Generally, the temperature increase due to a doubling of CO2 (280 ppm to 560 ppm) should result in a global average temperature increase of about 1.5 degrees C.
Most of the predictions for future warming lie in the 3 to 11 degree C range, although a few people predict that we could have uncontrolled warming.
Given this difference between CO2 forcing and predicted temperature increases, there is an implication that there is some sort of positive feedback mechanism.
Is it reasonable to assume that there is a positive feedback mechanism at work? Especially in light of the fact that almost all natural systems exhibit negative feedback mechanisms (broad generalization) that tend to dampen the effects of perturbations to a system.
I realize that H2O is often cited as the mechanism, but there is a high degree of uncertainty due to the fact that water vapor can be a driver of cooling AND warming.
Is there historical precedence for assuming that these positive feedbacks exist?
Why do we assume that positive feedback mechanisms are driving global warming?
There are a lot of feedbacks in the climate system, both positive and negative.
1. Water vapor. Air at 50% relative humidity (RH) and 16°C holds more water than air at 50% RH and 15°C. Water is a greenhouse gas (not as potent as CO2 per molecule, but there's a lot more of it in the air). In general, the amount of water in the toposphere is very closely tied to the surface temperature, so increasing surface temps mean increased *absolute* humidity, even if the *relative* humidity does not change.
You are right that eventually this will result in a large negative feedback, but that's not until the temperature rise is large enough to impact evaporation enough to increase worldwide cloudiness. Given that the *immediate* effect of warmth is to decrease RH rather than increase it, and given that the overall average RH in the troposphere is something like 50%, we've got a long way to go to get there.
2. Ice and snow reflect 80% of the sunlight hitting them, increasing earth's albedo and thus cooling the planet. As the planet warms, there is less ice and snow, meaning more of the darker surface is exposed, leading to more absorbtion of solar energy. This is one of the factors driving the rapid decrease of Arctic sea ice we've been seeing over the last few years.
3. Ocean warming. Warm water holds less CO2 than cool water. Right now we're lucky, because much of the CO2 we create by burning fossil fuels is absorbed by the oceans. But as the oceans warm, it will hold less and less, and eventually (around 2100 according to current predictions) the ocean will reach equilibrium with CO2 -- and after that, as temps continue to warm, it will start emitting some of the CO2 it absorbed in the 20th and 21st centuries.
Here are some overviews of climate feedbacks:
http://www.globalchange.umich.edu/global...
http://www-ocean.tamu.edu/410/Lecture_ma...
Reply:Bob's answer is accurate. I don't recognize your numbers (1.5°C, etc.) either, but the IPCC doesn't take feedbacks into account. Because of this, their predictions on future global warming are almost certainly conservative.
As mentioned by Bob, we know that darker oceans absorb more sunlight than reflective ice. That's a simple feedback. There are many other more complex ones which we know will occur at a certain tipping point temperature, such as CO2 becoming less soluble in warmer water, so as the oceans warm they'll begin to emit more carbon dioxide and absorb less.
Reply:It is not assumed it is known. For example, as ice cover is reduced by temperature rises it changes its surface albedo. As ice is very reflective the exposed land, or water will be a positive feedback. This is certainly not a politically based assumption as some suggest.
Reply:The answer is that we don't. Or at least the IPCC report doesn't. I don't know exactly where you're getting 1.5C and 3C to 11C from.
But most scientists think (they don't assert it is proven fact) that positive feedbacks will occur.
At least one positive feedback seems like a no brainer. Reflective ice melts, exposes dark ground. Very hard to see how that wouldn't be important.
Still, the lack of including positive feedbacks in the latest IPCC report has many scientists concerned that global warming will be much worse, in speed and amount, than the IPCC predictions.
"TONY JONES: It’s interesting you should quote the IPCC report, because you are very critical of it, aren’t you? You think it's rather a conservative outlook and doesn't reflect what in fact is happening or the speed with which it is happening.
PROFESSOR TIM FLANNERY: Look, I'm not critical of it in a sense. It's a consensus document and it is conservative particularly from the perspective that it cuts off the science in early 2005 and a lot has happened since then. Of course, being a consensus document, a lot of the material that I think is reasonably well-supported also gets weeded out through that process. If the IPCC says it you better believe it and then leave room to think it is actually a lot worse than they have said.
TONY JONES: What do you believe that goes beyond what they think and what's the evidence for that?
PROFESSOR TIM FLANNERY: Look, what we've seen in the Arctic over the last two years has been such breathtaking change that you have to worry about stability for sea levels and for the entire Northern Hemisphere climate system. The rate of ice melt in 2005 increased by about five times over what it was previously and it's been very, very large again in 2006."
http://www.abc.net.au/lateline/content/2...
