Bob wrote:
On 10 Dec, 00:27, "dennis@home" wrote:
"Bob" wrote in message
A very odd feedback mechanism..
if it were simple then the first hot day we have will result in more water
vapour..
leading to higher temps..
leading to more water vapour..
leading to the end of life as we know it Jim.
First of all, do you accept that water vapour is a greenhouse gas?
I'll assume so because that was the premise of John Rumm's message.
The runaway feedback you describe clearly *doesn't* happen otherwise
we wouldn't be here to talk about it.
It has happened in the past and yes, we weren't around to talk about it.
Life as we know it depends more or less on the temperatures being
somehat between 0C and 50C. Maybe 60C. water needs to go up as vapour
and come down as rain. Too hot and it wont rain. Too cold and it never
goes up in the first place.
Her is an interesting clip, for a start.
(
http://en.wikipedia.org/wiki/Snowball_Earth)
"In the 1960s, Mikhail Budyko, a Russian climatologist, developed a
simple energy-balance climate model to investigate the effect of ice
cover on global climate. Using this model, Budyko found that if ice
sheets advanced far enough out of the polar regions a feedback ensued
where the increased reflectiveness (albedo) of the ice led to further
cooling and the formation of more ice until the entire Earth was covered
in ice and stabilized in a new ice-covered equilibrium.[6] While
Budyko's model showed that this ice-albedo stability could happen, he
concluded that it had never happened, because his model offered no way
to escape from such a scenario."
and
"Breaking out of global glaciation
The carbon dioxide levels necessary to unfreeze the Earth have been
estimated as being 350 times what they are today, about 13% of the
atmosphere.[52] Since the Earth was almost completely covered with ice,
carbon dioxide could not be withdrawn from the atmosphere by release of
alkaline metal ions weathering out of siliceous rocks. Over 4 to 30
million years, enough CO2 and methane, mainly emitted by volcanoes,
would accumulate to finally cause enough greenhouse effect to make
surface ice melt in the tropics until a band of permanently ice-free
land and water developed;[53] this would be darker than the ice, and
thus absorb more energy from the sun €” initiating a "positive feedback."
On the continents, the melting of glaciers would release massive amounts
of glacial deposit, which would erode and weather. The resulting
sediments supplied to the ocean would be high in nutrients such as
phosphorus, which combined with the abundance of CO2 would trigger a
cyanobacteria population explosion, which would cause a relatively rapid
reoxygenation of the atmosphere, which may have contributed to the rise
of the Ediacaran biota and the subsequent Cambrian explosion €” a higher
oxygen concentration allowing large multicellular lifeforms to develop.
This positive feedback loop would melt the ice in geological short
order, perhaps less than 1,000 years; replenishment of atmospheric
oxygen and depletion of the CO2 levels would take further millennia.
Destabilization of substantial deposits of methane hydrates locked up in
low-latitude permafrost may also have acted as a trigger and/or strong
positive feedback for deglaciation and warming.[54]
It is possible that carbon dioxide levels fell enough for Earth to
freeze again; this cycle may have repeated until the continents had
drifted to more polar latitudes."
So that's the theories surrounding global freezing
One of the best sites that i have found giving a broad overview of the
various theories attempting to account for the earth last 500 million
years of climate change has this to say also:
(
http://web.me.com/uriarte/Earths_Cli...alaeocene.html)
--------------------------------------------------------
"Right on the boundary between the Palaeocene and the Eocene, around 55
million years ago, the temperature rose even higher, prompting a short
temperature peak known as the Palaeocene-Eocene Thermal Maximum.
It was a sudden global warming event which only lasted around 80,000
years, but nevertheless it had an enormous influence on the evolution of
animal life. The episode coincided with a major wave of extinctions
among the existing fauna, both on the continents and in the oceans, and
is coincident with the emergence of many new mammalian orders which have
dominated the animal kingdom ever since. Flora adapted by changing the
physiognomy of their leaves and by migrating to higher latitudes (Wing,
2005).
Continental temperatures, already high, rose again by between 5ΒΊ C and
7ΒΊ C. In the seas, the temperature of coastal surface waters in the
Antarctic rose from 13ΒΊ C to 20ΒΊ C, and in the Arctic, they reached as
high as 24ΒΊ C. Although the waters of subtropical regions also became
warmer, the effect was much more noticeable in the higher latitudes.
