is important to do as much as possible to understand what is likely to happen, it is even more important to ask whether we can do anything about it, or better simply to learn to roll with it.



George
« Last Edit: 02/06/2006 23:45:57 by another_someone »
 

Offline ukmicky

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Re: Global Warming
« Reply #8 on: 02/06/2006 23:45:50 »
I Remember seeing aprogram or reading somewhere , cant think where .anyway a study was done into the methane hydrate problem .

what they did was look at the warm periods in the earths history which could be tested through the melting OF core samples of ice in order to release the traps gases. They wanted to see if any of the worlds periods of global warming were caused by or affected by the release of methane hydrates. They found no evidence of elevated methane in the atmosphere during any period of global warming.

I will look to se if there is something about it on the web , there must be something

Michael
« Last Edit: 02/06/2006 23:46:36 by ukmicky »
 

another_someone

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Re: Global Warming
« Reply #9 on: 02/06/2006 23:50:07 »
quote:
Originally posted by ukmicky
what they did was look at the warm periods in the earths history which could be tested through the melting core samples of ice to release the traps gases. They wanted to see if any of the worlds periods of global warming were caused by or affected by the release of methane hydrates. They found no evidence of elevated methane in the atmosphere during any period of global warming.



Would you actually find traces of methane?  Would one not expect that the methane would quickly oxidise and mostly be manifest in raised CO2 and water levels as a product of methane oxidation.



George
 

Offline ukmicky

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Re: Global Warming
« Reply #10 on: 03/06/2006 00:04:42 »
There was also a program last week on sky digital bbc 4 channel where they were looking into global warming and they looked at the most recent historical periods . They did the same thing and tested the gases released from melted ice taken from core samples looking for evidence of raised methane and found nothing. So their must be someway which they can test for it.

Michael
« Last Edit: 03/06/2006 00:21:24 by ukmicky »
 

Offline JimBob

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Re: Global Warming
« Reply #11 on: 03/06/2006 04:06:09 »
First, to clarify one issue - Water does not lubricate faults. Not even oil can do it. Lift the hood (bonnet) of a car to get an idea of the effect of a small amount of force on either substance.

Methane hydrate is a problem and has been historically (Geolologic history, that is.)

A mass extinction, such as has happened in the past, does not happen all at once. It happens over a period of time - somewhere between 500,000 years and 1,000,000 years. This is a blink of the eye in geologic time but an eternity to an amphibian. There have been mass extinctions many times in the past. The chart below shows the events that can be documented.



The Permian event is a labeled with a "P".  (Wikipedia.) We are presently on the left side of the chart.  

The Permian was also a time of intense glaciation, as intense as or greater than the last set of glaciations (There have been a minimum of six recognized pulses of glaciations and 4-5 interglacial periods in the last million years.

Several ideas for these have been proposed but none have enough evidence to be proven scientifically. To me the most promissing is the methane hydreate theory. See the thread http://www.thenakedscientists.com/forum/topic.asp?TOPIC_ID=4443)

Methane hydrate is, in my opinion, a better cause for this than a meteorite. The effects of a meteorite ar catastrophic. Extinctions are only catastrophic in geologic time. The amount of Oxygen 18 can be determined from fossiles (if an oxygen compound is involved in fossilization) and the earth's heat (and possible methane hydrate) can be deduced from this fossil evidence.



I cannot say for sure if there is a reason for extinctions or glaciation. There is a ROUGH correlation between climate tempuraturr.

Other Ideas are needed.



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another_someone

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Re: Global Warming
« Reply #12 on: 03/06/2006 10:05:47 »
quote:
Originally posted by JimBob
First, to clarify one issue - Water does not lubricate faults. Not even oil can do it. Lift the hood (bonnet) of a car to get an idea of the effect of a small amount of force on either substance.



My understanding is that there is often observed an increase in seismic activity around areas where large water dams have been built, and I believe that this is supposed to be caused by the water seeping down into the rock and lubricating the underlying faults.

Clearly, a slight increase in rainfall is unlikely to have a dramatic effect, but an increase in sea level and the increasing number of inland areas covered by water (e.g. new or increased lakes) might well have the same effect as these dams.