The historical precedent is the very rapid climate shifts that have occurred in the past. Their rapidity is hard (maybe impossible) to explain other than by positive feedback mechanisms.
http://www.aip.org/history/climate/rapid...
http://www.esd.ornl.gov/projects/qen/tra...
The well respected scientists James Lovelock, has written extensively (within and without the peer reviewed literature) about the powerful negative feedback mechanisms of Earth. He calls Earth Gaia, in respect of it's self regulating nature.
The data on global warming has changed his mind, convincing him that this time is different. His latest book "The Revenge of Gaia" is a powerful plea for nuclear power plants to fight global warming.
It's one of the more amazing conversions. Here's the book:
http://www.amazon.com/Revenge-Gaia-Earth...
EDIT - Here's some info about the complicated effects volcanoes can have on climate. The bottom line is that the effects are predominantly cooling, but there can be localized warming, particularly shortly after the eruption.
Kirchner, I., G. Stenchikov, H.-F. Graf, A. Robock. J. Antuna, Climate model simulation of winter warming and summer cooling following the 1991 Mount Pinatubo volcanic eruption, J. Geophys. Res., 104, 19,039-19,055, 1999
http://volcano.und.edu/vwdocs/Gases/clim...
http://www.agu.org/pubs/crossref/1992/92...
The cause of the Permian extinction is controversial. Here's one article about it:
http://www.terradaily.com/reports/What_A...
Note that the volcanic eruptions some people think may have been the cause were "supervolcanoes" occuring in that time, and unlike anything now in existence.
Reply:"We" don't, you do; remember you are only speaking for yourself unless you can provide a specific person's statement to that effect..
There are both positive and negative feedback loops involved, as have been stated. The variety in numbers only reflects the different models used to predict future events. The models chosen would determine the parameters and feedback loops used.
The Permian Extinction (the one BEFORE the dinosaurs) was thought to be due to a huge volcano that spanned the Asian Continent from the top to the bottom of Russia, but that event was not the norm for volcanoes, that was the largest eruption in history, and it was not the only driving force of Global Warming. It erupted for years, the amount of CO2 produced was massive and the effects were seen over thousands of years (let's hope the same is not true of GW). But it was not enough on its own to drive the extinction event. The release of submarine deposits of methane due to rising ocean temperature finished the job (as recorded by the quantity of Carbon 12 in the fossil record).
That is one potential model based on historical evidence, but it is not the only model.
In determining which feedback loops to base the model on judgement must be used, i.e. airplane contrails reflect energy, so it is a negative feedback loop, it is recorded as a force factor; but the effect is so small and the error so large it is generally ignored.
But if you really want to compare the models used I would suggest you try the Hadley Center for Atmospheric Research. They are pretty much at the center of it all.
Reply:If they didn't assume a positive feedback then global warming would not be much of a problem, and what kind of political campaign can you make from a small problem?
Reply:It's true that absent positive feedback, GW is not a serious problem.
However, there is fossil evidence of positive feedback. As Mr. Jello points out, CO2 lags temperature at the end of every ice age, and that means that warming begets CO2, which causes even more warming. That's positive feedback, but CO2 alone can't lead to catastrophe, because it saturates.
The real danger is that other feedback mechanisms will run away. Methane hydrate on the ocean floor may release potent amounts of methane upon warming.
If the entire infrared spectrum becomes blanketed with saturated greenhouse gas absorption, the temperature could rise 30 C. This seems to have happened in the great Permian extinction, in reaction to a rapid warming caused by a major volcanic event. That may very well be the best historical analog to the current, rapid, anthropgenic warming.
(But I hope not.)
Reply:"Generally, the temperature increase due to a doubling of CO2 (280 ppm to 560 ppm) should result in a global average temperature increase of about 1.5 degrees C"
A CO2 increase to 440 PPM was observed in the late 1930's only to see temperatures decline. Climate models cannot explain what has already occurred, I don't think it would make any sense whatsoever to believe future predictions.
http://www.anenglishmanscastle.com/archi...
Full Paper:
http://www.anenglishmanscastle.com/180_y...
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Reply:The truth is that CO2 is a lagging factor of temps. Temps rise about 800 years before co2 starts to rise, and temps cool 800 years before co2 starts to reduce. Co2 STILL INCREASES durring the time the demps are rapidly decreasing.
The fact is that these are only guesses. No one knows. There guess isn't any better than a flip of a coin.
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