Deep water temperatures also rose (as during the warm mid-period of the
Cretaceous) to around 12ΒΊ C higher than the current day mean (Lear,
2000). This was probably due to a change in the principal location at
which deep waters were formed, which moved from the cold seas of the
southern hemisphere to the warmer ones of the northern hemisphere.
Carbon-13 analyses of sediments provide evidence pointing to this abrupt
circulatory change (Nunes, 2006).
It is believed that the PETM peak may have been caused by a sudden
increase in methane or carbon dioxide. The most reliable evidence of
this sudden increase in methane seems to lie in an abrupt high-low
oscillation of sedimentary carbon-13, since methane, due to its
biological origin, is very poor in this isotope.
The sudden release of methane into the atmosphere would have come from
the methane enclosed in ice crystals located in the sediments of the
ocean floor. The eruption of the gas may have occurred after the
temperature of oceans' deep waters passed a specific heat threshold,
thus enabling the defrosting of methane hydrates. It is possible that a
change in ocean circulation triggered this process (Tripati, 2005).
Nevertheless, the abundance of methane may also have been the result of
intense bacterial production in either the wetlands that covered vast
areas of tropical and mid-level regions during that period or the peat
bogs which formed in higher latitudes. However, the suddenness of the
episode seems to support the theory of the fusion of hydrates frozen in
the marine subsoil (Bains, 1999; Katz, 2000)."
-------------------------------------------------------------------
Its this particular event that worries climate changers the most.
Since we are already releasing large quantities of methane from the now
thawing siberian permafrost.
Also, the nearest period that seems to correspond with what we might be
doing now is the Eemian interglacial, when the world was about 4-5
degrees warmer than now, and the sea level 4-5 meters higher
From the same source
-----------------------------------------------------------------
Thanks to astronomical calculations related to the Earths movements, we
know that the solar radiation received during the summer months in the
northern hemisphere reached a maximum 127,000 years ago. This would then
have been the trigger for the melting of the northern ice sheets, a
process that subsequently gained momentum due to the fact that the melt
diminished the terrestrial albedo.
-----------------------------------------------------------------
In that case, it was earth's orbit that led to the warm period, but the
feedback mechanism - less ice=hotter planet=less ice still holds.
That plus the methane hydrate scenario, can lead to extremely rapid
variations in climate. Until some other mechanism like algal blooms,
takes over and starts sucking the CO2 back into depositable oil and tar
formations..
What emerges from that source, is not necessarily that CO2 is the
greatest driver, but that the climate is very capable if triggered, to
quite rapidly flip between different regimes of temperature with drastic
alteration to ice, snow, rainfall and sea levels. CO2 historically has
more accompanied the changes than caused them. HOWEVER by itself, in the
sorts of levels we have to day, it is quite capable of causing them.
Tht is where climate scientists are coming from There are at least ten
mechanisms interlinked - geological, astronomical, ecological..ocean
currents, land sea and air temperatures, CO2, methane and water vapour,
and their interplay through various biological agents like alga..and to
presume that just because no one burnt all the fossil fuel 1 million
years ago and caused a heatwave, doing it today wont, is deeply stupid.
CO2 by itself wont cause that much of an effect: But then neither does a
couple of percent hotter sun. Its the knock on effects that really
accelerate the thing. I dont know how much arctic ice has retreated in
the last decades, but its a lot. That in itself multiplies the effects
of whatever caused it a few tens of times, as those areas become warmer
without reflective ice cover. Thawing permafrost releases methane,
methane oxidise to yes, more CO2 and more water vapour. All three are
greenhouse gases.
So we might say that every gramme of industrial CO2 produces another 10g
of released CO2, and every degree that lot produces gives us another
million square miles of permafrost to thaw..and there is linkage
proposed between the little ice age and the black death
http://news.bbc.co.uk/1/hi/4755328.stm although I would more suppose
that a sharp drop in winter temperatures would more likel have led to
conditions ripe for a pandemic..
One thing is clear. CO2 levels are now higher than they ever have been
for hundreds of thousands of years. And oddly, asthma is more prevalent
than it has ever been. Ho hum.
And life has modified this planet before, and will do again. The only
difference is we may be smart enough to know its happening and why.
Not that that will make a blind bit of difference: Not with certain
folks around.