George
 

another_someone

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Re: Global Warming
« Reply #13 on: 03/06/2006 10:10:58 »
quote:
Originally posted by ukmicky
There was also a program last week on sky digital bbc 4 channel where they were looking into global warming and they looked at the most recent historical periods . They did the same thing and tested the gases released from melted ice taken from core samples looking for evidence of raised methane and found nothing. So their must be someway which they can test for it.



People look for radio signals sent by extra-terrestrial civilisations, but it does not mean that they must inevitably have a means of correctly identifying them.  They do the simplest of tests, because that is all they are capable of doing, and a positive result of such a simple test would still be meaningful, but it does not mean that a negative result to the test would prove it was never there, only that they have not yet found the evidence of it being there.



George
 

Offline JimBob

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Re: Global Warming
« Reply #14 on: 03/06/2006 18:23:21 »
Michael is correct. Inclusions in the ice cores contain the atmosphere of the earth when the ice around such inclusion was formed. These inclusions are easily analysised by normal conventional methods and it is positive proof of atmosphere composition. Rigorous testing and discussion of the value of the gas analyisies by concerned scientist, both practical and theoretical, has allowed gas inclusions to become the benchmark for scientif analysis of ancient atmospheres.


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another_someone

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Re: Global Warming
« Reply #15 on: 03/06/2006 19:15:23 »
quote:
Originally posted by JimBob
Michael is correct. Inclusions in the ice cores contain the atmosphere of the earth when the ice around such inclusion was formed. These inclusions are easily analysised by normal conventional methods and it is positive proof of atmosphere composition. Rigorous testing and discussion of the value of the gas analyisies by concerned scientist, both practical and theoretical, has allowed gas inclusions to become the benchmark for scientif analysis of ancient atmospheres.



I did not doubt that they would have known if  there were substantial differences in the amount of methane in the atmosphere.  My conjecture is that a large increase in methane would quickly be oxidised to CO2 and water, so even if the hydrates did suddenly escape, how long would that methane still remain detectable in the atmosphere?

What is the average lifetime of methane in the atmosphere?

We do know there are times when there was massive increase in CO2, with no presently known cause, and oxidation of methane being one possible cause.

Now, if we were talking about CFC's, those remain intact in the atmosphere for decades after release, and would be very easy to directly detect in historic ice sheets.



George
« Last Edit: 03/06/2006 19:24:34 by another_someone »
 

Offline JimBob

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Re: Global Warming
« Reply #16 on: 03/06/2006 20:46:43 »
You are correct, as always. Methane lasts in the atmosphere a minimum of 12 years, then is oxidized. But the last time I checked, CO2 was also a greenhouse gas. And the oxidation reduces the amount of free oxygen in the atmosphere, leaving us all gasping for breath.

The effects of methane in the atmosphere and the resultant rise in carbon lasts for hundreds of years.






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Offline ukmicky

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Re: Global Warming
« Reply #17 on: 03/06/2006 21:17:13 »
They can tell by measuring the levels of certain stable carbon isotopes released when they melt the core samples. I'm still investigating, But it seems their was a large increase of methane in the atmosphere after the last ice age but whether it was made up of the type which comes from the release of hydrates or through rotting vegetation is the big question.


 
quote:
molecule of methane consists of four hydrogen atoms and one carbon atom. Carbon comes in several isotopic varieties, with different weights. Most of it is an isotope known as carbon-12 (12C), but there is always a smattering of a heavier isotope, 13C, mixed in. Because 13C is heavier, organisms tend to absorb it less readily than they do 12C. This means that carbon which comes from a plant, or a swamp-dwelling bacterium that is digesting that plant, will have less 13C in it than did the carbon in the atmosphere from which the plant drew its sustenance.

