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Glom
2004-Oct-30, 04:32 PM
I've been thinking about the roles of two greenhouse gases.

Water vapour is the most potent greenhouse gas in the atmosphere. It is also the weirdest. Unlike other gases, it is incredible variable throughout the atmosphere, which adds so much uncertainty and difficulty for climate models because you have to know the distribution of water vapour for the conditions to be accurate. It just exacerbates the problems with the non-linear dynamics of the biosphere. The initial conditions are not even very predictable. Moreover, water vapour causes clouds, which introduce another variable in feedback, generally negative, making the non-linear aspect even more non-linear. Therefore, there is a major problem with accurate climate modelling.

Carbon dioxide is contained in vast quantities in the oceans. When the oceans heat up, they discharge carbon dioxide into the atmosphere. If carbon dioxide was the major temperature driver as is claimed, then this would lead to catastrophic positive feedback. Therefore, carbon dioxide is either not a major temperature driver, or it trigger a more powerful feedback, probably in water, that overcomes its influence. The result is that carbon dioxide's holy grail role in climate change does not make sense.

dgruss23
2004-Oct-30, 05:44 PM
I think you raise some good points Glom!

My suspicion is that the observed correlation between CO2 levels and climate changes has been interpreted backwards. Its possible that CO2 levels respond to climate changes rather than cause them.

Think about all the CO2 sinks that we're told will release more CO2 if the planet warms (Oceans, permafrost ...). Shouldn't that happen with solar forced climate warming too?

Its already been shown (http://www.sciencedaily.com/releases/2002/06/020607073439.htm) that the Sun-climate connection goes back at least 100,000 years. Now if warming causes CO2 to be released from the oceans, then wouldn't we predict that CO2 levels should also mirror those Solar forced climate changes?

aurora
2004-Oct-30, 06:25 PM
Now if warming causes CO2 to be released from the oceans, then wouldn't we predict that CO2 levels should also mirror those Solar forced climate changes?

CO2 levels have dropped over geologic time, as carbon was sequestered in rocks.

There's a pretty good discussion of this in Ward and Brownlee's last book Not Rare Earth, a later one the title of which escapes me at the moment but a search on Amazon should easily turn it up).

teddyv
2004-Oct-30, 06:41 PM
I have been interested in the years of talk of "global warming" in the media and in my experience no one ever brings up the subject of the role the Sun has in inducing weather and climate. Especially lately as (I believe) we are coming off a sunspot maximum which based on my limited knowledge usually corresponds to warmer and stranger weather patterns.

Also, regarding CO2 sequestering, if the oceans warm, should there not be a general increased deposition of carbonates in the form of corals and other carbonate skeletoned organisms (i.e. microfossils).

Glom
2004-Oct-30, 06:56 PM
I have been interested in the years of talk of "global warming" in the media and in my experience no one ever brings up the subject of the role the Sun has in inducing weather and climate. Especially lately as (I believe) we are coming off a sunspot maximum which based on my limited knowledge usually corresponds to warmer and stranger weather patterns.

I believe that we past a maximum in the eleven year cycle a couple of years ago. I'm not sure how much difference the eleven year cycle makes to the climate patterns overall. However, given the media's propensity for blaming every hurricane these days on AAGW, I am willing to entertain the idea that solar maxima do result in more extreme weather. However, I'm not willing to bet that the actual record will bear that out. I think the eleven year cycle happens too quickly to have much noticeable effect.

However, there are cycles far more macroscopic than the eleven year cycle which is seen by the envelope containing the eleven year oscillations. The envelope is reaching a 1500 year high at the moment, like it was during the Medieval warm period and had dropped to virtually zero during the Little Ice Age. There appears to be a good correlation.


Also, regarding CO2 sequestering, if the oceans warm, should there not be a general increased deposition of carbonates in the form of corals and other carbonate skeletoned organisms (i.e. microfossils).

The oceans themselves are a vast, vast sink of carbon, far greater than the atmosphere, in the form of the dissolved carbon dioxide. At warmer temperatures, the ability to a liquid solvent to dissolve a gas decreases so an increase in sea temperatures prompts a discharge of carbon dioxide.

Diamond
2004-Oct-30, 07:23 PM
I think you raise some good points Glom!

My suspicion is that the observed correlation between CO2 levels and climate changes has been interpreted backwards. Its possible that CO2 levels respond to climate changes rather than cause them.

Think about all the CO2 sinks that we're told will release more CO2 if the planet warms (Oceans, permafrost ...). Shouldn't that happen with solar forced climate warming too?

Its already been shown (http://www.sciencedaily.com/releases/2002/06/020607073439.htm) that the Sun-climate connection goes back at least 100,000 years. Now if warming causes CO2 to be released from the oceans, then wouldn't we predict that CO2 levels should also mirror those Solar forced climate changes?

As far as I can tell there is no evidence that rising carbon dioxide causes warming at all. The consistent result of every ice core taken is that the climate warms and then 8-10 centuries later, carbon dioxide starts to rise.

The current rising trend can be most simply described as a result of the warming that happened during the Medieval Warm Period roughly 800-1000 years ago.

The positive feedbacks do not exist. The Earth has had ten times the carbon dioxide concentration that it has today without a catastrophic "runaway greenhouse effect" occurring. If such positive feedbacks existed, we would not exist.

The rantings of global warming proponents is filled with the most lurid and blatently false scare-mongering, with phrases such as "tipping points" conveying an idea that the climate is on the cusp of some sort of collapse, when the actual evidence shows nothing of the kind. Throw in that the most influential study of climate (Mann, Bradley and Hughes 1998) has been shown to be fake, and you begin to wonder whatever happened to sober, reliable science and scientific reporting.

Glom
2004-Oct-30, 10:15 PM
The current rising trend can be most simply described as a result of the warming that happened during the Medieval Warm Period roughly 800-1000 years ago.

Interesting thought. What bit of evidence do you suggest would be good for validating it?

frogesque
2004-Oct-31, 10:32 AM
Is the level of disolved CO2 in the oceans anywhere near its saturation limit? If not (and I don't think it is) then a rise in ocean temperature of a degree or so wouldn't cause much extra release of gas. Also, the marine environment will react to a rise in temperature and micro organisims such as those that produce oil shales will increase. When Earth was a lot warmer there were also vast areas of palm forrest laying down what are now coal measures. Chalk, coral and limestone also tie up huge quantities of CO2, think about how they were/are formed. To me the real problem is de-aforestation and pollution which could interfere with Earth's ability to respond to any climatic change.

I don't think CO2 is the bad guy it's made out to be.

Diamond
2004-Oct-31, 11:44 AM
The current rising trend can be most simply described as a result of the warming that happened during the Medieval Warm Period roughly 800-1000 years ago.

Interesting thought. What bit of evidence do you suggest would be good for validating it?

Well, the evidence from every ice core showing the delay between the temperature rising and carbon dioxide rising. Here is one, for example:

http://www.john-daly.com/press/lag-time.gif

which comes from `Atmospheric CO2 Concentrations over the Last Glacial Termination' by Monnin et al, (Science, vol.291, p.112, 5 Jan 2001)

In any case, the global warming industry has spent most of its time re-writing the past so that the Little Ice Age and Medieval Warm Periods were no global events at all, but limited to the North Atlantic basin. This re-writing of history to smooth out all previous global climatic events means, of course, that the instrumental record from 1880 looks anomalous and therefore threatening.

Glom
2004-Oct-31, 12:05 PM
Cheers Diamond.

dgruss23
2004-Oct-31, 02:17 PM
Ahh ... gotta be careful reading that graph. The present is on the left not the right.