However, methane from undersea hydrates has an even bigger 13C deficit than that from surface bacteria. This is because it has been generated by bugs called archaeobacteria. Since this “deep biosphere” is such an inhospitable environment, these archaeobacteria need to be even more discriminating in their choice of carbon isotopes than organisms at the surface. So looking at the amount of 13C in bubbles formed at the end of the ice age should help to resolve the issue of where the methane came from.
http://www.economist.com/science/displayStory.cfm?story_id=1714858

Michael
« Last Edit: 03/06/2006 22:01:37 by ukmicky »
 

Offline VAlibrarian

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Re: Global Warming
« Reply #18 on: 04/06/2006 03:01:23 »
JimBob, to respond to your original question: "Why do you view the data as you do?"
I see the same data that any curious/somewhat educated person sees. To me, the data represents a danger that my great-grandkids (if any) will inherit a world of higher temperatures and a higher likelihood of both floods and droughts. I view the data through this lens because, like most atmospheric scientists, I see a connection between the 30% increase in atmospheric CO2 resulting from human activities and the 1 degree increase in observed temperatures. Others obviously look at these two things and see a coincidence. There is no causality here for them, and they look for explanations elsewhere.
I do not presume to explain to the atmospheric scientists that I know science better than they do, because I do not. But when I stir pepper into a stew and notice a more pungent taste, I think I know why.

chris wiegard
 

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Re: Global Warming
« Reply #19 on: 04/06/2006 09:45:21 »
quote:
Originally posted by VAlibrarian
But when I stir pepper into a stew and notice a more pungent taste, I think I know why.



The problem with this analogy is that you have a long experience of cooking stews, and of using black pepper.

If a black cat crosses your path, and you then get run over by a motor car, you may imply a causal link, although others may consider it premature to do so.  After the 10th such similar event, and if you have never been run over by a motor car when there was no black cat crossing your path, it may become at least reasonable to suggest that a black cat crossing your path is a good predictor of your being run over by a motor car, but it does not mean that it is a causal link, and it does not mean that if you set about to kill the black cat then it will make you immune from being run over by motor cars.

The proponents of the human cause of global warming are looking only at a single instance in history, and that covering only a few decades in time.  Scarcely what you might regard as a clear pattern.  The trouble is that to look for patterns in weather, one has to look at time spans that extend beyond the industrial revolution, which rather undermines the very causal link the proponents of industrially caused global warming would like to show.  The patterns do exist (albeit, we evidently still do not fully understand them), and they do clearly show that what we see in the climate today is not at all unusual in periods of time long before the industrial revolution.



George
« Last Edit: 04/06/2006 10:37:59 by another_someone »
 

another_someone

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Re: Global Warming
« Reply #20 on: 04/06/2006 10:02:38 »
quote:
Originally posted by VAlibrarian
 To me, the data represents a danger that my great-grandkids (if any) will inherit a world of higher temperatures and a higher likelihood of both floods and droughts.



Although there is constantly a shifting pattern of weather, and some areas will become drought ridden where they were not before, but global warming as such is unlikely to cause that.  Historic data, as well as common sense, associates widespread drought with global cooling.  Drought is not caused by heat but by lack of water, which infers lack of rainfall, which when taken on a global scale will relate to reduced evaporation caused by lower temperatures.




George
 

Offline JimBob

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Re: Global Warming
« Reply #21 on: 05/06/2006 13:14:12 »
Not quite accurate. 10,000 years ago earth was cooloer than it is now and there was much less desert than now. All the area of central asia was much more lush: areas that are now the worst deserts of the world (such as the Taklamakan) were irrigated, had no erosion and no sand. Today you die in them. This is true for all of Africa as well. There was no Sahara Desert, no Namibian Desert, etc.

All of these deserts were well watered, had a diverse fauna and flora as well as human habitation.

The warming of the earth during this interglacial is real. The question is whether or not man has accelerated this process so: Again I ask ---

Is Global Warming real? Is the data coming in now only reflective of normal changes? WHY do you view the data as you do?


 


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Re: Global Warming
« Reply #22 on: 05/06/2006 14:42:53 »
quote:
Originally posted by JimBob

Not quite accurate. 10,000 years ago earth was cooloer than it is now and there was much less desert than now. All the area of central asia was much more lush: areas that are now the worst deserts of the world (such as the Taklamakan) were irrigated, had no erosion and no sand. Today you die in them. This is true for all of Africa as well. There was no Sahara Desert, no Namibian Desert, etc.