Launch window
2004-Oct-31, 02:51 PM
When we are trying to investigate climite change one very important factor must be looked at and that is the global mean standard temperature or GMST, scientists can predict how the water cycle affects the climate by looking over date which shows water vapour in the air, levels of precipitation, humidity and satellite photos of cloud formation. They can then create models and graphs when show the exchange of water between the atmosphere , land and ocean. Water vapour is a greenhouse gas and over the medium term the water cycle is effectively in a steady state however the distribution of water among the reservoirs may change. No this ain't the ice polar caps on Mars it's home
http://btc.montana.edu/ceres/Images/si_var.gif
Another things we must keep in mind is the possible bizzare actions of the current administartion, with many top scientists being very vocal on the issue. Some have been speaking out in the recent past, one of NASA's top scientists Hansen remarked the Administartion was Suppressing information on Climate-Change. How true is this statement, who knows but we must keep it in mind when dealing with the next report on climate change from the administration. Neither Republican or Democrat should be allowed go ahead in Suppressing our Scientists
http://www.badastronomy.com/phpBB/viewtopic.php?t=17293
When looking at the water cycle we could see how it can have an impact on the climate, focusing on another greenhouse gas - CO2 we can also look at it as part of the global carbon cycle. Today it seems mankind is having an effect on the entirecarbon cycle, it may be slow but the change seems to be coming. Global surface temperatures in 1999 set a new record by a wide margin for the period of instrumental measurements, report researchers at the NASA Goddard Institute for Space Studies who analyze data collected from several thousand meteorological stations around the world.Glaciers along the southeastern coast of Greenland are thinning by more than 3 feet a year -- possibly because of global warming, according to a new study by NASA scientists. A report released by the EPA confirms something that scientists have been insisting for years: human activities are largely to blame for the problem of global warming. Oil refining, power plants and auto emissions are singled out as major contributors to the problem. "Greenhouse gases are accumulating in the Earth's atmosphere as a result of human activities, causing global mean surface air temperatures and subsurface ocean temperatures to rise," the EPA states in the report. The inter-agency report was sent to the United Nations. It includes the forecast that total greenhouse emissions by the United States will increase 43 percent between 2000 and 2020. Pre-industrial levels of carbon dioxide (prior to the start of the Industrial Revolution) were about 280 parts per million by volume (ppmv), and current levels are about 370 ppmv. The concentration of CO2 in our atmosphere today, has not been exceeded in the last 420,000 years, and likely not in the last 20 million years. According to the IPCC Special Report on Emission Scenarios (SRES), by the end of the 21st century, we could expect to see carbon dioxide concentrations of anywhere from 490 to 1260 ppm The United States is the largest contributor to global warming. The secretariat of the United Nations Framework Convention on Climate Change (UNFCCC) relaunched its website on 11 October 2004 so you can check out the most up to dtae version. Nations have strongly criticized teh president's decision in 2001 to abandon the Kyoto treaty. Meanwhile Russia's upper house of parliament ratified the Kyoto Protocol and sent it to President Vladimir Putin for the final stamp of approval that would bring the global climate pact into force early next year. The Russians government system known as the Russian Federation Council voted 139-1 with one abstention to endorse the protocol, which aims to stem global warming by reducing greenhouse-gas emissions, thank goodness for the Ruskies at least some one gives a damn about Mother Earth. A number of people have stated afterward that without Russia's participation, the world community's efforts for many years to establish a global mechanism for solving environmental problems would be doomed to failure

Glom
2004-Oct-31, 05:20 PM
Russia's signing of Kyoto has absolutely nothing to do with "giving a damn about Mother Earth". It has to do with the fact that the EU bullied and bribed them to comply by obstructing their admission into the WTO unless they played ball.

The increase surface temperatures cannot be trusted because when adding in the uncertainty caused by the urban heat island effect, the increase measured is smaller than the error. In addition, a hundred or so years ago, extensive use of dirty coal burning meant that particulate pollution was everywhere blocking sunlight. This has been virtually eliminated in many areas in recent decades and so a greater amount of sunlight is reaching the surface. In other words, a temperature increase is expected from cleaning up human activities.

Further, less than 5% of carbon dioxide emissions are anthropogenic.

It's also interesting that you should say that we shouldn't trust any politicians' on this issue. I agree. But didn't Al Gore have a role in the shaping of IPCC reports?

Zero Signal
2004-Oct-31, 11:49 PM
Glom, Diamond, dgruss23, et al. I have a question that I consider relevant to this and other similar threads.

What exactly are your political philosophies? Are you all by any chance libertarians or otherwise strongly pro-free market people? Do you support the Cato Institute or the Junk Science webpage?

frogesque
2004-Nov-01, 12:30 AM
Zero Signal I would not presume to speak for others who are well able to speak for themselves.

I come to BA with a view to reading (and discussing if I think I have a valid point) items of technical, scientific and astronomical interest. The message board is well balanced, all are welcome as long as the board guidelines are observed and there is also the opportunity to let my hair down a bit sometimes on BABB, I have no agenda.

Zero Signal
2004-Nov-01, 12:38 AM
Zero Signal I would not presume to speak for others who are well able to speak for themselves.

I apologize if I came across that way :oops:. I meant not to speak for other, just mere ask about political persuasions. Every person who I've come across who denies an anthropogenic cause of global warming and/or claims that global warming is benificial are libertarians or otherwise ardent free-market supporters. I'm just curious as to whether or not this is the case here. If I'm wrong, then I digress. I'm sorry if I came across as presumptuous in my last post.

ChaosInc
2004-Nov-01, 12:53 AM
Finally a discussion of global warming with a little science to back it up! Nice.
A few questions: Diamond, how do they estimate the temperature that many years in the past? Also, I see the 800 year delay in the points of inflection, but if you didn't highlight them, I don't think I would have noticed. You are saying this occurs in other evidencs?

Launch, there is an EPA report that says (abridging your quote) humans are causing global warming? Would you cite the report?

frogesque
2004-Nov-01, 12:57 AM
No need to apologise Zero Signal. I think what you may find is that there is a balance, point and counterpoint that make the board lively and interesting. There are many shades of grey and green, so long as your own stance is technically valid and scientifically accurate (ie. the numbers must add up!) then you are free to persuade others to your own point of view. It's what debate is all about.

loandbehold
2004-Nov-01, 10:40 AM
The current rising trend can be most simply described as a result of the warming that happened during the Medieval Warm Period roughly 800-1000 years ago.

Are you saying that the CO2 rise that we are seeing today is of the same origin as the rise seen during deglaciations? If so, wouldn't you expect a much smaller increase? After all, during deglaciations the temperature rise is something like 6 deg C and the CO2 rise is approximately 90ppm. During the last 200 years the CO2 concentration has increased by about 90ppm, so would that mean that the MWP was 6 degrees warmer than the previous period? If so, do you have any evidence that the temperature increase was of this order?

And if the present CO2 rise is predominately natural in origin, what is happening to the CO2 that is being released by fossil fuel burning? And what about the evidence of fossil fuel sources in the Carbon-13 and Carbon-14 concentrations of the atmosphere (the so-called "Suess effect")?

Glom
2004-Nov-01, 01:14 PM
Fossil fuels have something to do with it. They do account for almost 5% of carbon dioxide emissions. Good thing too. It'll make the world a greener place.

loandbehold
2004-Nov-01, 01:38 PM
Fossil fuels have something to do with it. They do account for almost 5% of carbon dioxide emissions. Good thing too. It'll make the world a greener place.

But how much of the current increase do fossil fuels account for? It's true that natural sources of CO2 outweigh anthropogenic sources, but the earth also acts as a large sink of CO2. That's why in the absence of anthropogenic influences or large-scale perturbations to the climate system the concentrations of CO2 are more or less constant- the natural sinks usually balance the natural sources.

A more meaningful question to ask in this context is whether nature is currently acting as a net sink, or as a net source of CO2. Taking a look here (http://www.grida.no/climate/ipcc_tar/wg1/110.htm#351) you'll see that during the 90's the anthropogenic emissions were 6.4 PgC/yr, while the atmospheric increase was only 3.2 PgC/yr. This means that nature must be currently acting as a net sink of CO2 (i.e. it is absorbing more than it is emitting). This basically means that if we stopped emitting CO2 tomorrow, the atmospheric concentration would actually start going down (at least at first, anyway).

Glom
2004-Nov-01, 03:24 PM
But think what would happen to the plants if that happened. Good thing we're here.

You speak of dynamic equilibrium. There are so many external factors that alter the position of the equilibrium. At the moment, the planet may be sucking in carbon (not a good thing), but that will be highly variable as we've seen from that fact that carbon dioxide levels have changed markedly throughout time. Still, this doesn't solve the mystery of why the present increase in levels seems to be greater than what could be accounted for by Diamond's suggestion that it is the result of the MWP.

In any event, it has still not been established that carbon dioxide is a determiner of climate since Diamond's graph shows that carbon dioxide lags temperature. The fact that carbon dioxide is greater than expected now may show that something else is up, but it doesn't show it is a major climate driver.

loandbehold
2004-Nov-01, 04:45 PM
In any event, it has still not been established that carbon dioxide is a determiner of climate since Diamond's graph shows that carbon dioxide lags temperature. The fact that carbon dioxide is greater than expected now may show that something else is up, but it doesn't show it is a major climate driver.

Well, what the graph actually shows is that CO2 increases aren't the cause for the initial onset of deglaciation. Which isn't really surprising, since the timing of ice ages is thought to be regulated by Milankovich oscillations, which basically act to vary the timing and distribution of insolation at high latitudes. This impacts the ice albedo and hence the temperature. If the temperatures then change the CO2 concentration, it could in turn influence the temperature. So it's perfectly plausable that you have a two-way relationship between CO2 and temperature, which act as a positive feedback loop during deglaciations.