All of these deserts were well watered, had a diverse fauna and flora as well as human habitation.

The warming of the earth during this interglacial is real. The question is whether or not man has accelerated this process so: Again I ask ---



http://www.scienceblog.com/community/older/1996/A/199600627.html
quote:

MADISON -- Water would seem like a mirage today in the sweltering Sahara Desert, but climate researchers at the University of Wisconsin-Madison are finding the ancient Sahara was a wetter, greener place than ever imagined.
Writing in today's (Dec. 19) edition of the British journal Nature, UW-Madison climatologist John Kutzbach and colleagues report that the Sahara and Sahel regions of northern Africa were much wetter 12,000 to 5,000 years ago than earlier climate models predicted.


A slight shift in the earth's orbit forced those changes, causing stronger summer monsoons to sweep through the region. This naturally produced more vegetation and increased water content in the soil.



http://www.innovations-report.com/html/reports/earth_sciences/report-54055.html
quote:

The earliest humans in the area were Palaeolithic hunter-gatherers, who lived in the Fazzan between about 400,000 and 70,000 years ago. They survived by hunting large and small game in a landscape that was considerably wetter and greener than it is now. A prolonged arid phase from about 70,000 to 12,000 years ago apparently drove humans out of the region, but then the rains returned – along with the people.

Around 5,000 years ago the climate began to dry out again, but this time people adapted by developing an agricultural civilization with towns and villages based around oases. This process culminated with the emergence of the Garamantian society in the first millennium BC.



http://upload.wikimedia.org/wikipedia/en/c/ca/Holocene_Temperature_Variations.png


You will see that around 10,000 the weather was not much different from what it is today, and was still getting warmer.  If you go back to beyond 12,000 years ago, the weather was indeed cooler than it is today, but that was the period in which the sahara was just coming out of its long arid phase.

The weather started to become slightly cooler around 5,000 years ago, as the sahara started to dry out again.



George
« Last Edit: 05/06/2006 14:45:29 by another_someone »
 

Offline JimBob

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Re: Global Warming
« Reply #23 on: 07/06/2006 01:10:13 »
quote:
Originally posted by another_someone

...  Historic data, as well as common sense, associates widespread drought with global cooling.  Drought is not caused by heat but by lack of water, which infers lack of rainfall, which when taken on a global scale will relate to reduced evaporation caused by lower temperatures.




George




This just isn't the way we earth scientists believe that desert climates are brought about and sustained. Although arid, the Sahara was not a desert until recently (I am not going to carp about dates). Common sense, my 40 years as an earth scientist and personal observations have convinced me cold is NOT the cause of the majority of desert climates.

The majority of land area covered by desert is HOT. I have long been aware that a hot desert is caused not by cold or lack of rain but primarily by excessive heat evaporaiting rainfall before it can reach the ground. This mechanism is what forms all deserts except those at high latitudes, the cold deserts. By deifnition, I live in one of the hot deserts, the Sonoran Desert - average rainfall less than 15 cm of rain a year and high heat. It is to be 100 degrees F and above this weekend. I live amid grass, deer, cougars, trees including pine trees, rabbits, and cattle in profusion, etc, etc. Arid does not mean void of life. I live at about the same latitude as the deep Sahara, about 32 deg N. To the west of where I live is dessicated desert but this is also the real-life location of the huge cattle ranches of the classic western movies. Billions of pounds of beef have been produced off this land and driven north to the 1860-70 rail lines. It was originally grass land, had a higher rainfall, but is now dessicated desert due to over-grazing.

"It is a common misconception that droughts cause desertification."
http://en.wikipedia.org/wiki/Desertification

I can assure you that man has also lived here for much more than 10,000 years. The area where I live was the home of Karankawa, Tonkawa, and Comanche Indians. The classic Clovis point of the Native Americans is found 600 miles north-west of where I am, in the deep desert. Clovis, New Mexico is in the heart of cattle country.