I agree, though, that the you can't categorically state that CO2 is a factor in the temperature changes based on such paleoclimatic evidence, since there are usually many other factors at work and the data is patchy. The climate models seem to fit the observed temperatures with the changes in CO2 included, though, so this might be taken as indicating that CO2 is important in driving these changes.

Really, though, the reason why CO2 is thought to change the climate is based on understanding of the radiative energy budget of the earth, as well as analysis of the warming over the past century.

dgruss23
2004-Nov-01, 06:17 PM
Glom, Diamond, dgruss23, et al. I have a question that I consider relevant to this and other similar threads.

What exactly are your political philosophies? Are you all by any chance libertarians or otherwise strongly pro-free market people? Do you support the Cato Institute or the Junk Science webpage?

My evaluation of this issue is based first upon the scientific evidence. But the political ramifications are important. I see no reason to hamstring our economy for a problem that hasn't been shown scientifically to exist.

And yes I'm pro-free market - its the best way to freedom for all.

Glom
2004-Nov-01, 07:56 PM
Well, what the graph actually shows is that CO2 increases aren't the cause for the initial onset of deglaciation. Which isn't really surprising, since the timing of ice ages is thought to be regulated by Milankovich oscillations, which basically act to vary the timing and distribution of insolation at high latitudes. This impacts the ice albedo and hence the temperature. If the temperatures then change the CO2 concentration, it could in turn influence the temperature. So it's perfectly plausable that you have a two-way relationship between CO2 and temperature, which act as a positive feedback loop during deglaciations.

So what caused the change in the nature of the feedback? My bet would be on water vapour. During an ice age, a lot of water is frozen and therefore is not taken part in the hydrological cycle. When the planet recovers from an ice age, a lot of water is liberated and enters the hydrological cycle, creating clouds, dramatically effecting the heat flow the atmosphere, and swamping the effects of the puny carbon dioxide as a greenhouse gas. It may be the case that carbon dioxide is the major player in the absence of anything else.


I agree, though, that the you can't categorically state that CO2 is a factor in the temperature changes based on such paleoclimatic evidence, since there are usually many other factors at work and the data is patchy. The climate models seem to fit the observed temperatures with the changes in CO2 included, though, so this might be taken as indicating that CO2 is important in driving these changes.

I thought the climate models were rubbish. They predicted a present about five degrees warmer than it was when inputted with data from thirty years ago or something.


Really, though, the reason why CO2 is thought to change the climate is based on understanding of the radiative energy budget of the earth, as well as analysis of the warming over the past century.

But carbon dioxide is not that powerful a greenhouse gas. Water vapour is far more dominant and since carbon dioxide is found closer to the surface, while the hydrological cycle carries heat bearing water far above the carbon dioxide layer, the ability of carbon dioxide to imprison vast amounts of heat in the atmosphere is limited.

Diamond
2004-Nov-01, 08:39 PM
Glom, Diamond, dgruss23, et al. I have a question that I consider relevant to this and other similar threads.

What exactly are your political philosophies? Are you all by any chance libertarians or otherwise strongly pro-free market people? Do you support the Cato Institute or the Junk Science webpage?

Irrelevant. No. No.

Since when has a view of science been allied to a political viewpoint? Global Warming has become a political viewpoint and a religious belief by some. I find it personally offensive that the response to scientific evidence appears to be to characterise them according to some extreme political opinion.

This website and this forum are about science. The scientific evidence, in my view, does not support the notion that global climate is affected in any measureable way by human activities.

Diamond
2004-Nov-01, 08:44 PM
Finally a discussion of global warming with a little science to back it up! Nice.
A few questions: Diamond, how do they estimate the temperature that many years in the past? Also, I see the 800 year delay in the points of inflection, but if you didn't highlight them, I don't think I would have noticed. You are saying this occurs in other evidencs?

Launch, there is an EPA report that says (abridging your quote) humans are causing global warming? Would you cite the report?

The answer to the question is in the graph. In order to estimate temperature they use isotope ratios, on the assumption that these ratios are strongly correlated with temperature.