The Sahara as we know is new. It was inhabited and much more productive than present, although an arid place, until primarily the pig was domesticated. The pig is what is thought to be the main caused the denudation and desertification of the Sahara. Up until about the time of the pig domestication it is believed the Sahara ws only arid, not a sand waste. Even as it is now the Sahara is inhabited and productive or people could not live there. Yes, I know what your references say. The man from Reading is in a small minority.

"Neolithic rock engravings, or 'petroglyphs' and the megaliths in the Sahara desert of Libya attest to early hunter-gatherer culture in the dry grasslands of North Africa during the glacial age. The region of the present Sahara was an early site for the practice of agriculture (Wavy-line ceramics). However, after the desertification of the Sahara, settlement in North Africa became concentrated in the valley of the Nile, where the pre-literate Nomes of Egypt laid a base for the culture of ancient Egypt. Archeological findings show that primitive tribes lived along the Nile long before the dynastic history of the pharaohs began. By 6000 B.C., organized agriculture had appeared."

http://en.wikipedia.org/wiki/History_of_Africa

Present day -  

"2.5 million people live in the Sahara, most of these in Egypt, Mauritania, Morocco and Algeria. Dominant ethnicities in the Sahara are various Berber groups, ..."

http://en.wikipedia.org/wiki/Sahara.

And from http://www.worldwildlife.org/wildworld/profiles/terrestrial/pa/pa1327_full.html

"The Sahara is the largest desert in the world and occupies approximately 10 percent of the African Continent. The ecoregion includes the hyper-arid central portion of the Sahara where rainfall is minimal and sporadic. Although species richness and endemism are low, some highly adapted species do survive with notable adaptations. Only a few thousand years ago the Sahara was significantly wetter, and a large mammal fauna resided in this area. Climatic desiccation over the past 5000 years, and intense human hunting over the past 100 years, has obliterated these faunas. Now only rock, sand and sparse vegetation exist over huge areas. The remnant large mammal fauna is highly threatened by over-hunting." BOLD IS MINE.

I am also aware you will argue that the Sahara was wetter ONLY during the period from ~12,000 to ~5000 years ago. That just isn't supported by the amount of data gathered.  

Again I ask, do you believe man has accelerated global warming?



The mind is like a parachute. It works best when open.  -- A. Einstein
« Last Edit: 07/06/2006 01:11:40 by JimBob »
 

another_someone

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Re: Global Warming
« Reply #24 on: 07/06/2006 02:32:09 »
There are a number of issues you have mentioned, and it is not easy to disentangle them.

quote:

Although arid, the Sahara was not a desert until recently



quote:

By deifnition, I live in one of the hot deserts, the Sonoran Desert - average rainfall less than 15 cm of rain a year and high heat. It is to be 100 degrees F and above this weekend. I live amid grass, deer, cougars, trees including pine trees, rabbits, and cattle in profusion, etc, etc. Arid does not mean void of life.



So, arid does not mean devoid of life (no problem with that), but is a desert defined by its aridity or its life?

You say you live in a desert because it is hot and arid, yet is teaming with life.

You say the sahara was not a desert until recently, although it was hot and arid (I assume you are saying it was not a desert because it was, like the sonoran, teaming with life).

You seem to be saying that until recently the sahara was like the sonoran is today, yet the sahara was not a desert but the sonoran is?

Semantics aside, the issue I was raising was not so much about whether man has effected the flora and fauna over the millennia, but only what the relationship between temperature and rainfall is (not between the flora and fauna and temperature), and what the relationship between human activity and global temperature is.

Clearly, man has had a major impact upon the flora and fauna, in all sorts of ways, and continues to do so.  That overuse of the soil, and that denuding the soil of its vegetable cover, has had a major impact, and that man has been a major component in the impact, is (at least by me) accepted without question.  None of this is to do with global warming, nor with global rainfall (whether it impacts on local rainfall – possibly so, although even your own comments do not seem to suggest that man's activity made the deserts arid, only that man denuded the soil; but I would contend that man's impact on global rainfall is marginal, if exists at all).





George
« Last Edit: 07/06/2006 02:38:16 by another_someone »
 

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Re: Global Warming
« Reply #24 on: 07/06/2006 02:32:09 »

 

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