The other piece of evidence I would point out is that contrary to what you'd expect, ALL ice cores show the same pattern: the temperature rises and then carbon dioxide levels rise some 8 -10 centuries later (on average). Not one ice core has ever shown a carbon dioxide rise preceding temperature rise, in defiance of greenhouse warming hypothesis.

bobjohnston
2004-Nov-01, 08:47 PM
A more meaningful question to ask in this context is whether nature is currently acting as a net sink, or as a net source of CO2. Taking a look here (http://www.grida.no/climate/ipcc_tar/wg1/110.htm#351) you'll see that during the 90's the anthropogenic emissions were 6.4 PgC/yr, while the atmospheric increase was only 3.2 PgC/yr. This means that nature must be currently acting as a net sink of CO2 (i.e. it is absorbing more than it is emitting). This basically means that if we stopped emitting CO2 tomorrow, the atmospheric concentration would actually start going down (at least at first, anyway).

Here's a graphical look at the point raised above:

http://www.johnstonsarchive.net/environment/g-delco2.gif

Of the anthropogenic CO2 emissions since 1940, about 53% is still in the atmosphere and the rest has been sequestered.

Diamond
2004-Nov-01, 09:00 PM
The most relevant question is whether carbon dioxide, a trace gas, causes warming at all. For the all the tedious accounting of gigatons of carbon (which as someone has already pointed out, is a very small fraction of the natural output), no-one has yet produced evidence that more carbon dioxide actuall produces warming at all.

Glom
2004-Nov-01, 09:44 PM
You to prove my dedication to the P&R rule, I took a potential political discussion about libertarians to PM. That's bought me another three months without being banned.

bobjohnston
2004-Nov-01, 10:46 PM
The most relevant question is whether carbon dioxide, a trace gas, causes warming at all. For the all the tedious accounting of gigatons of carbon (which as someone has already pointed out, is a very small fraction of the natural output), no-one has yet produced evidence that more carbon dioxide actuall produces warming at all.

I don't think there's much argument that, everything else being equal, more CO2 gives a higher planetary temperature. The physics there is pretty clear. However, it is not clear for the CO2 increases we're discussing that a temperature increase would be observable. This involves two aspects: first, we may not be able to distinguish the temperature increase; and two, feedback effects could negate the increase.

Diamond
2004-Nov-01, 10:53 PM
The most relevant question is whether carbon dioxide, a trace gas, causes warming at all. For the all the tedious accounting of gigatons of carbon (which as someone has already pointed out, is a very small fraction of the natural output), no-one has yet produced evidence that more carbon dioxide actuall produces warming at all.

I don't think there's much argument that, everything else being equal, more CO2 gives a higher planetary temperature. The physics there is pretty clear.

No it isn't. The Greenhouse analogy is faulty, but even so, if you microscopically changed the properties of the glass would the temperature rise in the greenhouse?

Or alternatively, if the temperature rose and caused more water to evaporate, wouldn't the increased cloudiness block out more sunlight and reduce the temperature?


However, it is not clear for the CO2 increases we're discussing that a temperature increase would be observable. This involves two aspects: first, we may not be able to distinguish the temperature increase; and two, feedback effects could negate the increase.

Which is why, in order to justify the economic suicide of Kyoto, lots of mythic "positive feedbacks" and " tipping points" are put in

bobjohnston
2004-Nov-02, 02:02 AM
No it isn't. The Greenhouse analogy is faulty, but even so, if you microscopically changed the properties of the glass would the temperature rise in the greenhouse?

Or alternatively, if the temperature rose and caused more water to evaporate, wouldn't the increased cloudiness block out more sunlight and reduce the temperature?


Yes, the greenhouse analogy should be trashed. What atmospheric CO2 does is prevent the escape of thermal blackbody radiation into space because it absorbs light at a variety of fixed wavelengths. Since equilibrium requires incoming energy to be matched by outgoing radiation, the Earth's temperature must be higher if there is more absorption by CO2 (or H2O, etc.).

So if you could ignore every other effect, more CO2 would mean a higher temperature. This was my point. But your valid point is that we can't ignore every other effect, and your example of a possible negative feedback mechanism is also valid.

So one issue is that feedback mechanisms are not fully understood; some are potentially positive, some potentially negative. Second, we have good reason to believe that the effect of CO2 alone is small compared to other climate influences. Finally, the effect of CO2 when everything else is ignored is possibly overestimated: some observational evidence suggests that the effect of a given CO2 increase is overestimated in climate models by a factor of 3-4 or more.

loandbehold
2004-Nov-02, 08:38 AM
Well, what the graph actually shows is that CO2 increases aren't the cause for the initial onset of deglaciation. Which isn't really surprising, since the timing of ice ages is thought to be regulated by Milankovich oscillations, which basically act to vary the timing and distribution of insolation at high latitudes. This impacts the ice albedo and hence the temperature. If the temperatures then change the CO2 concentration, it could in turn influence the temperature. So it's perfectly plausable that you have a two-way relationship between CO2 and temperature, which act as a positive feedback loop during deglaciations.

So what caused the change in the nature of the feedback? My bet would be on water vapour. During an ice age, a lot of water is frozen and therefore is not taken part in the hydrological cycle. When the planet recovers from an ice age, a lot of water is liberated and enters the hydrological cycle, creating clouds, dramatically effecting the heat flow the atmosphere, and swamping the effects of the puny carbon dioxide as a greenhouse gas. It may be the case that carbon dioxide is the major player in the absence of anything else.

I think the point is that the ability of the oceans to absorb CO2 doesn't only decrease with the temperature, it also increases with the partial pressure of the CO2 in the atmosphere. So you tend to have two effects that compete with each other, and the whether one wins out over the other depends on the conditions. In particular the initial conditions are different- during deglaciations the temperature change came first, while in the present case the carbon dioxide is increasing first. So this could be the reason why the oceans were acting as a net source, while at the moment they are acting as a net sink. The latter could well change if there is significant warming in the future.



I agree, though, that the you can't categorically state that CO2 is a factor in the temperature changes based on such paleoclimatic evidence, since there are usually many other factors at work and the data is patchy. The climate models seem to fit the observed temperatures with the changes in CO2 included, though, so this might be taken as indicating that CO2 is important in driving these changes.

I thought the climate models were rubbish. They predicted a present about five degrees warmer than it was when inputted with data from thirty years ago or something.

I doubt that the climate models would predict a warming of that magnitude. It's true, though, that in the past climate models tended to overestimate the present warming, but then the point was that they didn't include the cooling effects of sulphate aerosols. Once these were included the agreement became better.



Really, though, the reason why CO2 is thought to change the climate is based on understanding of the radiative energy budget of the earth, as well as analysis of the warming over the past century.

But carbon dioxide is not that powerful a greenhouse gas. Water vapour is far more dominant and since carbon dioxide is found closer to the surface, while the hydrological cycle carries heat bearing water far above the carbon dioxide layer, the ability of carbon dioxide to imprison vast amounts of heat in the atmosphere is limited.

I'm not sure I understand your point. Even if the water vapour tends to be higher in the atmosphere than the carbon dioxide (and I don't know whether this is true) I don't see how this would effect the greenhouse effect of CO2. The greenhouse effect arises from absorption of infrared radiation emitted from the surface and propagating upwards into the atmosphere. So if the water vapour is higher than the CO2 then the absorption of the IR should be unaffected by whether or not there is water vapour present. Also, although there's some overlap, CO2 and water vapour tend to absorb IR at different frequencies. So I don't see how water vapour in this case would limit the ability of carbon dioxide to trap heat.

Glom
2004-Nov-02, 10:25 AM
It would limit heat capture by carrying heat through convection above the layers of the most carbon dioxide where it could be radiated into space. Carbon dioxide, and greenhouse gases in general, can only trap heat being radiated from below. If the heat is carried by some other means, such as convection above where the gas is most abundant, the heat can escape.

The other thing I understand is that as climate models have improved, and there is still a long way to go before they can be considered reliable enough to be used to justify drastic action, the date of our impending doom has been delayed further and further.

loandbehold
2004-Nov-02, 10:47 AM
It would limit heat capture by carrying heat through convection above the layers of the most carbon dioxide where it could be radiated into space. Carbon dioxide, and greenhouse gases in general, can only trap heat being radiated from below. If the heat is carried by some other means, such as convection above where the gas is most abundant, the heat can escape.

Ah, sorry, I misunderstood. You're saying that some of the energy will be stored as latent heat, rather than being available to heat the surface? Yes, that's certainly something that happens. It basically acts to reduce the lapse rate (the rate of temperature decrease with increasing altitude). This then reduces the greenhouse effect, though not completely (otherwise there wouldn't be a natural greenhouse effect).